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 lan- guages 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. In the following article, 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. Whichever assembler you decide to use.
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.