Make your own free website on Tripod.com
TOP NEXT HOME

IBM Personal Computer Assembly
Language Tutorial


This talk is for people who are just getting started with the PC MACRO Assembler. Maybe you are just contemplating doing some coding in assembler, maybe you have tried it with mixed success. If you are here to get aimed in the right direction, to get off to a good start with the assembler, then you have come for the right reason. I can't promise you'll get what you want, but I'll do my best.

On the other hand, if you have already turned out some working assembler code, then this talk is likely to be on the elementary side for you. If you want to review a few basics and have no where else pressing to go, then by all means stay.

Why Learn Assembler?

The reasons for LEARNING assembler are not the same as the reasons for USING it in a particular application. But, we have to start with some of the reasons for using it and then I think the reasons for learning it will become clear.

First, let's dispose of a bad reason for using it. Don't use it just because you think it is going to execute faster. A particular sequence of ordinary bread-and-butter computations written in PASCAL, C, FORTRAN, or compiled BASIC can do the job just about as fast as the same algorithm coded in assembler. Of course, interpretive BASIC is slower, but if you have a BASIC application which runs too slow you probably want to try compiling it before you think too much about translating parts of it to
another language.

On the other hand, high level languages do tend to isolate you from the machine. That is both their strength and their weakness. Usually, when implemented on a micro, a high level language provides an escape mechanism to the underlying operating system or to the bare machine. So, for example, BASIC has its PEEK and POKE. But, the route to the bare machine is often a circuitous one, leading to tricky programming which is hard to follow.

For those of us working on PC's connected to SHARE-class mainframes, we are generally concerned with three interfaces: the keyboard, the screen, and the communication line or lines. All three of these entities raise machine dependent issues which are imperfectly addressed by the underlying operating system or by high level languages.

Sometimes, the system or the language does too little for you. For example, with the asynch adapter, the system provides no interrupt handler, no buffer, and no flow control. The application is stuck with the responsibility for monitoring that port and not missing any characters, then deciding what to do with all errors. BASIC does a reasonable job on some of this, but that is only BASIC. Most other languages do less.

Sometimes, the system may do too much for you. System support for the keyboard is an example. At the hardware level, all 83 keys on the keyboard send unique codes when they are pressed, held down, and released. But, someone has decided that certain keys, like Num Lock and Scroll Lock are going to do certain things before the application even sees them and can't therefore be used as ordinary keys.

Sometimes, the system does about the right amount of stuff but does it less efficiently then it should. System support for the screen is in this class. If you use only the official interface to the screen you sometimes slow your application down unacceptably. I said before, don't use assembler just to speed things up, but there I was talking about mainline code, which generally can't be speeded up much by assembler coding. A critical system interface is a different matter: sometimes we may have to use assembler to bypass a hopelessly inefficient implementation. We don't want to do this if we can avoid it, but sometimes we can't.

Assembly language code can overcome these deficiencies. In some cases, you can also overcome these deficiencies by judicious use of the escape valves which your high level language provides. In BASIC, you can PEEK and POKE and INP and OUT your way around a great many issues. In many other languages you can issue system calls and interrupts and usually manage, one
way or other, to modify system memory. Writing handlers to take real-time hardware interrupts from the keyboard or asynch port, though, is still going to be a problem in most languages. Some languages claim to let you do it but I have yet to see an acceptably clean implementation done that way.

The real reason while assembler is better than "tricky POKEs" for writing machine-dependent code, though, is the same reason why PASCAL is better than assembler for writing a payroll package: it is easier to maintain.

Let the high level language do what it does best, but recognize that there are some things which are best done in assembler code. The assembler, unlike the tricky POKE, can make judicious use of equates, macros, labels, and appropriately placed comments to show what is really going on in this machine-dependent realm where it thrives.

So, there are times when it becomes appropriate to write in assembler; given that, if you are a responsible programmer or manager, you will want to be "assembler-literate" so you can decide when assembler code should be written.

What do I mean by "assembler-literate?" I don't just mean understanding the 8086 architecture; I think, even if you don't write much assembler code yourself, you ought to understand the actual process of turning out assembler code and the various ways to incorporate it into an application. You ought to be able to tell good assembler code from bad, and appropriate assembler code from inappropriate.

Steps to becoming ASSEMBLER-LITERATE

1. Learn the 8086 architecture and most of the instruction set. Learn what you need to know and ignore what you don't. Reading: The 8086 Primer by Stephen Morse, published by Hayden. You need to read only two chapters, the one on machine organization and the one on the instruction set.

2. Learn about a few simple DOS function calls. Know what services the operating system provides. If appropriate, learn a little about other systems too. It will aid portability later on. Reading: appendices D and E of the PC DOS manual.

3. Learn enough about the MACRO assembler and the LINKer to write some simple things that really work. Here, too, the main thing is figuring out what you don't need to know. Whatever you do, don't study the sample programs distributed with the assembler unless you have nothing better!

4. At the same time as you are learning the assembler itself, you will need to learn a few tools and concepts to properly combine your assembler code with the other things you do. If you plan to call assembler subroutines from a high level language, you will need to study the interface notes provided in your language manual. Usually, this forms an appendix of some sort. If you plan to package your assembler routines as .COM programs you will need to learn to do this. You should also learn to use DEBUG.

5. Read the Technical Reference, but very selectively. The most important things to know are the header comments in the BIOS listing. Next, you will want to learn about the RS 232 port and maybe about the video adapters.

Notice that the key thing in all five phases is being selective. It is easy to conclude that there is too much to learn unless you can throw away what you don't need. Most of the rest of this talk is going to deal with this very important question of what you need and don't need to learn in each phase. In some cases, I will have to leave you to do almost all of the learning, in others, I will teach a few salient points, enough, I hope, to get you started. I hope you understand that all I can do in an hour is get you started on the way.