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1988-06-03
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IBM Personal Computer Assembly
Language Tutorial
Joshua Auerbach
Yale University
Yale Computer Center
175 Whitney Avenue
P. O. Box 2112
New Haven, Connecticut 06520
Installation Code YU
Integrated Personal Computers Project
Communications Group
Communications and Data Base Division
Session C316
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?
____________________
Why Learn Assembler?
Why Learn Assembler?
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 com-
IBM PC Assembly Language Tutorial 1
piling 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 operat-
ing 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 respon-
sibility 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 key-
board 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 assem-
bler 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 lan-
guages 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.
IBM PC Assembly Language Tutorial 2
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; giv-
en 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 assem-
bler 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
____________________________________
Steps to becoming ASSEMBLER-LITERATE
Steps to becoming ASSEMBLER-LITERATE
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 sam-
ple 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 assem-
bler 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 rou-
tines 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.