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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?
____________________
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.
Phase 1: Learn the architecture and instruction set
____________________________________________________
The Morse book might seem like a lot of book to buy for just two really
important chapters; other books devote a lot more space to the instruction set
and give you a big beautiful reference page on each instruction. And, some of
the other things in the Morse book, although interesting, really aren't very
vital and are covered too sketchily to be of any real help. The reason I like
the Morse book is that you can just read it; it has a very conversational
style, it is very lucid, it tells you what you really need to know, and a
little bit more which is by way of background; because nothing really gets
belabored to much, you can gracefully forget the things you don't use. And, I
very much recommend READING Morse rather than studying it. Get the big picture
at this point.
Now, you want to concentrate on those things which are worth fixing in
memory. After you read Morse, you should relate what you have learned to this
outline.
1. You want to fix in your mind the idea of the four segment registers CODE,
DATA, STACK, and EXTRA. This part is pretty easy to grasp. The 8086 and the
8088 use 20 bit addresses for memory, meaning that they can address up to 1
megabyte of memory. But, the registers and the address fields in all the
instructions are no more that 16 bits long. So, how to address all of that
memory? Their solution is to put together two 16 bit quantities like this:
calculation SSSS0 ---- value in the relevant segment register SHL 4
depicted in AAAA ---- apparent address from register or instruction
hexadecimal --------
RRRRR ---- real address placed on address bus
In other words, any time memory is accessed, your program will supply a
sixteen bit address. Another sixteen bit address is acquired from a segment
register, left shifted four bits (one nibble) and added to it to form the real
address. You can control the values in the segment registers and thus access
any part of memory you want. But the segment registers are specialized: one
for code, one for most data accesses, one for the stack (which we'll mention
again) and one "extra" one for additional data accesses.
Most people, when they first learn about this addressing scheme become
obsessed with converting everything to real 20 bit addresses. After a while,
though, you get use to thinking in segment/offset form. You tend to get your
segment registers set up at the beginning of the program, change them as little
as possible, and think just in terms of symbolic locations in your program, as
with any assembly language.
EXAMPLE:
MOV AX,DATASEG
MOV DS,AX ;Set value of Data segment
ASSUME DS:DATASEG ;Tell assembler DS is usable
.......
MOV AX,PLACE ;Access storage symbolically by 16 bit address
In the above example, the assembler knows that no special issues are involved
because the machine generally uses the DS register to complete a normal data
reference.
If you had used ES instead of DS in the above example, the assembler would
have known what to do, also. In front of the MOV instruction which accessed
the location PLACE, it would have placed the ES segment prefix. This would
tell the machine that ES should be used, instead of DS, to complete the
address.
Some conventions make it especially easy to forget about segment registers.
For example, any program of the COM type gets control with all four segment
registers containing the same value. This program executes in a simplified 64K
address space. You can go outside this address space if you want but you don't
have to.
2. You will want to learn what other registers are available and learn their
personalities:
AX and DX are general purpose registers. They become special only when
accessing machine and system interfaces.
CX is a general purpose register which is slightly specialized for counting.
BX is a general purpose register which is slightly specialized for forming
base-displacement addresses.
AX-DX can be divided in half, forming AH, AL, BH, BL, CH, CL, DH, DL.
SI and DI are strictly 16 bit. They can be used to form indexed addresses
(like BX) and they are also used to point to strings.
SP is hardly ever manipulated. It is there to provide a stack.
BP is a manipulable cousin to SP. Use it to access data which has been
pushed onto the stack.
Most sixteen bit operations are legal (even if unusual) when performed in SI,
DI, SP, or BP.
3. You will want to learn the classifications of operations available
WITHOUT getting hung up in the details of how 8086 opcodes are constructed.
8086 opcodes are complex. Fortunately, the assembler opcodes used to
assemble them are simple. When you read a book like Morse, you will learn some
things which are worth knowing but NOT worth dwelling on.
a. 8086 and 8088 instructions can be broken up into subfields and bits with
names like R/M, MOD, S and W. These parts of the instruction modify the basic
operation in such ways as whether it is 8 bit or 16 bit, if 16 bit, whether all
16 bits of the data are given, whether the instruction is register to register,
register to memory, or memory to register, for operands which are registers,
which register, for operands which are memory, what base and index registers
should be used in finding the data.
b. Also, some instructions are actually represented by several different
machine opcodes depending on whether they deal with immediate data or not, or
on other issues, and there are some expedited forms which assume that one of
the arguments is the most commonly used operand, like AX in the case of
arithmetic.
There is no point in memorizing any of this detail; just distill the bottom
line, which is, what kinds of operand combinations EXIST in the instruction set
and what kinds don't. If you ask the assembler to ADD two things and the two
things are things for which there is a legal ADD instruction somewhere in the
instruction set, the assembler will find the right instruction and fill in all
the modifier fields for you.
I guess if you memorized all the opcode construction rules you might have a
crack at being able to disassemble hex dumps by eye, like you may have learned
to do somewhat with 370 assembler. I submit to you that this feat, if ever
mastered by anyone, would be in the same class as playing the "Minute Waltz" in
a minute; a curiosity only. Here is the basic matrix you should remember:
Two operands: One operand:
R <-- M R
M <-- R M
R <-- R S *
R|M <-- I
R|M <-- S *
S <-- R|M *
* -- data moving instructions (MOV, PUSH, POP) only
S -- segment register (CS, DS, ES, SS)
R -- ordinary register (AX, BX, CX, DX, SI, DI, BP, SP,
AH, AL, BH, BL, CH, CL, DH, DL)
M -- one of the following
pure address
[BX]+offset
[BP]+offset
any of the above indexed by SI
any of the first three indexed by DI
4. Of course, you want to learn the operations themselves. As I've
suggested, you want to learn the op codes as the assembler presents them, not
as the CPU machine language presents them. So, even though there are many MOV
op codes you don't need to learn them. Basically, here is the instruction set:
a. Ordinary two operand instructions. These instructions perform an
operation and leave the result in place of one of the operands. They are:
1) ADD and ADC -- addition, with or without including a carry from a previous
addition 2) SUB and SBB -- subtraction, with or without including a borrow from
a previous subtraction 3) CMP -- compare. It is useful to think of this as a
subtraction with the answer being thrown away and neither operand actually
changed 4) AND, OR, XOR -- typical boolean operations 5) TEST -- like an AND,
except the answer is thrown away and neither operand is changed. 6) MOV --
move data from source to target 7) LDS, LES, LEA -- some specialized forms of
MOV with side effects
b. Ordinary one operand instructions. These can take any of the operand
forms described above. Usually, the perform the operation and leave the result
in the stated place:
1) INC -- increment contents
2) DEC -- decrement contents
3) NEG -- twos complement
4) NOT -- ones complement
5) PUSH -- value goes on stack (operand location itself unchanged)
6) POP -- value taken from stack, replaces current value
c. Now you touch on some instructions which do not follow the general
operand rules but which require the use of certain registers. The important
ones are:
1) The multiply and divide instructions 2) The "adjust" instructions which
help in performing arithmetic on ASCII or packed decimal data 3) The shift and
rotate instructions. These have a restriction on the second operand: it must
either be the immediate value 1 or the contents of the CL register. 4) IN and
OUT which send or receive data from one of the 1024 hardware ports. 5) CBW and
CWD -- convert byte to word or word to doubleword by sign extension
d. Flow of control instructions. These deserve study in themselves and we
will discuss them a little more. They include:
1) CALL, RET -- call and return
2) INT, IRET -- interrupt and return-from-interrupt
3) JMP -- jump or "branch"
4) LOOP, LOOPNZ, LOOPZ -- special (and useful) instructions which
implement a counted loop similar to the 370 BCT instruction
5) various conditional jump instructions
e. String instructions. These implement a limited storage-to-storage
instruction subset and are quite powerful. All of them have the property that:
1) The source of data is described by the combination DS and SI.
2) The destination of data is described by the combination ES and DI.
3) As part of the operation, the SI and/or DI register(s) is(are) incremented
or decremented so the operation can be repeated.
They include:
1) CMPSB/CMPSW -- compare byte or word
2) LODSB/LODSW -- load byte or word into AL or AX
3) STOSB/STOSW -- store byte or word from AL or AX
4) MOVSB/MOVSW -- move byte or word
5) SCASB/SCASW -- compare byte or word with contents of AL or AX
6) REP/REPE/REPNE -- a prefix which can be combined with any of
the above instructions to make them execute repeatedly across a
string of data whose length is held in CX.
f. Flag instructions: CLI, STI, CLD, STD, CLC, STC. These can set or
clear the interrupt (enabled) direction (for string operations) or
carry flags.
The addressing summary and the instruction summary given above masks a lot of
annoying little exceptions. For example, you can't POP CS, and although the R
<-- M form of LES is legal, the M <-- R form isn't etc. etc. My advice is:
a. Go for the general rules
b. Don't try to memorize the exceptions
c. Rely on common sense and the assembler to teach you about
exceptions over time. A lot of the exceptions cover things you
wouldn't want to do anyway.
5. A few instructions are rich enough and useful enough to warrent careful
study. Here are a few final study guidelines:
a. It is well worth the time learning to use the string instruction set
effectively. Among the most useful are:
REP MOVSB ;moves a string
REP STOSB ;initializes memory
REPNE SCASB ;look up occurance of character in string
REPE CMPSB ;compare two strings
b. Similarly, if you have never written for a stack machine before, you will
need to exercise PUSH and POP and get very comfortable with them because they
are going to be good friends. If you are used to the 370, with lots of general
purpose registers, you may find yourself feeling cramped at first, with many
fewer registers and many instructions having register restrictions. But, you
have a hidden ally: you need a register and you don't want to throw away
what's in it? Just PUSH it, and when you are done, POP it back. This can lead
to abuse. Never have more than two "expedient" PUSHes in effect and never
leave something PUSHed across a major header comment or for more than 15
instructions or so. An exception is the saving and restoring of registers at
entrance to and exit from a subroutine; here, if the subroutine is long, you
should probably PUSH everything which the caller may need saved, whether you
will use the register or not, and POP it in reverse order at the end. Be aware
that CALL and INT push return address information on the stack and RET and IRET
pop it off. It is a good idea to become familiar with the structure of the
stack.
c. In practice, to invoke system services you will use the INT instruction.
It is quite possible to use this instruction effectively in a cookbook fashion
without knowing precisely how it works.
d. The transfer of control instructions (CALL, RET, JMP) deserve careful
study to avoid confusion. You will learn that these can be classified as
follows:
1) all three have the capability of being either NEAR (CS register unchanged)
or FAR (CS register changed)
2) JMPs and CALLs can be DIRECT (target is assembled into instruction) or
INDIRECT (target fetched from memory or register)
3) if NEAR and DIRECT, a JMP can be SHORT (less than 128 bytes away) or LONG
In general, the third issue is not worth worrying about. On a forward jump
which is clearly VERY short, you can tell the assembler it is short and save
one byte of code:
JMP SHORT CLOSEBY
On a backward jump, the assembler can figure it out for you. On a forward
jump of dubious length, let the assembler default to a LONG form; at worst you
waste one byte.
Also leave the assembler to worry about how the target address is to be
represented, in absolute form or relative form.
e. The conditional jump set is rather confusing when studied apart from the
assembler, but you do need to get a feeling for it. The interactions of the
sign, carry, and overflow flags can get your mind stuttering pretty fast if you
worry about it too much. What is boils down to, though, is
JZ means what it says
JNZ means what it says
JG reater this means "if the SIGNED difference is positive"
JA bove this means "if the UNSIGNED difference is positive"
JL ess this means "if the SIGNED difference is negative"
JB elow this means "if the UNSIGNED difference is negative"
JC arry assembles the same as JB; it's an aesthetic choice
You should understand that all conditional jumps are inherently DIRECT, NEAR,
and "short"; the "short" part means that they can't go more than 128 bytes in
either direction. Again, this is something you could easily imagine to be more
of a problem than it is. I follow this simple approach:
1) When taking an abnormal exit from a block of code, I always use an
unconditional jump. Who knows how far you are going to end up jumping by the
time the program is finished. For example, I wouldn't code this:
TEST AL,IDIBIT ;Is the idiot bit on?
JNZ OYVEY ;Yes. Go to general cleanup
Rather, I would probably code this:
TEST AL,IDIBIT ;Is the idiot bit on?
JZ NOIDIOCY ;No. I am saved.
JMP OYVEY ;Yes. What can we say...
NOIDIOCY:
The latter, of course, is a jump around a jump. Some would say it is evil,
but I submit it is hard to avoid in this language.
2) Otherwise, within a block of code, I use conditional jumps freely. If the
block eventually grows so long that the assembler starts complaining that my
conditional jumps are too long I:
a) consider reorganizing the block but
b) also consider changing some conditional jumps to their opposite and use
the "jump around a jump" approach as shown above.
Enough about specific instructions!
6. Finally, in order to use the assembler effectively, you need to know the
default rules for which segment registers are used to complete addresses in
which situations.
a. CS is used to complete an address which is the target of a NEAR DIRECT
jump. On an NEAR INDIRECT jump, DS is used to fetch the address from memory
but then CS is used to complete the address thus fetched. On FAR jumps, of
course, CS is itself altered. The instruction counter is always implicitly
pointing in the code segment.
b. SS is used to complete an address if BP is used in its formation.
Otherwise, DS is always used to complete a data address.
c. On the string instructions, the target is always formed from ES and DI.
The source is normally formed from DS and SI. If there is a segment prefix, it
overrides the source not the target.
Learning about DOS
__________________
I think the best way to learn about DOS internals is to read the technical
appendices in the manual. These are not as complete as we might wish, but they
really aren't bad; I certainly have learned a lot from them. What you don't
learn from them you might eventually learn via judicious disassembly of parts
of DOS, but that shouldn't really be necessary.
From reading the technical appendices, you learn that interrupts 20H through
27H are used to communicate with DOS. Mostly, you will use interrupt 21H, the
DOS function manager.
The function manager implements a great many services. You request the
individual services by means of a function code in the AH register. For
example, by putting a nine in the AH register and issuing interrupt 21H you
tell DOS to print a message on the console screen.
Usually, but by no means always, the DX register is used to pass data for the
service being requested. For example, on the print message service just
mentioned, you would put the 16 bit address of the message in the DX register.
The DS register is also implicitly part of this argument, in keeping with the
universal segmentation rules.
In understanding DOS functions, it is useful to understand some history and
also some of the philosophy of MS-DOS with regard to portability. Generally,
you will find, once you read the technical information on DOS and also the IBM
technical reference, you will know more than one way to do almost anything.
Which is best? For example, to do asynch adapter I/O, you can use the DOS
calls (pretty incomplete), you can use BIOS, or you can go directly to the
hardware. The same thing is true for most of the other primitive I/O (keyboard
or screen) although DOS is more likely to give you added value in these areas.
When it comes to file I/O, DOS itself offers more than one interface. For
example, there are four calls which read data from a file.
The way to decide rationally among these alternatives is by understanding the
tradeoffs of functionality versus portability. Three kinds of portability need
to be considered: machine portability, operating system portability (for
example, the ability to assemble and run code under CP/M 86) and DOS version
portability (the ability for a program to run under older versions of DOS>.
Most of the functions originally offered in DOS 1.0 were direct descendents
of CP/M functions; there is even a compatibility interface so that programs
which have been translated instruction for instruction from 8080 assembler to
8086 assembler might have a reasonable chance of running if they use only the
core CP/M function set. Among the most generally useful in this original
compatibility set are:
09 -- print a full message on the screen
0A -- get a console input line with full DOS editing
0F -- open a file
10 -- close a file (really needed only when writing)
11 -- find first file matching a pattern
12 -- find next file matching a pattern
13 -- erase a file
16 -- create a file
17 -- rename a file
1A -- set disk transfer address
The next set provide no function above what you can get with BIOS calls or
more specialized DOS calls. However, they are preferable to BIOS calls when
portability is an issue.
00 -- terminate execution
01 -- read keyboard character
02 -- write screen character
03 -- read COM port character
04 -- write COM port character
05 -- print a character
06 -- read keyboard or write screen with no editing
The standard file I/O calls are inferior to the specialized DOS calls but
have the advantage of making the program easier to port to CP/M style systems.
Thus they are worth mentioning:
14 -- sequential read from file
15 -- sequential write to file
21 -- random read from file
22 -- random write to file
23 -- determine file size
24 -- set random record
In addition to the CP/M compatible services, DOS also offers some specialized
services which have been available in all releases of DOS. These include:
27 -- multi-record random read.
28 -- multi-record random write.
29 -- parse filename
2A-2D -- get and set date and time
All of the calls mentioned above which have anything to do with files make
use of a data area called the "FILE CONTROL BLOCK" (FCB). The FCB is anywhere
from 33 to 37 bytes long depending on how it is used. You are responsible for
creating an FCB and filling in the first 12 bytes, which contain a drive code,
a file name, and an extension.
When you open the FCB, the system fills in the next 20 bytes, which includes
a logical record length. The initial lrecl is always 128 bytes, to achieve
CP/M compatibility. The system also provides other useful information such as
the file size.
After you have opened the FCB, you can change the logical record length. If
you do this, your program is no longer CP/M compatible, but that doesn't make
it a bad thing to do. DOS documentation suggests you use a logical record
length of one for maximum flexibility. This is usually a good recommendation.
To perform actual I/O to a file, you eventually need to fill in byte 33 or
possibly bytes 34-37 of the FCB. Here you supply information about the record
you are interested in reading or writing. For the most part, this part of the
interface is compatible with CP/M.
In general, you do not need to (and should not) modify other parts of the
FCB.
The FCB is pretty well described in appendix E of the DOS manual. Beginning
with DOS 2.0, there is a whole new system of calls for managing files which
don't require that you build an FCB at all. These calls are quite incompatible
with CP/M and also mean that your program cannot run under older releases of
DOS. However, these calls are very nice and easy to use. They have these
characteristics
1. To open, create, delete, or rename a file, you need only a character
string representing its name.
2. The open and create calls return a 16 bit value which is simply placed in
the BX register on subsequent calls to refer to the file.
3. There is not a separate call required to specify the data buffer.
4. Any number of bytes can be transfered on a single call; no data area must
be manipulated to do this.
The "new" DOS calls also include comprehensive functions to manipulate the
new chained directory structure and to allocate and free memory.
Learning the assembler
______________________
It is my feeling that many people can teach themselves to use the assembler
by reading the MACRO Assembler manual if:
1. You have read and understood a book like Morse and thus have a feeling
for the instruction set
2. You know something about DOS services and so can communicate with the
keyboard and screen and do something marginally useful with files. In the
absence of this kind of knowledge, you can't write meaningful practice programs
and so will not progress.
3. You have access to some good examples (the ones supplied with the
assembler are not good, in my opinion. I will try to supply you with some more
relevant ones.
4. You ignore the things which are most confusing and least useful. Some of
the most confusing aspects of the assembler include the facilities combining
segments. But, you can avoid using all but the simplest of these facilities in
many cases, even while writing quite substantial applications.
5. The easiest kind of assembler program to write is a COM program. They
might seem harder, at first, then EXE programs because there is an extra step
involved in creating the executable file, but COM programs are structurally
very much simpler.
At this point, it is necessary to talk about COM programs and EXE programs.
As you probably know, DOS supports two kinds of executable files. EXE programs
are much more general, can contain many segments, and are generally built by
compilers and sometimes by the assembler. If you follow the lead given by the
samples distributed with the assembler, you will end up with EXE programs. A
COM program, in contrast, always contains just one segment, and receives
control with all four segment registers containing the same value. A COM
program, thus, executes in a simplified environment, a 64K address space. You
can go outside this address space simply by temporarily changing one segment
register, but you don't have to, and that is the thing which makes COM programs
nice and simple. Let's look at a very simple one.
The classic text on writing programs for the C language says that the first
thing you should write is a program which says
HELLO, WORLD.
when invoked. What's sauce for C is sauce for assembler, so let's start with
a HELLO program of our own. My first presentation of this will be bare bones,
not stylistically complete, but just an illustration of what an assembler
program absolutely has to have:
HELLO SEGMENT ;Set up HELLO code and data section
ASSUME CS:HELLO,DS:HELLO ;Tell assembler about conditions at entry
ORG 100H ;A .COM program begins with 100H byte prefix
MAIN: JMP BEGIN ;Control must start here
MSG DB 'Hello, world.