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-LITERATESteps to becoming ASSEMBLER-LITERATESteps 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. 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.
IBM PC Assembly Language Tutorial 3
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____________________________________________________Phase 1: Learn the architecture and instruction setPhase 1: Learn the architecture and instruction setPhase 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 study-
ing it. Get the big picture at this point.
Now, you want to concentrate on those things which are worth fixing in mem-
ory. 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
IBM PC Assembly Language Tutorial 4
tend to get your segment registers set up at the beginning of the pro-
gram, 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 regis-
ters. For example, any program of the COM type gets control with all
four segment registers containing the same value. This program exe-
cutes 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 per-
formed in SI, DI, SP, or BP.
IBM PC Assembly Language Tutorial 5
3. You will want to learn the classifications of operations available
WITHOUT getting hung up in the details of how 8086 opcodes are con-
structed.
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 differ-
ent 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:
IBM PC Assembly Language Tutorial 6
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
With FAR linkage, it doesn't really matter what you call the segment.
you must declare the name by which you will be called in a PUBLIC pseu-
do-op and also show that it is a FAR procedure. Only CS will be ini-
tialized to your segment when you are called. Generally, the other
segment registers will continue to point to the caller's segments.
With NEAR linkage, you are executing in the same segment as the caller.
Therefore, you must give the segment a specific name as instructed by
the language manual. However, you may be able to count on all segment
registers pointing to your own segment (sometimes the situation can be
more complicated but I cannot really go into all of the details). You
should be aware that the code you write will not be the only thing in
the segment and will be physically relocated within the segment by the
linker. However, all OFFSET references will be relocated and will be
correct at execution time.
2. Parameters passed on the stack. Usually, high level languages pass
parameters to subroutines by pushing words onto the stack prior to
calling you. What may differ from language to language is the nature
of what is pushed (OFFSET only or OFFSET and SEGMENT) and the order in
which it is pushed (left to right, right to left within the CALL state-
ment). However, you will need to study the examples to figure out how
to retrieve the parameters from the stack. A useful fact to exploit is
the fact that a reference involving the BP register defaults to a ref-
erence to the stack segment. So, the following strategy can work:
ARGS STRUC
DW 3 DUP(?) ;Saved BP and return address
ARG3 DW ?
ARG2 DW ?
ARG1 DW ?
ARGS ENDS
...........
PUSH BP ;save BP register
MOV BP,SP ;Use BP to address stack
MOV ...,[BP].ARG2 ;retrieve second argument
(etc.)
IBM PC Assembly Language Tutorial 22
This example uses something called a structure, which is only available
in the large assembler; furthermore, it uses it without allocating it,
which is not a well-documented option. However, I find the above
approach generally pleasing. The STRUC is like a DSECT in that it
establishes labels as being offset a certain distance from an arbitrary
point; these labels are then used in the body of code by beginning them
with a period; the construction ".ARG2" means, basically, " +
(ARG2-ARGS)."
What you are doing here is using BP to address the stack, accounting
for the word where you saved the caller's BP and also for the two words
which were pushed by the CALL instruction.
3. How big is the stack? BASIC only gives you an eight word stack to play
with. On the other hand, it doesn't require you to save any registers
except the segment registers. Other languages give you a liberal
stack, which makes things a lot easier. If you have to create a new
stack segment for yourself, the easiest thing is to place the stack at
the end of your program and:
CLI ;suppress interrupts while changing the stack
MOV SSAVE,SS ;save old SS in local storage (old SP
; already saved in BP)
MOV SP,CS ;switch
MOV SS,SP ;the
MOV SP,OFFSET STACKTOP ;stack
STI ;(maybe)
Later, you can reverse these steps before returning to the caller. At
the end of your program, you place the stack itself:
DW 128 DUP(?) ;stack of 128 words (liberal)
STACKTOP LABEL WORD
4. Make sure you save and restore those registers required by the caller.
5. Be sure to get the right kind of addressibility. In the FAR call exam-
ple, only CS addresses your segment. If you are careful with your
ASSUME statements the assembler will keep track of this fact and gener-
ate CS prefixes when you make data references; however, you might want
to do something like
MOV AX,CS ;get current segment address
MOV DS,AX ;To DS
ASSUME DS:THISSEG
Be sure you keep your ASSUMEs in synch with reality.
IBM PC Assembly Language Tutorial 23
Learning about BIOS and the hardware____________________________________Learning about BIOS and the hardwareLearning about BIOS and the hardwareLearning about BIOS and the hardware
You can't do everything with DOS calls. You may need to learn something
about the BIOS and about the hardware itself. In this, the Technical Ref-
erence is a very good thing to look at.
The first thing you look at in the Technical Reference, unless you are
really determined to master the whole ball of wax, is the BIOS listings
presented in Appendix A. Glory be: here is the whole 8K of ROM which deals
with low level hardware support layed out with comments and everything.
In fact, if you are just interested in learning what BIOS can do for you,
you just need to read the header comments at the beginning of each section
of the listing.
BIOS services are invoked by means of the INT instruction; the BIOS occu-
pies interrupts 10H through 1FH and also interrupt 5H; actually, of these
seventeen interrupts, five are used for user exit points or data pointers,
leaving twelve actual services.
In most cases, a service deals with a particular hardware interface; for
example, BIOS interrupt 10H deals with the screen. As with DOS function
calls, many BIOS services can be passed a function code in the AH register
and possible other arguments.
I am not going to summarize the most useful BIOS features here; you will
see some examples in the next sample program we will look at.
The other thing you might want to get into with the Tech reference is the
description of some hardware options, particularly the asynch adapter,
which are not well supported in the BIOS. The writeup on the asynch adapt-
er is pretty complete.
Actually, the Tech reference itself is pretty complete and very nice as far
as it goes. One thing which is missing from the Tech reference is informa-
tion on the programmable peripheral chips on the system board. These
include
the 8259 interrupt controller
the 8253 timer
the 8237 DMA controller and
the 8255 peripheral interface
To make your library absolutely complete, you should order the INTEL data
sheets for these beasts.
I should say, though, that the only I ever found I needed to know about was
the interrupt controller. If you happen to have the 8086 Family User's
Manual, the big book put out by INTEL, which is one of the things people
sometimes buy to learn about 8086 architecture, there is an appendix there
which gives an adequate description of the 8259.
IBM PC Assembly Language Tutorial 24
A final example_______________A final exampleA final exampleA final example
I leave you with a more substantial example of code which illustrates some
good elementary techniques; I won't claim its style is perfect, but I think
it is adequate. I think this is a much more useful example than what you
will get with the assembler:
PAGE 61,132
TITLE SETSCRN -- Establish correct monitor use at boot time
;
; This program is a variation on many which toggle the equipment flags
; to support the use of either video option (monochrome or color).
; The thing about this one is it prompts the user in such a way that he
; can select the use of the monitor he is currently looking at (or which
; is currently connected or turned on) without really having to know
; which is which. SETSCRN is a good program to put first in an
; AUTOEXEC.BAT file.
;
; This program is highly dependent on the hardware and BIOS of the IBMPC
; and is hardly portable, except to very exact clones. For this reason,
; BIOS calls are used in lieu of DOS function calls where both provide
; equal function.
;
OK. That's the first page of the program. Notice the PAGE statement,
which you can use to tell the assembler how to format the listing. You
give it lines per page and characters per line. I have mine setup to print
on the host lineprinter; I routinely upload my listings at 9600 baud and
print them on the host; it is faster than using the PC printer.
There is also a TITLE statement. This simply provides a nice title for
each page of your listing. Now for the second page:
SUBTTL -- Provide .COM type environment and Data
PAGE
;
; First, describe the one BIOS byte we are interested in
;
BIOSDATA SEGMENT AT 40H ;Describe where BIOS keeps his data
ORG 10H ;Skip parts we are not interested in
EQUIP DB ? ;Equipment flag location
MONO EQU 00110000B ;These bits on if monochrome
COLOR EQU 11101111B ;Mask to make BIOS think of the color board
BIOSDATA ENDS ;End of interesting part
;
; Next, describe some values for interrupts and functions
;
DOS EQU 21H ;DOS Function Handler INT code
PRTMSG EQU 09H ;Function code to print a message
KBD EQU 16H ;BIOS keyboard services INT code
GETKEY EQU 00H ;Function code to read a character
SCREEN EQU 10H ;BIOS Screen services INT code
MONOINIT EQU 02H ;Value to initialize monochrome screen
IBM PC Assembly Language Tutorial 25
;COLORINIT EQU 03H ;Value to initialize color screen (80x25)
COLORINIT EQU 01H ;Value to initialize color screen (40X25)
;
; Now, describe our own segment
;
SETSCRN SEGMENT ;Set operating segment for CODE and DATA
