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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. 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 set Phase 1: Learn the architecture and instruction set 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 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 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 sug- gested, 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 nei- ther 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 oper- and forms described above. Usually, the perform the operation and leave the result in the stated place: 1) INC -- increment contents IBM PC Assembly Language Tutorial 7 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. IBM PC Assembly Language Tutorial 8 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 IBM PC Assembly Language Tutorial 9