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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 effec- tively in a cookbook fashion without knowing precisely how it works. d. The transfer of control instructions (CALL, RET, JMP) deserve care- ful 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 instruc- tion) 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 for- ward 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 IBM PC Assembly Language Tutorial 10 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 some- thing 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 assem- bler 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 seg- ment. 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. IBM PC Assembly Language Tutorial 11 Learning about DOS __________________ Learning about DOS Learning about DOS 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 inter- rupt 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. General- ly, 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 porta- bility need to be considered: machine portability, operating system porta- bility (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 IBM PC Assembly Language Tutorial 12 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 preferabƒâƒ9$ ƒαßƒ ƒƒƒüαßƒƒÅÅƒƒƒÇÇƒƒ ƒαßƒƒÅîƒƒƒÇÇƒƒ ƒ ƒ αßƒƒÅëƒƒƒÇÇƒƒ ƒ ƒ αßƒƒÅêƒƒƒÇÇƒƒ ƒ8 ƒ ƒ αßƒƒÅâƒƒƒÇÇƒƒ ƒ8 ƒ ƒ αßƒƒÅÇƒƒƒÇÇƒƒ ƒƒ αßƒƒÅüƒƒƒÇÇƒƒ ƒnstruction 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