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.. 90 lines per page .. leave 6/12 lines for article headline .. put cursor on ruler line & hit Ctrl-OF .. !------!----!----!----!----!----!----!----!----- IBM Personal Computer Assembly Language Tutorial Joshua Auerbach, Yale University Yale Computer Center 175 Whitney Avenue P. O. Box 2112 New Haven, Connecticut 06520 Learning the assembler _____________________ It is my feeling that many people can teach them- selves 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, than 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.$' ;But it is ; generally useful ; to put data first BEGIN: MOV DX,OFFSET MSG ;Let DX --> message. MOV AH,9 ;Set DOS function ; code for printing ; a message INT 21H ;Invoke DOS RET ; Return to system HELLO ENDS ;End of code and ; data section END MAIN ;Terminate assem- ; bler and specify ; entry point First, let's attend to some obvious points. The macro assembler uses the general form name opcode operands Unlike the 370 assembler, though, comments are NOT set off from operands by blanks. The syntax uses blanks as delimiters within the operand field (see line 6 of the example) and so all comments must be set off by semi-colons. Line comments are frequently set off with a semi- colon in column 1. I use this approach for block comments too, although there is a COMMENT statement which can be used to introduce a block comment. Being an old 370 type, I like to see assembler code in upper case, although my comments are mixed case. Actually, the assembler is quite happy with mixed case anywhere. As with any assembler, the core of the opcode set consists of opcodes which generate machine instruc- tions but there are also opcodes which generate data and ones which function as instructions to the assembler itself, sometimes called pseudo-ops. In the example, there are five lines which generate machine code (JMP, MOV, MOV, INT, RET), one line which generates data (DB) and five pseudo-ops (SEGMENT, ASSUME, ORG, ENDS, and END). We will discuss all of them. Now, about labels. You will see that some labels in the example end in a colon and some don't. This is just a bit confusing at first, but no real mystery. If a label is attached to a piece of code (as opposed to data), then the assembler needs to know what to do when you JMP to or CALL that label. By convention, if the label ends in a colon, the assembler will use the NEAR form of JMP or CALL. If the label does not end in a colon, it will use the FAR form. In practice, you will always use the colon on any label you are jumping to inside your program because such jumps are always NEAR; there is no reason to use a FAR jump within a single code section. I mention this, though, because leaving off the colon isn't usually trapped as a syntax error, it will generally cause something more abstruse to go wrong. On the other hand, a label attached to a piece of data or a pseudo-op never ends in a colon. Machine instructions will generally take zero, one or two operands. Where there are two operands, the one which receives the result goes on the left as in 370 assembler. I tried to explain this before, now maybe it will be even clearer: there are many more 8086 machine opcodes then there are assembler opcodes to repre- sent them. For example, there are five kinds of JMP, four kinds of CALL, two kinds of RET, and at least five kinds of MOV depending on how you count them. The macro assembler makes a lot of decisions for you based on the form taken by the operands or on attributes assigned to symbols elsewhere in your program. In the example above, the assembler will generate the NEAR DIRECT form of JMP because the target label BEGIN labels a piece of code instead of a piece of data (this makes the JMP DIRECT) and ends in a colon (this makes the JMP NEAR). The assembler will generate the immediate forms of MOV because the form OFFSET MSG refers to immediate data and because 9 is a constant. The assembler will generate the NEAR form of RET because that is the default and you have not told it otherwise. The DB (define byte) pseudo-op is an easy one: it is used to put one or more bytes of data into storage. There is also a DW (define word) pseudo- op and a DD (define doubleword) pseudo-op; in the PC MACRO assembler, the fact that a label refers to a byte of storage, a word of storage, or a double- word of storage can be very significant in ways which we will see presently. About that OFFSET operator, I guess this is the best way to make the point about how the assembler decides what instruction to assemble: an analogy with 370 assembler: PLACE DC ...... ... LA R1,PLACE L R1,PLACE In 370 assembler, the first instruction puts the address of label PLACE in register 1, the second instruction puts the contents of storage at label PLACE in register 1. Notice that two different opcodes are used. In the PC assembler, the analogous instructions would be PLACE DW ...... ... MOV DX,OFFSET PLACE MOV DX,PLACE If PLACE is the label of a word of storage, then the second instruction will be understood as a desire to fetch that data into DX. If X is a label, then "OFFSET X" means "the ordinary number which represents X's offset from the start of the segment." And, if the assembler sees an ordinary number, as opposed to a label, it uses the instruction which is equivalent to LA. If PLACE were the label of a DB pseudo-op, instead of a DW, then MOV DX,PLACE would be illegal. The assembler worries about length attributes of its operands. Next, numbers and constants in general. The assembler's default radix is decimal. You can change this, but I don't recommend it. If you want to represent numbers in other forms of notation such as hex or bit, you generally use a trailing letter. For example, 21H is hexidecimal 21, 00010000B is the eight bit binary number pictured. The next elements we should point to are the SEGMENT...ENDS pair and the END instruction. Every assembler program has to have these elements. SEGMENT tells the assembler you are starting a section of contiguous material (code and/or data). The symmetrically named ENDS statement tells the assembler you are finished with a section of conti- guous material. I wish they didn't use the word SEGMENT in this context. To me, a "segment" is a hardware construct: it is the 64K of real storage which becomes addressable by virtue of having a particular value in a segment register. Now, it is true that the "segments" you make with the assem- bler often correspond to real hardware "segments" at execution time. But, if you look at things like the GROUP and CLASS options supported by the linker, you will discover that this correspondence is by no means exact. So, at risk of maybe confusing you even more, I am going to use the more informal term "section" to refer to the area set off by means of the SEGMENT and ENDS instructions. The sections delimited by SEGMENT...ENDS pairs are really a lot like CSECTs and DSECTs in the 370 world. I strongly recommend that you be selective in your study of the SEGMENT pseudo-op as described in the manual. Let me just touch on it here. name SEGMENT name SEGMENT PUBLIC name SEGMENT AT nnn Basically, you can get away with just the three forms given above. The first form is what you use when you are writing a single section of assembler code which will not be combined with other pieces of code at link time. The second form says that this assembly only contains part of the section; other parts might be assembled separately and combined later by the linker. I have found that one can construct reasonably large modular applications in assembler by simply making every assembly use the same segment name and declaring the name to be PUBLIC each time. If you read the assembler and linker documentation, you will also be bombarded by information about more complex options such as the GROUP statement and the use of other "combine types" and "classes." I don't recommend getting into any of that. I will talk more about the linker and modular construction of programs a little later. The assembler manual also implies that a STACK segment is required. This is not really true. There are numerous ways to assure that you have a valid stack at execution time. Of course, if you plan to write applications in assembler which are more than 64K in size, you will need more than what I have told you; but who is really going to do that? Any application that large is likely to be coded in a higher level language. The third form of the SEGMENT statement makes the delineated section into something like a "DSECT;" that is, it doesn't generate any code, it just describes what is present somewhere already in the computer's memory. Sometimes the AT value you give is meaningful. For example, the BIOS work area is located at location 40 hex. So, you might see BIOSAREA SEGMENT AT 40H ;Map BIOS work area ORG BIOSAREA+10H EQUIP DB ? ;Location of equipment ;flags, first byte BIOSAREA ENDS in a program which was interested in mucking around in the BIOS work area. At other times, the AT value you give may be arbi- trary, as when you are mapping a repeated control block: PROGPREF SEGMENT AT 0 ;Really a DSECT mapping ;the program prefix ORG PROGPREF+6 MEMSIZE DW ? ;Size of available ;memory PROGPREF ENDS Really, no matter whether the AT value represents truth or fiction, it is your responsibility, not the assembler's, to set up a segment register so that you can really reach the storage in question. So, you can't say MOV AL,EQUIP unless you first say something like MOV AX,BIOSAREA ;BIOSAREA becomes a ;symbol with value 40H MOV ES,AX ASSUME ES:BIOSAREA Enough about SEGMENT. The END statement is simple. It goes at the end of every assembly. When you are assembling a subroutine, you just say END but when you are assembling the main routine of a program you say END label where 'label' is the place where execution is to begin. Another pseudo-op illustrated in the program is ASSUME. ASSUME is like the USING statement in 370 assembler. However, ASSUME can ONLY refer to segment registers. The assembler uses ASSUME information to decide whether to assemble segment override prefixes and to check that the data you are trying to access is really accessible. In this case, we can reassure the assembler that both the CS and DS registers will address the section called HELLO at execution time. Actually, the SS and ES registers will too, but the assembler never needs to make use of this information. I guess I have explained everything in the program except that ORG pseudo-op. ORG means the same thing as it does in many assembly languages. It tells the assembler to move its location counter to some particular address. In this case, we have asked the assembler to start assembling code hex 100 bytes from the start of the section called HELLO instead of at the very beginning. This simply reflects the way COM programs are loaded. When a COM program is loaded by the system, the system sets up all four segment registers to address the same 64K of storage. The first 100 hex bytes of that storage contains what is called the program prefix; this area is described in appendix E of the DOS manual. Your COM program physically begins after this. Execution begins with the first physical byte of your program; that is why the JMP instruction is there. Wait a minute, you say, why the JMP instruction at all? Why not put the data at the end? Well, in a simple program like this I probably could have gotten away with that. However, I have the habit of putting data first and would encourage you to do the same because of the way the assembler has of assembling different instructions depending on the nature of the operand. Unfortunately, sometimes the different choices of instruction which can assemble from a single opcode have different lengths. If the assembler has already seen the data when it gets to the instruc- tions it has a good chance of reserving the right number of bytes on the first pass. If the data is at the end, the assembler may not have enough information on the first pass to reserve the right number of bytes for the instruction. Sometimes the assembler will complain about this, something like "Forward reference is illegal" but at other times, it will make some default assumption. On the second pass, if the assumption turned out to be wrong, it will report what is called a "Phase error," a very nasty error to track down. So get in the habit of putting data and equated symbols ahead of code. OK. Maybe you understand the program now. Let's walk through the steps involved in making it into a real COM file. 1. The file should be created with the name HELLO.ASM (actually the name is arbitrary but the extension .ASM is conventional and useful) 2. ASM HELLO,,; (this is just one example of invoking the assembler; it uses the small assembler ASM, it produces an object file and a listing file with the same name as the source file. I am not going exhaustively into how to invoke the assembler, which the manual goes into pretty well. I guess this is the first time I mentioned that there are really two assemblers; the small assembler ASM will run in a 64K machine and doesn't support macros. I used to use it all the time; now that I have a bigger machine and a lot of macro libraries I use the full function assembler MASM. You get both when you buy the package). 3. If you issue DIR at this point, you will discover that you have acquired HELLO.OBJ (the object code resulting from the assembly) and HELLO.LST (a listing file). I guess I can digress for a second here concerning the listing file. It contains TAB characters. I have found there are two good ways to get it printed and one bad way. The bad way is to use LPT1: as the direct target of the listing file or to try copying the LST file to LPT1 without first setting the tabs on the printer. The two good ways are to either a. direct it to the console and activate the printer with CTRL-PRTSC. In this case, DOS will expand the tabs for you. b. direct to LPT1: but first send the right escape sequence to LPT1 to set the tabs every eight columns. I have found that on some early serial numbers of the IBM PC printer, tabs don't work quite right, which forces you to the first option. 4. LINK HELLO; (again, there are lots of linker options but this is the simplest. It takes HELLO.OBJ and makes HELLO.EXE). HELLO.EXE? I thought we were making a COM program, not an EXE program. Right. HELLO.EXE isn't really executable; its just that the linker doesn't know about COM programs. That requires another utility. You don't have this utility if you are using DOS 1.0; you have it if you are using DOS 1.1 or DOS 2.0. Oh, by the way, the linker will warn you that you have no stack segment. Don't worry about it. 5. EXE2BIN HELLO HELLO.COM This is the final step. It produces the actual program you will execute. Note that you have to spell out HELLO.COM; for a nominally rational but actually perverse reason, EXE2BIN uses the default extension BIN instead of COM for its output file. At this point, you might want to erase HELLO.EXE; it looks a lot more useful than it is. Chances are you won't need to recreate HELLO.COM unless you change the source and then you are going to have to redo the whole thing. 6. HELLO You type hello, that invokes the program, it says HELLO YOURSELF!!! (oops, what did I do wrong....?) What about subroutines? ______________________ I started with a simple COM program because I actually think they are easier to create than subroutines to be called from high level languages, but maybe its really the latter you are interested in. Here, I think you should get comfortable with the assembler FIRST with little exercises like the one above and also another one which I will finish up with. Next you are ready to look at the interface information for your particular language. You usually find this in some sort of an appendix. For example, the BASIC manual has Appendix C on Machine Language Subroutines. The PASCAL manual buries the information a little more deeply: the interface to a separately compiled routine can be found in the Chapter on Procedures and Functions, in a subsection called Internal Calling Conventions. Each language is slightly different, but here are what I think are some common issues in subroutine construction. 1. NEAR versus FAR? Most of the time, your language will probably call your assembler routine as a FAR routine. In this case, you need to make sure the assembler will generate the right kind of return. You do this with a PROC...ENDP statement pair. The PROC statement is probably a good idea for a NEAR routine too even though it is not strictly required: FAR linkage: ARBITRARY SEGMENT PUBLIC THENAME ASSUME CS:ARBITRARY THENAME PROC FAR ..... code and data THENAME ENDP ARBITRARY ENDS END NEAR linkage: SPECIFIC SEGMENT PUBLIC PUBLIC THENAME ASSUME CS:SPECIFIC,DS:SPECIFIC ASSUME ES:SPECIFIC,SS:SPECIFIC THENAME PROC NEAR ..... code and data .... THENAME ENDP SPECIFIC ENDS END 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 pseudo-op and also show that it is a FAR procedure. Only CS will be initialized 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 statement). 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 reference 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 ........... (continued at top of next column) PUSH BP ;save BP ; register MOV BP,SP ;Use BP to ; address stack MOV ...,[BP].ARG2 ;retrieve second ; argument (etc.) 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 addressibili- ty. In the FAR call example, only CS addresses your segment. If you are careful with your ASSUME statements the assembler will keep track of this fact and generate 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. Learning 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 Reference 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 occupies 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 adapter 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 information 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 one 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. A 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 ;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 ; ASSUME CS:SETSCRN,DS:SETSCRN ASSUME ES:SETSCRN,SS:SETSCRN ; All segments ; ORG 100H ;Begin assembly at ; standard .COM offset ; MAIN PROC NEAR ;COM files use NEAR ; linkage JMP BEGIN ;And, it is helpful to ; put the data first, ; but then you must ; branch around it. ; ; Data used in SETSCRN ; CHANGELOC DD EQUIP ;Location of the ; EQUIP, recorded as ; far pointer MONOPROMPT DB 'Please press the plus ( + ) key.$' ; User sees on mono COLORPROMPT DB 'Please press the minus ( - ) key.$' ; User sees on color Several things are illustrated on this page. First, in addition to titles, the assembler supports subtitles: hence the SUBTTL pseudo-op. Second, the PAGE pseudo-op can be used to go to a new page in the listing. You see an example here of the DSECT-style segment in the "SEGMENT AT 40H". Here, our interest is in correctly describing the location of some data in the BIOS work area which really is located at segment 40H. You will also see illustrated the EQU instruction, which just gives a symbolic name to a number. I don't make a fetish of giving a name to every single number in a program. I do feel strongly, though, that interrupts and function codes, where the number is arbitrary and the function being performed is the thing of interest, should always be given symbolic names. One last new element in this section is the define doubleword (DD) instruction. A doubleword constant can refer, as in this case, to a location in another segment. The assembler will be happy to use information at its disposal to properly assemble it. In this case, the assembler knows that EQUIP is offset 10 in the segment BIOSDATA which is at 40H. SUBTTL -- Perform function PAGE BEGIN: CALL MONOON ;Turn on ; mono display MOV DX,OFFSET MONOPROMPT ;GET MONO ; PROMPT MOV AH,PRTMSG ;ISSUE INT DOS ;IT CALL COLORON ;Turn on ; color display MOV DX,OFFSET COLORPROMPT ;GET COLOR ; PROMPT MOV AH,PRTMSG ;ISSUE INT DOS ;IT MOV AH,GETKEY ;Obtain user ; response INT KBD CMP AL,'+' ;Does he ; want MONO? JNZ NOMONO CALL MONOON ;yes. give ; it to him NOMONO: RET MAIN ENDP The main code section makes use of subroutines to keep the basic flow simple. About all that's new to you in this section is the use of the BIOS interrupt KBD to read a character from the keyboard. Now for the subroutines, MONOON and COLORON: SUBTTL -- Routines to turn monitors on PAGE MONOON PROC NEAR ;Turn mono on LES DI,CHANGELOC ;Get location to ; change ASSUME ES:BIOSDATA ;TELL ASSEMBLER ABOUT ; CHANGE TO ES OR EQUIP,MONO MOV AX,MONOINIT ;Get screen ; initialization value INT SCREEN ;Initialize screen RET MONOON ENDP COLORON PROC NEAR ;Turn color on LES DI,CHANGELOC ;Get location to ; change ASSUME ES:BIOSDATA ;TELL ASSEMBLER ABOUT ; CHANGE TO ES AND EQUIP,COLOR MOV AX,COLORINIT ;Get screen ; initialization value INT SCREEN ;Initialize screen RET COLORON ENDP SETSCRN ENDS ;End of segment END MAIN ;End of assembly; ; execution at MAIN The instructions LES and LDS are useful ones for dealing with doubleword addresses. The offset is loaded into the operand register and the segment into ES (for LES) or DS (for LDS). By telling the assembler, with an ASSUME, that ES now addresses the BIOSDATA segment, it is able to correctly assemble the OR and AND instructions which refer to the EQUIP byte. An ES segment prefix is added. To understand the action here, you simply need to know that flags in that particular byte control how the BIOS screen service initializes the adapters. BIOS will only work with one adapter at a time; by setting the equipment flags to show one or the other as installed and calling BIOS screen initialization, we achieve the desired effect. The rest is up to you. r the other as installed and calling BIOS scr