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The stack should be considered a place to stash things for the short term Items stored on the stack have no names, and in general must be taken off the stack in the reverse order that they were put on Last in, first out, remember LIFO! One excellent use of the stack allows the all-too-few registers to do multiple duty If you need a register to temporarily hold some value to be operated on by the CPU and all the registers are in use, push one of the busy registers onto the stack Its value will remain safe on the stack while you use the register for other things When you're finished using the register, pop its old value off the stack and you've gained the advantages of an additional register without really having one (The cost, of course, is the time you spend moving that register's value onto and off of the stack It's not something you want to do in the middle of an often-repeated loop!)
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Using DOS Services through INT
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I think of EATASM as something of a Tom Sawyer program It doesn't do much, and it does what it does in time-honored Tom Sawyer fashion by getting somebody else to do all the work All that EAT does is display a character string on your screen The visible part of that string is the advertising slogan itself: Eat at Joe's! The other part is the pair of invisible characters we call newline or EOL: carriage return (0DH) followed by line feed (0AH) (For more on EOL markers and how they interact with text, see 4) The EOL marker does nothing more than return the display cursor to the left margin of the next screen line, so that any subsequent text displayed will begin at the left margin and not nipping at the heels of Eat at Joe's! Both parts of our advertising slogan are sent to the display at once, and via the same mechanism: through a DOS service As I explain in 4, DOS is both a god and a troll It controls all the most important elements of the machine in godlike fashion: the disk drives, the printer, and (to some extent) the display At the same time, DOS is like a troll living under a bridge to all those parts of your machine: You tell the troll what you want done, and the troll will go out and do it for you There is another troll guarding the bridges to other components of your machine called the BIOS, to which we'll return in a little while DOS and BIOS both offer services, which are simple tasks that your programs would have to do themselves if the services were not provided Quite apart from saving you the programmer a lot of work, having DOS and BIOS services helps guarantee that certain things will be done in identical fashion on all machines, which (especially in terms of disk storage) is a major reason software written for DOS runs on so many different machines: All the machine-dependent stuff is done the same way One of the services DOS provides is simple (far too simple, actually) access to your machine's display For the purposes of EATASM (which is just a lesson in getting your first assembly language program written and operating), simple services are enough So how do we use DOS and BIOS services The way is as easy to use as it is tricky to understand: through software interrupts
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As one new to the x86 family of processors back in 1981, the notion of a software interrupt drove me nuts I kept looking and looking for the interrupter and interruptee Nothing was getting interrupted The name is unfortunate, even though I admit that there was some reason for calling software interrupts what they are They are, in fact, courteous interrupts if you can still call an interrupt an interrupt when it is so courteous that it does no interrupting at all The nature of software interrupts and DOS services is best explained by a real example illustrated twice in EATASM As I hinted previously, DOS keeps little sequences of machine instructions tucked away within itself Each sequence does something useful read something from a disk file, display something to the screen, send something to the printer DOS uses them to do its own work, and it also makes them available (with its troll hat on) to you the programmer to access from your programs Well, there is the critical question: How do you find something tucked away inside of DOS All code sequences, of course, have addresses, and Microsoft or IBM could publish a booklet of addresses indicating where all the code is hidden There are numerous good reasons not to pass out the addresses of the code itself, however DOS is evolving and (we should hope) being repaired on an ongoing basis Repairing and improving code involves adding, changing, and removing machine instructions, which changes the size of those hidden code sequences and also, in consequence, changes their location Add a dozen instructions to one sequence, and all the other sequences upmemory from that one sequence will have to shove over, to make room Once they shove over, they'll be at different addresses, so instantly the booklets are obsolete Even one byte added to or removed from a code sequence in DOS could change everything What if the first code sequence has a bug that must be fixed The solution is ingenious At the very start of real mode memory, down at segment 0, offset 0, is a
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special table with 256 entries Each entry is a complete address including segment and offset portions, for a total of 4 bytes per entry The first 1,024 bytes of memory in any x86 machine are reserved for this table, and no code or data may be placed there Each of the addresses in the table is called an interrupt vector The table as a whole is called the interrupt vector table Each vector has a number, from 0 to 255 The vector occupying bytes 0 through 3 in the table is vector 0 The vector occupying bytes 4 through 7 is vector 1, and so on, as shown in Figure 83
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Figure 83: The interrupt vector table None of the addresses is burned into permanent memory the way BIOS routines are When your machine starts up, DOS and BIOS fill many of the slots in the interrupt vector table with addresses of certain service routines within themselves Each version of DOS knows the location of its innermost parts, and when you upgrade to a new version of DOS, that new version will fill the appropriate slots in the vector table with upgraded and accurate addresses What doesn't change from DOS version to DOS version is the number of the interrupt that holds a particular address In other words, since the PC first began, interrupt 21H has pointed the way into darkest DOS to DOS's services dispatcher, a sort of multiple-railway switch with spurs heading out to the many (over 50) individual DOS service routines The address of the dispatcher has changed with every DOS version, but regardless of version, programs can find the address of the dispatcher in slot 21H of the interrupt vector table Furthermore, programs don't have to go snooping the table for the address themselves The x86 CPUs include a machine instruction that makes use of the interrupt vector table The INT (INTerrupt) instruction is used by EATASM to request the services of DOS in displaying two strings on the screen At two places, EATASM has an INT 21H instruction When an INT 21H instruction is executed, the CPU goes down to the interrupt vector table, fetches the address from slot 21H, and then jumps
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execution to the address stored in slot 21H Since the DOS services dispatcher lies at the address in slot 21H, the dispatcher gets control of the machine and does the work that it knows how to do The process is shown in Figure 84 When DOS loads itself at boot time, one of the many things it does to prepare the machine for use is put correct addresses in several of the vectors in the interrupt vector table One of these addresses is the address of the dispatcher, which goes into slot 21H
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Figure 84: Riding the interrupt vector into DOS Later on, when you type the name of your program MYPROG on the DOS command line, DOS loads MYPROGEXE into memory and gives it control of the machine MYPROGEXE does not know the address of the DOS dispatcher MYPROG does know that the dispatcher's address will always be in slot 21H of the vector table, so it executes an INT 21 instruction The correct address lies in vector 21H, and MYPROG is content to remain ignorant and simply let the INT 21 instruction and vector 21H take it where it needs to go Back on the Northwest Side of Chicago, where I grew up, there was a bus that ran along Milwaukee Avenue All Chicago bus routes had numbers, and the Milwaukee Avenue route was number 56 It started somewhere in the tangled streets just north of Downtown, and ended up in a forest preserve just inside the city limits The Forest Preserve District ran a swimming pool called Whelan Pool in that forest preserve Kids all along Milwaukee Avenue could not necessarily have told you the address of Whelan Pool But come summer, they'd tell you in a second how to get there: Just hop on bus number 56 and take it to the end of the line It's like that with software interrupts Find the number of the vector that reliably points to your destination, and ride that vector to the end of the line, without worrying about the winding route or the address of your destination Note that the INT 21 instruction does something else: It pushes the address of the next instruction (that is, the instruction immediately following the INT 21 instruction) on the stack before it follows vector 21H into the depths of DOS Like Hansel and Gretel, the INT 21 was pushing some breadcrumbs to the stack as a way of helping execution find its way back to MYPROGEXE after the excursion down into DOS but more on that later
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Now, the DOS dispatcher controls access to dozens of individual service routines How does it know which one to execute You have to tell the dispatcher which service you need, and you do so by placing the service's number in 8-bit register AH The dispatcher may require other information as well and will expect you to provide that information in the correct place before executing INT 21 Look at the following three lines of code from EATASM: mov mov int dx,eatmsg ah,09H 21H ; Mem data ref without [] loads the ADDRESS! ; Function 9 displays text to standard output ; INT 21H makes the call into DOS
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This sequence of instructions requests that DOS display a string on the screen The first line sets up a vital piece of information: the offset address of the string to be displayed on the screen Without that, DOS will not have any way to know what it is that we want to display The dispatcher expects the offset address to be in DX and assumes that the segment address will be in DS In flat model, DS is initialized by DOS at execution time In segmented model, the address of the data segment was loaded into DS earlier in the program by these two instructions: mov mov ax,data ds,ax ; Move segment address of data segment into AX ; Copy address from AX into DS
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Once loaded, DS is not disturbed during the full run of the program, so the DOS dispatcher's assumption is valid even though DS is loaded at the start of program execution and not each time we want to display a string In moving 09H into register AH, we tell the dispatcher which service we want performed Service 09H is DOS's Print String service This is not the fastest nor in other ways the best way to display a string on the PC's screen, but it is most certainly the easiest DOS service 09H has a slightly odd requirement: That the end of the string be marked with a dollar sign ($) This is the reason for the dollar sign hung incongruously on the end of EATASM's advertising slogan string Given that DOS does not ask us to pass it a value indicating how long the string is, the end of the string has to be marked somehow, and the dollar sign is DOS's chosen way It's a lousy way, unfortunately, because with the dollar sign acting as a marker, there is no way to display a dollar sign If you intend to talk about money on the PC's screen, don't use DOS service 9! As I said, this is the easiest, but certainly not the best way to display text on the screen With the address of the string in DS:DX and service number 09H in AH, we take a trip to the dispatcher by executing INT 21H The INT instruction is all it takes boom!, and DOS has control, reading the string at DS:DX and sending it to the screen through mechanisms it keeps more or less to itself
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