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One problem with assembly language is that it's tough knowing where to put things There are only so many registers to go around Having variables in a data segment is helpful, but it isn't the whole story People who come to assembly from higher-level languages such as Pascal and Basic find this particularly jarring, since they're used to being able to create new variables at any time as needed The x86 CPUs contain the machinery to create and manage a vital storage area called the stack The name is appropriate, and for a usable metaphor I can go back to my high school days, when I was a dishwasher for Resurrection Hospital on Chicago's Northwest Side
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Five Hundred Plates an Hour
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What I did most of the time was pull clean plates from a moving conveyor belt of little prongs that emerged endlessly from the steaming dragon's mouth of a 180 dishwashing machine This was hot work, but it was a lot less slimy than stuffing the dirty plates into the other end of the machine When you pull 500 plates an hour out of a dishwashing machine, you had better have some place efficient to stash them Obviously, you could simply stack them on a table, but stacked ceramic plates in any place habituated by rowdy teenage boys is asking for fragments What the hospital had instead was an army of little wheeled stainless steel cabinets equipped with one or more spring-loaded circular plungers accessed from the top When you had a handful of plates, you pushed them down into the plunger The plunger's spring was adjusted such that the weight of the added plates pushed the whole stack of plates down just enough to make the new top plate flush with the top of the cabinet Each plunger held about 50 plates We rolled one up next to the dragon's mouth, filled it with plates, and then rolled it back into the kitchen where the clean plates were used at the next meal shift to set patients' trays It's instructive to follow the path of the first plate out of the dishwashing machine on a given shift That plate got into the plunger first and was subsequently shoved down into the bottom of the plunger by the remaining 49 plates that the cabinet could hold After the cabinet was rolled into the kitchen, the kitchen girls pulled plates out of the cabinet one by one as they set trays The first plate out of the cabinet was the last plate in The last plate out of the cabinet had been the first plate to go in The x86 stack is like that We call it a last in, first out, or LIFO stack
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Two of the x86 registers team up to create and maintain the stack Like everything else in 86-land, the stack must exist within a segment The SS (Stack Segment) register holds the segment address of the segment chosen to be the stack segment, and the SP (Stack Pointer) register points to locations within the stack segment As with all other segments in real mode, the stack segment may be as much as 65,536 bytes long, but it may be any length less than that as well You'll find in practice that the stack rarely needs to be larger than a thousand bytes or so unless you're doing some really peculiar things The stack segment begins at SS:0, but the truly odd thing about it is that all the stack action happens at the opposite end of the stack segment When a stack segment is set up, the SS register points to the base or beginning of the stack segment, and SP is set to point to the end of the stack segment To store something in the stack segment (which we usually call "pushing something onto the stack"), we move SP "down the stack" (that is, closer to SS) and then copy the item to the memory location pointed to by SS:SP This takes some getting used to Figure 81 provides the big picture of the stack segment and the two pointers that give it life In real mode flat model, SS is set to the base of the stack segment by DOS when the program is loaded and begins running (And all the other segment registers are set to the same address) In real mode segmented model, you set SS from the address of the segment that you define within the program in two steps, first using NASM's SEGMENT directive:
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Figure 81: The big picture of the real mode stack SEGMENT stack stack Then you need a couple of MOV instructions to get the address of segment stack into SS: mov mov ax,stack ss,ax ; Move segment address of stack segment into AX ; Copy address from AX into SS
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Defining a stack segment just provides a starting point address for that segment No room is actually reserved for the stack by the SEGMENT directive That requires a new directive that we haven't discussed: resb 64 ; Reserve 64 bytes for the program stack
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RESB means "REServe Byte" And it means just that: It tells the assembler to set aside 64 bytes starting at the beginning of the stack segment and not to let anything else (such as memory variables) be defined in that reserved space You can use RESB to reserve as much stack as you think you'll need; 64 bytes is enough for simple experimentation If you're writing a more ambitious program, you may be better off looking at what it does and actually estimating a worst-case demand for stack space Note that you don't need to use RESB to reserve stack space if you're working in real mode flat model The stack in that model exists at the very highest addresses of the single segment the program lives in The space isn't reserved in the strictest sense, and you have to be careful not to let your code or data get so high in memory that it collides with your stack This is called a stack crash and you're not likely to see one in your own programs until you get a lot further along in your assembly experience SP is set to the far (that is, the high, address-wise) end of the stack segment (See Figure 81, where an arrow indicates the initial value of SP) Again, if you're working in real mode flat model, DOS does it when your program is loaded as you can see if you load EATCOM with DEBUG and display the registers with the R command SP will have a value something like 0FFFEH in any case, something fairly high rather than close to 0000H And if you're working in real mode segmented model, you have to set SP yourself This is done by first
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indicating the initial address to be contained in SP: resb 64 stacktop: ; Reserve 64 bytes for the program stack ; It's significant that this label points to ; the *last* of the reserved 64 bytes, and ; not the first!
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Note that the label stacktop: is immediately after the RESB 64 directive The label stacktop: represents an address at the very end of the block of reserved memory locations set aside by RESB Although the position of the two lines on the source code listing suggests that stacktop: points beyond the block of memory set aside by RESB, that's not the case The stacktop: label resolves to the offset of the last byte in that block of 64 bytes You load the address represented by the stacktop: label into SP when the program begins, typically right after you set up the segment registers: mov sp,stacktop ; Point SP to the top of the stack
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