Using Memory Data in .NET framework

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Using Memory Data
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With one or two important exceptions (the string instructions, which I cover to a degree-but not exhaustively-later on), only one of an instruction's two operands may specify a memory location In other words, you can move an immediate value to memory, or a memory value to a register, or some other similar combination, but you can't move a memory value directly to another memory value This is just an inherent limitation of the CPU, and we have to live with it, inconvenient as it gets at times Specifying a memory address as one of an instruction's operands is a little complicated The offset address must be resident in one of the general-purpose registers that can legally hold an offset address (Remember, that's only BP, BX, SI, and DI-not any of the others such as AX, CX, or DX) To
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specify that we want the data at the memory location contained in the register rather than the data in the register itself, we use square brackets around the name of the register In other words, to move the word at address DS:BX into register AX, we would use the following instruction: MOV AX,[BX] Similarly, to move a value residing in register DX into the word at address DS:DI, you would use this instruction: MOV [DI],DX
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The only problem with these examples is this: "DS" isn't anywhere in either instruction Where does it say to use DS as the segment register It doesn't To keep addressing notation simple, the x86 CPUs in real mode make certain assumptions about certain instructions in combinations with certain registers There is no comprehensible system to these assumptions, and like dates in history or Spanish irregular verbs, you'll just have to memorize them, or at least know where to look them up (The where is in Appendix B in this book) One of these assumptions is that in working with memory data, the MOV instruction uses the segment address stored in segment register DS unless you explicitly tell it otherwise In the case of the two preceding examples, we did not tell the MOV instruction to use some segment register other than DS, so it fell back on its assumptions and used DS However, had you specified the offset as residing in register SP instead of BX or DI, the MOV instruction would have assumed the use of segment register SS instead This assumption involves a memory mechanism known as the stack, which we won't really address until the next chapter
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Overriding Segment Assumptions for Memory Data
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But what if you want to use ES as a segment register for memory data addressed in the MOV instruction It's not difficult The instruction set includes what are called segment override prefixes These are not precisely instructions, but are more like the filters that may be snapped in front of a camera lens The filter is not itself a lens, but it alters the way the lens operates There is one segment override prefix for each of the four segment registers: CS, DS, SS, and ES In assembly language they are written as the name of the segment register followed by a colon, as shown in Table 72 Table 72: Segment Override Prefixes SEGMENT OVERRIDE PREFIX CS: DS: SS: ES: FUNCTION Forces use of code segment register CS Forces use of the data segment register DS Forces use of the stack segment register SS Forces use of the extra segment register ES
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In use, the segment override prefix is placed immediately in front of the memory data reference whose segment register assumption is to be overridden For example, to force a MOV instruction to copy a value from the AX register into a location at some offset (contained in SI) into the code segment, you would use this instruction: MOV [CS:SI],AX Without the CS: override prefix, this instruction would move the value of AX into the data segment, at an address specified as DS:SI
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Prefixes in use are very reminiscent of how an address is written; in fact, understanding how prefixes work will help you keep in mind that in every reference to memory data within an instruction, there is a ghostly segment register assumption floating in the air You may not see the ghostly DS: assumption in your MOV instruction, but if you forget that it's there, the whole concept of memory data will begin to seem arbitrary and magical Every reference to memory data includes either an assumed segment register or else a segment override prefix to specify a segment register other than the assumed segment register At the machine-code level, a segment override prefix is a single binary byte The prefix byte is placed in front of rather than within a machine instruction In other words, if the binary bytes comprising a MOV AX,[BX] instruction are 8BH 07H, adding the ES segment override prefix to the instruction (MOV AX,[ES:BX]) places a single 26H in front of the opcode bytes, giving us 26H 8BH 07H as the full binary equivalent If you're sharp, the question will already have occurred to you: What about the flat models Recall that in both real mode flat model and protected mode flat model, the segment registers all point to the same place and are not changed during the run of the program In the flat models you do not use segment overrides What I have explained previously about segment overrides applies only to the real mode segmented model!
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