Real Mode Memory Data Summary in VS .NET

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Real Mode Memory Data Summary
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Real mode memory data consists of a single byte or word in memory, addressed by way of a segment value and an offset value The register containing the offset address is enclosed in square brackets to indicate that the contents of memory, rather than the contents of the register, are being addressed The segment register used to address memory data is usually assumed according to a complex set of rules Optionally, a segment override prefix may be placed in the instruction to specify some segment register other than the default segment register Figure 71 shows diagrammatically what happens during a MOV AX,[ES:BX] instruction The segment address component of the full 20-bit memory address is contained inside the CPU in segment register ES Ordinarily, the segment address would be in register DS, but the MOV instruction contains the ES: segment override prefix The offset address component is specified to reside in the BX register
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Figure 71: How memory data is addressed The CPU sends out the values in ES and BX to the memory system side by side Together, the two
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values pin down one memory location where MyWord begins MyWord is actually two bytes, but that's fine-all the x86 CPUs working in real mode (except for the 8088) can bring both bytes into the CPU at once, while the 8088 brings both bytes in separately, one after the other The CPU handles details like that and you needn't worry about it Because AX is a 16-bit register, of course, two 8-bit bytes can fit into it quite nicely The segment address may reside in any of the four segment registers: CS, DS, SS, or ES However, the offset address may reside only in registers BX, BP, SP, SI, or DI AX, CX, and DX may not be used to contain an offset address during real mode memory addressing
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Limitations of the MOV Instruction
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The MOV instruction can move nearly any register to any other register For reasons having to do with the limited budget of transistors on the 8086 and 8088 chips, MOV can't quite do any move you can think of-in real mode, at least Here's a list of MOV's real mode limitations: 1 MOV cannot move memory data to memory data In other words, an instruction like MOV [SI],[BX] is illegal Either of MOV's two operands may be memory data, but both cannot be at once 2 MOV cannot move one segment register into another Instructions like MOV CS,SS are illegal This could have been handy, but it simply can't be done 3 MOV cannot move immediate data into a segment register You can't code up MOV CS,0B800H Again, it would be handy but you just can't do it 4 MOV cannot move one of the 8-bit register halves into a 16-bit register, nor vice versa There are easy ways around any possible difficulties here, and preventing moves between operands of different sizes can keep you out of numerous kinds of trouble These limitations, of course, are over and above those situations that simply don't make sense: moving a register or memory into immediate data, moving immediate data into immediate data, specifying a general-purpose register as a segment register to contain a segment, or specifying a segment register to contain an offset address Table 73 shows numerous illegal MOV instructions that illustrate these various limitations and nonsense situations Table 73: Rogue MOV Instructions ILLEGAL MOV INSTRUCTION MOV 17,1 MOV 17,BX MOV CX,DH MOV [DI],[SI] MOV DI,[DX:BX] MOV ES,0B800 MOV DS,CS MOV [AX],BP MOV SI,[CS] WHY IT'S ILLEGAL Only one operand may be immediate data Only the source operand may be immediate data The operands must be the same size Only one operand may be memory data DX is not a segment register Segment registers may not be loaded from immediate data Only one operand may be a segment register AX may not address memory data (nor may CX or DX) Segment registers may not address memory data
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