Reading and Changing Registers with DEBUG in Visual Studio .NET

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Reading and Changing Registers with DEBUG
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Much or most of what defines your assembly language programs lies in your use of registers Machine instructions act on registers, and registers define how memory is addressed and what is read from or placed there While you're developing and debugging your programs, a lot of what you'll be looking at is the contents of your registers The DOS DEBUG utility provides a handy window into the CPU's hidden world of registers How DEBUG does this is the blackest of all black arts and I can't begin to explain it in an introductory text For now, just consider DEBUG a magic box One thing to keep in mind is that DEBUG is a real mode creature It doesn't work in protected mode You can only use it while debugging real mode programs, whether segmented or flat model Protected mode debuggers do exist, but DEBUG isn't one of them Looking at the registers from DEBUG doesn't even require that you load a program into DEBUG Simply run DEBUG, and at the dash prompt type the single-letter command R The display will look something very close to this: - R AX=0000 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=1980 ES=1980 SS=1980 CS=1980 IP=0100 NV UP EI PL NZ NA PO NC 1980:0100 389A5409 CMP [BP+SI+0954],BL SS:0954=8A I say "something very close" because details of the display will vary depending on what resident programs you have loaded in memory, which version of DOS you're using, and so on What will vary will be the values listed as present in the various registers, and the machine instruction shown in the third line of the display (here, CMP [BP+SI+0954],BL) What will not vary is the fact that every CPU register has its place in the display, along with its current value shown to the right of an equals sign The characters "NV UP EI PL NZ NA PO NC" are a summary of the current values of the flags in the flags register The preceding display is that of the registers when no program has been loaded All of the generalpurpose registers except for SP have been set to 0, and all of the segment registers have been set to the value 1980H These are the default conditions set up by DEBUG in the CPU when no program has been loaded (The 1980H value will probably be different for you-it represents the first available segment in memory above DOS, and where that segment falls depends on what else exists in memory both above and below DOS) Changing a register is done very simply, again using DEBUG's R command To change the value of AX, type R AX and press Enter: -R AX AX:0000 :0A7B DEBUG will respond by displaying the current value of AX (here, "0000") and then, on the following line, a colon prompt It will wait for you to either enter a new numeric value for AX, or else for you to press Enter If you press Enter, the current value of the register will not be changed In the preceding example, I typed "0A7B" (you needn't type the H indicating hex) followed by Enter Once you do enter a new value and then press Enter, DEBUG does nothing to verify that the change has been made To see the change to register AX, you must display all the registers again using the R command: - R AX=0A7B BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=1980 ES=1980 SS=1980 CS=1980 IP=0100 NV UP EI PL NZ NA PO NC 1980:0100 389A5409 CMP [BP+SI+0954],BL SS:0954=8A
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Take a few minutes to practice entering new values for the general-purpose registers, then display the registers as a group to verify that the changes were made While exploring, you might find that the IP register can be changed, even though I said earlier that it can't be changed directly The key word is directly; DEBUG knows all the dirty tricks
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