Executing a MOV Instruction with the Trace Command in Visual Studio .NET

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Executing a MOV Instruction with the Trace Command
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Note that you haven't executed anything You've simply used DEBUG's Assemble command to write a machine instruction into a particular location in memory There are two ways to execute machine instructions from within DEBUG One way is to execute a program in memory, starting at CS:IP This means that DEBUG will simply start the CPU executing whatever sequence of instructions begins at CS:IP We looked at the G command very briefly at the end of the last chapter, when we found the JMP instruction that reboots your PC on power-up, and used G to execute that instruction The command is quite evocative: Go But don't type G just yet Here's the reason: You haven't entered a program You've entered one instruction, and one instruction does not a program make The instruction after your MOV instruction could be anything at all, recalling that DEBUG is simply interpreting garbage values in memory as random machine instructions A series of random machine instructions could easily go berserk, locking your system into an endless loop or writing zeroes over an entire segment of memory that may contain part of DOS or Windows, or of DEBUG itself We'll use DEBUG's G command a little later, once we've constructed a complete program in memory Go executes programs in memory starting at CS:IP; Trace executes the single instruction at CS:IP For now, let's consider the mechanism DEBUG has for executing one machine instruction at a time It's called Trace, and you invoke it by typing T The Trace command will execute the machine instruction at CS:IP, then give control of the machine back to DEBUG Trace is generally used to single-step a machine-code program one instruction at a time, in order to watch what it's up to every step of the way For now, it's a fine way to execute a single instruction and examine that instruction's effects So type T DEBUG will execute the MOV instruction you entered at CS:IP, and then immediately display the registers before returning to the dash prompt You'll see this: AX=0001 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000 DS=1980 ES=1980 SS=1980 CS=1980 IP=0103 NV UP EI PL NZ NA PO NC 1980:0103 6E DB 6E
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Look at the first line DEBUG says AX is now equal to 0001 It held the default value 0000 before; obviously, your MOV instruction worked And there's something else to look at here: The third line shows an instruction called DB at CS:IP Not quite true DB is not a machine instruction, but an assembly language directive that means Define Byte DB has other uses, but in this case it's simply DEBUG's way of saying that the number 6EH does not correspond to any machine instruction It is truly a garbage byte sitting in memory, doing nothing Executing a 6EH byte as though it were an instruction, however, could cause your machine to do unpredictably peculiar things, up to and including locking up hard Remember, of course, that the 6EH was what happened to lie in memory one address up from the MOV AX,1 instruction on my machine at that particular time You almost certainly encountered something else when you tried the experiment just now In fact, I just rebooted my machine and tried it again and found an XCHG BP,[8001] instruction there instead There's nothing meaningful about the instructions you find in memory with DEBUG this way DEBUG is interpreting random values in memory as instructions, so almost any instruction may turn up and if the random values do not represent a legal machine instruction, you'll see a DB directive instead
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Machine Instructions and Their Operands
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As we said earlier, MOV copies data from a source to a destination MOV is an extremely versatile instruction, and understanding its versatility demands a little study of this notion of source and a destination
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