Hello,

Could you guys please hlep me to understand the purpose of the following VisualC++ ASM code that I found in every VC++ disassembly code?

mov eax, 0CCCCCCCCh
rep stos dword ptr [edi]

Thank you.

Basically it initilizes all local variables used by the current function with the value 0CCCCCCCCh this is really pointless because you will most likely initilize the function anyway. my opinion is that it's a waste of processor time. but it is a good example of how to quickly fill a section of memory with a certan value.

not really to do with VC++, but for disassembling most C

mov eax, #
or xor eax, eax etc

then

mov edi, address
rep stos dword ptr [edi]/ES:[EDI]

is usually a call to memset

That code only shows up in DEBUG builds of vc++ software,
BUILD | CONFIG | Release option, will build a more optimized executable without the debugging stuff like mov eax,0CCCCCCCCh.

0CCh is int 3 opcode and it is put there, because if the program has some bug and if the excution flow jumps somewhere illegal the program will crash, and then it shows you all the registers' contents, stack, etc.

Often this memory filling also helps when debugging to see what memory is used during the program run time. You can optimise the stack allocation better when you know the expected maximum usage by seeing where the '0xcccccccc' values have been overwritten with code or data used during a test run.