Registars and Buffers

[Tipidy]

Hi all,

I have a quick question, whats wrong with this code below:

Code Select Expand


invoke lstrlen addr username
; eax now holds the number of characters in the username
mov dword ptr [eax], offset random


As you might of noticed, I want to move the value of eax into a buffer. I am having troubles finding the correct code, can someone please help me out?

Cheers, Tipidy.

[Neil]

mov buffer,eax

[travism]

I thought it would be?

mov dword ptr [buffer], eax

[starzboy]

@travism
that is more appropriate, but if the buffer is a local dword then it is also fine :)

[Neil]

I assumed that random was a dword variable, we need a bit more information as to what it is.

[dedndave]

mov dword ptr [buffer], eax

generally, if the size is defined by a register (eax in this case), a ptr is not needed
if it were an immediate value, then you would want one

mov buffer,eax - the assembler knows it is a dword because of the size of eax
mov dword ptr buffer,27h - the assembler does not know the width of 27h (27h could be a byte, word, or dword)

although, if buffer is defined as a dword, it may know - don't trust it

cmp word ptr [esi],27h - in this case, [esi] could be any size - so could 27h - so a ptr is needed

[starzboy]

Hey Neil
No offense intended brother.

[Neil]

None taken :bg, if Tipidy had posted a bit more code then we wouldn't be guessing ::) what exactly he means.

[travism]

mov dword ptr [buffer], eax

generally, if the size is defined by a register (eax in this case), a ptr is not needed
if it were an immediate value, then you would want one

mov buffer,eax - the assembler knows it is a dword because of the size of eax
mov dword ptr buffer,27h - the assembler does not know the width of 27h (27h could be a byte, word, or dword)

although, if buffer is defined as a dword, it may know - don't trust it

cmp word ptr [esi],27h - in this case, [esi] could be any size - so could 27h - so a ptr is needed

Hey thanks for the explanation on that :) Im sure he should get it now.