how to understand INC EDI and ADD EDI, 4?

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jacky_zz

Joined: 10 Oct 2007
Posts: 4

jacky_zz 11 Oct 2007, 06:12

i got a problem about INC EDI and ADD EDI, 4.
*****code begin*****
long test(void* p) {
__asm {
push edi
mov edi, p
mov [edi], 02h
add edi, 4 // inc edi

mov [edi], 20h
mov eax, [edi]

pop edi
}
}

long* long_array = new long[2];
long result = test(long_array);
*****code end*****
my question: when i use "INC EDI" replace statement "add edi, 4", the EDI's address add one, the pointer "long_array" points to first element of array or not? why?

11 Oct 2007, 06:12

Artlav

Joined: 23 Dec 2004
Posts: 188
Location: Moscow, Russia

Artlav 11 Oct 2007, 06:28

inc edi is add edi,1, not 4. You have array of long, 4 bytes per element, so add edi,4 seems right to access array[1].

That is, if i got it right what you were asking.

11 Oct 2007, 06:28

jacky_zz

Joined: 10 Oct 2007
Posts: 4

jacky_zz 11 Oct 2007, 06:35

yes, it is, when i call "inc edi", and edi's address add 1, if address before call "inc edi" is 0012FEA4, after call, the current address is 0012FEA5, then i call "mov [edi], 20H", i want to know what is value of adress 0012FEA5?

11 Oct 2007, 06:35

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 11 Oct 2007, 11:44

Code:

mov     edi, $0012FEA4   ; EDI = $0012FEA4
inc     edi              ; EDI = $0012FEA5
mov     dword [edi], $20 ; memory at address range $0012FEA5..$0012FEA8 = $00000020 ($0012FEA5 = $20, the other 3 addresses are $00)

11 Oct 2007, 11:44

jacky_zz

Joined: 10 Oct 2007
Posts: 4

jacky_zz 12 Oct 2007, 03:36

oh, i get it, thanks!

12 Oct 2007, 03:36

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