CODEWITHSUNDEEP
assemblyx8664-bitx86-64calling-convention
formateu
formateu

Reputation: 167

Intel x64 assembly function called from C program how to modify arguments passed by reference

I try to modify some of arguments passed to my assembly function in C program. I have tried it already in x86 assembly but in x64 it doesn't seem to work anymore.

section .text
global f
f:
  push rbp
  mov rbp, rsp
  ;[rbp+8]  bitmap beginning address (unsigned*) ?
  ;[rbp+12] bitmap width (int*) ?
  ;[rbp+16] bitmap height (int*) ?
  ;[rbp+20] current X pos (double*) ?
  ;[rbp+24] current Y pos (double*) ?

  mov rax, [rbp+12]

  mov rcx, [rax]
  inc rcx
  mov [rax], rcx 

  mov rsp, rbp
  pop rbp
  ret

Upvotes: 0

Views: 1080

Answers (1)

blatinox
blatinox

Reputation: 843

The way arguments are passed to a function in C depends on the calling convention.

Here, the way you modify arguments use the x86 calling convention properties (arguments are passed via the stack). But in x86_64, the calling convention is different (and may differ depending on your compiler). Arguments are generally passed via registers so modifying the stack does not modify arguments.

You can see the list of x86 calling convention on Wikipedia

Upvotes: 1

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