Popping 7th function parameter off the stack on AMD64?

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fpga

Joined: 22 Sep 2009
Posts: 36

fpga 05 Oct 2009, 18:54

The fasm function below is called by the c++ code underneath it.
It was ok until I added a 7nth parameter to both modules to work out
how to use the stack for function parameters. What am I doing wrong?

Code:

format ELF64 
section '.text' executable
public fn
fn:
   mov eax,edi
   add eax,esi
   add eax,edx
   add eax,ecx
   add eax,r8d    
   add eax,r9d
                         
   pop rdi        ;<=====!!! why's this an illegal instruction???

   add eax, edi
   ret

Code:

#include <iostream>
using namespace std;
//extern "C" int fn(int a,int b,int c, int d, int e, int f);
extern "C" int   fn(int a,int b,int c, int d, int e, int f, int g);
int main(){
   //cout << "params added = " << fn(1,2,3,4,5,6) << endl;
   cout << "params added = " <<   fn(1,2,3,4,5,6,7) << endl;
   return 0;

05 Oct 2009, 18:54

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 05 Oct 2009, 19:02

I think you get the error in RET actually because with POP RDI you removed the return address from the stack.

I don't remember how the calling convention was but check with this:

Code:

; remove pop rdi
;replace add eax, edi with this:
  add eax, [rsp+8]

05 Oct 2009, 19:02

fpga

Joined: 22 Sep 2009
Posts: 36

fpga 05 Oct 2009, 19:06

You are spot on!!!
Thanks very much
I need to read up on this

05 Oct 2009, 19:06

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 05 Oct 2009, 19:10

Take a look at this: http://www.x86-64.org/documentation/abi.pdf

05 Oct 2009, 19:10

fpga

Joined: 22 Sep 2009
Posts: 36

fpga 05 Oct 2009, 19:35

I had no awareness
a) that the function return address was before the parameters on the stack,
b) that rsp is the stack pointer
c) or that +8 means +8 bytes ie the next 64 bit word on stack
Thank you

05 Oct 2009, 19:35

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