CODEWITHSUNDEEP
functionassemblyx86callstack
MobDev
MobDev

Reputation: 1259

x86 assembly programming function calling

I am working on function calls and returns in x86 assembly. I can't seem to get this to go without crashing. I am calling the function MIRROR_BYTE from bit_operations. But every time i run the code it has unknown error and crashes. It happens only after I MIRROR_BYTE finishes Really lost here, any help is appreciated. url to full code

http://codepaste.net/av7ikf

 __declspec(naked) void
 bit_operations(unsigned long inputDWord, unsigned long *outputDWord)
 {
     __asm{
         // start code for part B here

         push eax
         push ebx

         mov ebx, [esp + 12] 
         push ebx                 //move inputDword into the stack
             call MIRROR_BYTE
         pop ebx                  //pop inputDword out of the stack

         pop ebx
         pop eax

         // end code for part B here
         ret
     }
 }

 /*
     This function takes 4 bytes as input and mirrors the value of Byte 4 (leftmost).
     For example, for a byte like 10110111, the mirrored byte value is 11101101.
 */

 __declspec(naked) unsigned long
 MIRROR_BYTE( unsigned long inputDWord )
 {
     __asm{

     // not sure what to do here just return dummy al from inputDword
     mov al, byte ptr[esp +4]


     }
 }

Upvotes: 0

Views: 1194

Answers (2)

Gunner
Gunner

Reputation: 5884

You need to return from MIRROR_BYTE!!! Without a ret or C return, the CPU will continue executing code after the MIRROR_BYTE proc.

Upvotes: 1

Jester
Jester

Reputation: 58762

Notice that your pop eax; pop ebx should be reversed to match the push eax; push ebx earlier. Can't say if that's causing your problems, though.

Upvotes: 0

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