Passing a var from c to asm

[Tk]

I need to pass a var ( or vars ) from c to asm.
ie:

_SetGMode:

   mov ah,0x00
   mov al,[var]
   int 0x10
   ret

and c:

extern void SetGMode( unsigned char var );
SetGmode(0x13);

Thanks

[mhaggag]

When your asm function is called, esp will point to the return address. esp + 4 will contain the variable in question. So, off the top of my head:

Code: Select all

   push ecx    ; Save this because we'll modify it

   mov ecx, [esp + 8]   ; + 8 because we have pushed ecx, so 4 + 4 = 8
   mov ah,0x00
   mov al, cl
   int 0x10
   pop ecx
   ret

Or something similar.

[Tk]

Thanks its working, now the reverse....i need to pass unsigned char back to the c function.

[mhaggag]

Thanks its working, now the reverse....i need to pass unsigned char back to the c function.

It's the same concept, really--you push the values on the stack. Something like (assuming nasm here):

Code: Select all

extern some_c_function
...
some_assembly_routine:
    push eax    ; Push this as a parameter to the function
    call some_c_function
    add esp, 4  ; Pop the parameter off the stack
                    ; but since we don't need its value
                    ; we just add 4 to the stack pointer (instead of popping)

The convention with return values, in case you need them, is to put the return value (if it's a primitive) or its address (if it's not a primitive) in eax. You can do that to return a value from asm to C or vice versa.

[Tk]

Thanks again, thats perfect.