call assembly function via VB

[lek_70]

does anyone know how to call the assembly function(MASM ) via vb?? if so, how is the assembly function should be written?

[CornedBee]

you must write the assembly function to follow the pascal calling convention. Then you make a dll out of it and call it like a API function.

[lek_70]

Can u give me a simple example for both vb and assembly code?? Thanks.

[CornedBee]

Well, I'm not good in ASM and have never done Win32 ASM, I also don't know much about VB, but here's some code, I can't guarantee it will work.

; asm file, compile it and make a dll out of it
; I'll use AT&T syntax because I usually only code for linux (if I code at all)
; the most important things are that source and target operator are exchanged
; and opcodes have suffixes specifying the data length
; and registers and values have prefixes
; you can find information via google
.section .data
ErrStr:
.ascii "An Error occured."

.section .code
.global ErrorMB

; this must follow __stdcall conventions
ErrorMB: .type .function
  pushl %ebp
  movl %esp, %ebp
  ; call MessageBox(NULL, ErrStr, ErrStr, 0)
  ; it is a __stdcall function
  ; args are passed from left to right
  pushl $0
  movl $ErrStr, %eax ; is the same as: leal ErrStr, %eax
  pushl %eax
  pushl %eax
  pushl $0
  call MessageBox
  ; __stdcall says that stack is cleared by called function
  ; so if I understand correctly, we don't need pops here
  ; but this function returns now, so we need to clear up our waste
  movl %ebp, %esp
  popl %ebp
  ret    ; you should also somehow clear the arguments passed to this funtion here
  ; but since there are no arguments I'll leave it to you to find out how. You might want to look at an asm listing of a C function

Now that you have created the dll AsmFuncs.dll, we can call the function from VB

Private Declare Sub ErrorMB Lib "AsmFuncs.dll" ()
 
'Call
ErrorMB

This should theoretically work.

[CornedBee]

Ok, so you write
ret (number of bytes to pop)
to clear the stack.
example:

; now in MASM syntax
; int __stdcall add(int, int)
; returns the sum of the arguments
PUBLIC add   ; correct name would be _add@8, but then you would need a .def file
add PROC NEAR ; maybe needs to be far, who knows
  push ebp
  mov  ebp, esp
  ; get parameters
  mov eax, DWORD PTR 8[ebp] ; get first int
  add eax, DWORD PTR 12[ebp] ; add second int
  ; result is in eax, this is the return value
  ; leave esp untouched, it is still needed
  pop ebp
  ret 8 ; this will pop the 8 bytes off the stack and decrease esp and return

This is about what VC++ makes of the function

int __stdcall add(int i, int j)
{
  return i + j;
}