use proc macro without OS

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Byte

Joined: 21 Mar 2026
Posts: 4

Byte 21 Mar 2026, 18:13

I need clean program code in 32-bit mode using 'proc' and 'invoke' macros.
An error occurs during compilation.

Code:

invoke _OutMsg, txtToOut, EAX
/inc/proc32.inc [20] invoke [6]:
   call [proc] }
processed: call[_OutMsgIn12h]
error: operand size not specified

Code:

format binary

_OutMsgIn12h% = 2

use32

include '../inc/proc32.inc'

_OutMsg   EQU   _OutMsgIn12h

txtToOut    DB   'Hello from protected mode', 0
.....
.....
.....
xor      eax, eax
mov    ax, 216
shl      eax, 16
mov   ax, 239
invoke _OutMsg, txtToOut, eax
....
....
....
proc _OutMsgIn12h stdcall, txtmsg:DWORD, xy:DWORD
   push  ecx

  mov  ecx, [txtmsg]
  mov  edx, [xy]
  ......
  ......
  ......
omsg_end:
   pop  ecx
   ret
endp

Can anyone help? I used MASM before, but I couldn't generate non-PE 32-bit code with it.

21 Mar 2026, 18:13

bitRAKE

Joined: 21 Jul 2003
Posts: 4409
Location: vpcmpistri

bitRAKE 21 Mar 2026, 19:30

You left out what tool you're actually using.

stdcall is the general macro to call functions.
invoke is engineered for indirect calls specifically (typically external).

Code:

macro invoke?: proc*,args&
        stdcall [proc],args
end macro

21 Mar 2026, 19:30

Byte

Joined: 21 Mar 2026
Posts: 4

Byte 21 Mar 2026, 20:47

bitRAKE wrote:

You left out what tool you're actually using.

Sorry. I use FASM for DOS. What prevents me from using invoke to call my own functions like I did in MASM?
I can't understand why the error is in the unspecified operand size?

21 Mar 2026, 20:47

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20913
Location: In your JS exploiting you and your system

revolution 21 Mar 2026, 21:03

invoke is an indirect call.
stdcall is a direct call.

Code:

call [my_proc] ; indirect call
call the_proc  ; direct call

my_proc dd the_proc

the_proc:
  ret

Indirect calls (invoke) is used for DLL linking.

21 Mar 2026, 21:03

bitRAKE

Joined: 21 Jul 2003
Posts: 4409
Location: vpcmpistri

bitRAKE 21 Mar 2026, 21:06

The unspecified operand size error is because the call instruction has many forms.

FASM didn't aim for parity with MASM. Of course, it's possible to write your own invoke - I don't use it myself. The way you're using it is little more than an extended CALL instruction.

_________________
¯\(°_o)/¯ AI may [not] have aided with the above reply.

21 Mar 2026, 21:06

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20913
Location: In your JS exploiting you and your system

revolution 21 Mar 2026, 21:13

Rewriting _OutMsg EQU _OutMsgIn12h can make invoke work

Code:

_OutMsg   dd   _OutMsgIn12h

But it is wasteful. Just use a direct call then the entire line can be removed.

21 Mar 2026, 21:13

Byte

Joined: 21 Mar 2026
Posts: 4

Byte 21 Mar 2026, 21:35

Thank you very much everyone for your answers!
This advice solved my issue.

revolution wrote:

Rewriting _OutMsg EQU _OutMsgIn12h can make invoke work
Code:
_OutMsg   dd   _OutMsgIn12h    
But it is wasteful. Just use a direct call then the entire line can be removed.

Using invoke avoids errors when working with variables on the stack. And using proc structures the code better than simply jumping to a label.
MASM is a good assembler. Unfortunately, it is not very suitable for writing OS-independent code (unless it is 16-bit code). I will study FASM.

P . S . Another question: can FASM create a listing file?

Last edited by Byte on 21 Mar 2026, 21:40; edited 1 time in total

21 Mar 2026, 21:35

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20913
Location: In your JS exploiting you and your system

revolution 21 Mar 2026, 21:39

stdcall is a less wasteful option, that provides all the same protections from errors as invoke.

Code:

stdcall _OutMsgIn12h, txtToOut, eax

21 Mar 2026, 21:39

Byte

Joined: 21 Mar 2026
Posts: 4

Byte 21 Mar 2026, 21:42

revolution wrote:

stdcall is a less wasteful option, that provides all the same protections from errors as invoke.

Thank you, I will let you know.

21 Mar 2026, 21:42

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20913
Location: In your JS exploiting you and your system

revolution 21 Mar 2026, 21:44

Byte wrote:

P . S . Another question: can FASM create a listing file?

Look under the "tools\listing" folder in the fasm download.

21 Mar 2026, 21:44

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