PRB: No Warning for INVOKE Between 16-Bit and 32-Bit Segments

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

The Microsoft Macro Assembler (MASM) version 6.0 does not support transfer of control between 16-bit and 32-bit code segments through the INVOKE command. If the assembler encounters an INVOKE in a 16-bit segment, it assumes that the procedure being invoked is also in a 16-bit segment and places the parameters on the stack using WORDs. On the other hand, if the assembler encounters an INVOKE in a 32-bit code segment, the parameters are placed on the stack as DWORDs with the assumption that the called routine will also be in a 32-bit code segment. MASM does not generate warnings when the segment being called does not match the calling segment.

CAUSE

The INVOKE command, introduced in MASM 6.0, is not designed to support transferring control between 16-bit and 32-bit code segments.

RESOLUTION

You can work around this problem by pushing the necessary parameters on the stack and making the appropriate call instead of using INVOKE.

STATUS

This feature is under review and will be considered for inclusion in a future release.

MORE INFORMATION

It is important to note that, in order to run code in 32-bit segments, the operating system that your program is running on must be able to support 32-bit code segments. Most PC-based operating systems such as MS-DOS version 5.0, Windows version 3.0, and OS/2 versions 1.x do not provide support for 32-bit code segments. Newer operating systems and MS-DOS extender packages may provide this support.

Sample Code

; Assemble options needed: none ; Note that this code does not work on present operating systems such ; as MS-DOS. This code is only provided to demonstrate how to replace ; an INVOKE statement with the appropriate push and call sequence.

.MODEL small, pascal .386

func PROTO FAR32 arg1:BYTE, arg2:WORD, arg3:DWORD

.CODE

  .STARTUP
  MOV dl, x
  MOV cx, y
  MOV ebx, z
  INVOKE func, dl, cx, ebx ; This doesn't work.

  ;MOV al, dl    ; Uncomment this code to set up the call to func.
  ;PUSH eax      ; Push DWORD instead of WORD.
  ;MOV ax, cx
  ;PUSH eax      ; Push DWORD instead of WORD.
  ;PUSH ebx
  ;CALL FAR32 PTR func

  .EXIT 0

  x DB 11h
  y DW 2222h
  z DD 33333333h

  MOV bl, arg1   ; func assumes that all variables
  MOV cx, arg2   ; on the stack are DWORD aligned.
  MOV edx, arg3
  RET

END