FIX: MASM 6.0 ALIGN Directive Requires Patch to Work Correctly

The information in this article applies to:

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

SYMPTOMS

The Microsoft Macro Assembler (MASM) version 6.0 may generate incorrect code with respect to the ALIGN directive. If the alignment value specified is larger than 2, the padding generated by the assembler may consist of incorrect instructions. Instead of generating an opcode for a no-operation "mov ax, ax" instruction (8Bh C0h), the assembler may reverse the bytes that are output (C0h 8Bh), resulting in code that fails to run correctly.

RESOLUTION

This problem may be corrected by patching ML.EXE with one of the following debug scripts. Before performing this operation on your EXE files, you might want to make a backup copy.

   Debug Script for DOS
   --------------------

   1: a 7d8a
   2: mov ax, ax
   3:
   4: w
   5: q

   Debug Script for OS/2
   ---------------------

   1: a 325a
   2: mov ax, ax
   3:
   4: w
   5: q

   REN ML.EXE ML.DAT
   DEBUG ML.DAT < PATCH.SCR
   REN ML.DAT ML.EXE

STATUS

Microsoft has confirmed this to be a problem in MASM version 6.0. This problem was corrected in MASM version 6.0a.

MORE INFORMATION

The ALIGN instruction is used to indicate the boundary on which to start the next instruction. For example, a statement such as

   ALIGN 4

MASM version 6.0 may use a 2-byte instruction (that has no effect) to pad the code. In this case, the opcode may be reversed as described above. This problem may be illustrated by assembling the sample code below.

Sample Code

; Assemble options needed: none

.MODEL large .STACK 4096

.DATA _data ENDS

_CodeSeg SEGMENT para public 'code' main PROC

   mov ax, dx
   ALIGN 8
   mov ax, 0