FIX: MASM 6.0 Always Uses EAX Register with BSWAP

The information in this article applies to:

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

SYMPTOMS

The Microsoft Macro Assembler (MASM) version 6.0 always generates code that uses the EAX register with the BSWAP instruction.

Attempting to use another register with the BSWAP instruction assembles without error; however, the code generated uses the EAX register.

RESOLUTION

A workaround for this problem is to embed the opcode (operation code) for the instruction directly in the code. The sample code below demonstrates this method.

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 following sample code can be used to demonstrate the problem.

Sample Code

; Assemble options needed : none

.MODEL small .486

.STACK 4096

.CODE start:

    bswap   eax            ; Code generated: 66| 0F C8 (bswap eax)

    bswap   ebx            ; Code generated: 66| 0F C8 (bswap eax)
    BYTE    66h            ; Embed a bswap ebx in the code
    WORD    0CB0Fh

    bswap   ecx            ; Code generated: 66| 0F C8 (bswap eax)
    BYTE    66h            ; Embed a bswap ecx in the code
    WORD    0C90Fh

    bswap   edx            ; Code generated: 66| 0F C8 (bswap eax)
    BYTE    66h            ; Embed a bswap edx in the code
    WORD    0CA0Fh