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Q78393: FIX: MASM 6.0 Always Uses EAX Register with BSWAP

Article: Q78393
Product(s): Microsoft Macro Assembler
Version(s): MS-DOS:6.0
Operating System(s): 
Keyword(s): 
Last Modified: 04-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Macro Assembler (MASM), 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
  END start

Additional query words: 6.00 buglist6.00 fixlist6.00a

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600
Version           : MS-DOS:6.0
Solution Type     : kbfix

=============================================================================

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