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Q120155: PRB: Error A2068 Using REP Prefix with CMPS Instructions

Article: Q120155
Product(s): Microsoft Macro Assembler
Version(s): 6.0,6.0a,6.0b,6.1,6.11,6.1a
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 6.0, 6.0a, 6.0b, 6.1, 6.1a 
- Microsoft Macro Assembler (MASM), version 6.11 
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SYMPTOMS
========

Assembling a line of code that uses the REP instruction prefix with one of the
string-compare instructions, such as

     REP CMPSB

produces the following error message:

  error A2068: instruction prefix not allowed

The same line of code assembles without error under MASM version 5.1.

CAUSE
=====

With MASM version 5.1, as well as when using MASM version 6.x with OPTION M510,
the REP CMPSB instruction is encoded as a REPE CMPSB.

NOTE: The difference between these instructions is that the REP instruction
repeats CX times regardless of the zero flag (ZF); however, the REPE and REPNE
instructions check the ZF flag as they repeat. Because there is virtually no
encoding for REP CMPSB, MASM versions 6.0 and 6.1 generate an error message.

RESOLUTION
==========

Assemble using the OPTION M510 directive, or use the REPE instruction prefix.

MORE INFORMATION
================

The following are legal forms of the string-compare instruction:

  CMPSB ; compare string byte
  CMPSW ; compare string word
  CMPSD ; compare string doubleword

Additional query words: rep 6.00 6.00a 6.00b 6.10 6.10a

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM610 kbMASM611 kbMASM610a kbMASM600a kbMASM600b
Version           : :6.0,6.0a,6.0b,6.1,6.11,6.1a

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

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