Microsoft KB Archive/120155

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

Article ID: 120155

Article Last Modified on 2/11/2004

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APPLIES TO


 * Microsoft Macro Assembler 6.0 Standard Edition
 * Microsoft Macro Assembler 6.0a
 * Microsoft Macro Assembler 6.0b
 * Microsoft Macro Assembler 6.1 Standard Edition
 * Microsoft Macro Assembler 6.1a
 * Microsoft Macro Assembler 6.11 Standard Edition

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This article was previously published under Q120155



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: KB120155

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