PRB: Error A2068 Using REP Prefix with CMPS Instructions

ID: Q120155

The information in this article applies to:

Microsoft Macro Assembler (MASM) for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, 6.1a
Microsoft Macro Assembler (MASM) for Windows NT, version 6.11

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 :
Version : :6.0,6.0a,6.0b,6.1,6.11,6.1a
Platform :
Issue type :