FIX: MASM 6.0 May Generate Wrong Listing for CMPXCHG

 Last reviewed: September 16, 1997
Article ID: Q79835
6.00 6.00a 6.00b 6.10 6.10a | 6.00 6.00a 6.00b 
MS-DOS                      | OS/2
kbtool kbfixlist kbbuglist

The information in this article applies to:

  • Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, and 6.1a
  • Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

When using the Microsoft Macro Assembler (MASM) to generate a listing file for an assembler source file that contains a CMPXCHG instruction, errors may occur in that listing. A colon is appended to bytes of values 26h, 2Eh, 36h, and 3Eh when they appear in an instruction. For example, the following are generated: 

   0000  0F B0 26: 00 00              cmpxchg data1, ah
   0005  0F B0 2E: 00 00              cmpxchg data1, ch
   000A  0F B0 36: 00 00              cmpxchg data1, dh
   000F  0F B0 3E: 00 00              cmpxchg data1, bh
instead of the following: 

   0000  0F B0 26 0000  R             cmpxchg data1, ah
   0005  0F B0 2E 0000  R             cmpxchg data1, ch
   000A  0F B0 36 0000  R             cmpxchg data1, dh
   000F  0F B0 3E 0000  R             cmpxchg data1, bh
In addition, the address of a memory location is not always shown; instead, a series of disjoint zero bytes appear. For example, the following is generated: 

   0014  0F B0 16 00 00               cmpxchg data2, dl
instead of the following: 

   0014  0F B0 16 0100  R             cmpxchg data2, dl
Note that this also occurs in the first example. Finally, extra trailing bytes may be placed in the listing file. For example, the following is generated: 

   0019  67& 0F B1 94 73 00000200  R  cmpxchg data3[ebx+2*esi],dx
         00 00 00 09
instead of the following: 

   0019  67& 0F B1 94 73 00000200  R  cmpxchg data3[ebx+2*esi],dx

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, 6.0b, 6.1, and 6.1a. This problem was corrected in MASM for MS-DOS version 6.11.

Sample Code

; Assemble options needed: /c /Fl

.MODEL small .486 .STACK 4096

.DATA data1 DB 100h DUP ('x') data2 DB 100h DUP ('y') data3 DW 100h DUP (1)

.CODE start: 

     cmpxchg data1, ah
     cmpxchg data1, ch
     cmpxchg data1, dh
     cmpxchg data1, bh
     cmpxchg data2, dl
     cmpxchg data3[ebx+2*esi],dx
END start

Additional reference words: 6.00 6.00a 6.00b 6.10 6.10a buglist6.00a
buglist6.00b buglist6.10 buglist6.10a fixlist6.11
KBCategory: kbtool kbfixlist kbbuglist
KBSubCategory: MLIss
Solution Type : kbfix

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Last reviewed: September 16, 1997
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