FIX: MASM 6.0 May Incorrectly Generate A4000 for CMP

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, version 6.0
• Microsoft Macro Assembler for OS/2, version 6.0

SYMPTOMS

Microsoft Macro Assembler (MASM) version 6.0 will incorrectly generate the following error when the sample code below is assembled:

   A4000: cannot modify READONLY segment

CAUSE

This problem occurs because the segment is tagged as READONLY, and MASM treats the CMP instruction as if it modifies memory. This is incorrect behavior because the CMP instruction does not modify any memory.

RESOLUTION

To work around this problem, either remove the READONLY tag from the segment or ignore the warning when it is generated for CMP instructions.

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 sample code below reproduces the problem.

Sample Code

; Assemble options needed: none

code    SEGMENT word public readonly 'code'
        ASSUME cs:code
        cmp word ptr cs:[di], 0
code    ENDS

END