FIX: Incorrectly Generated A4100: Impure Memory Reference (59567)


The information in this article applies to:

  • Microsoft Macro Assembler (MASM) 5.0
  • Microsoft Macro Assembler (MASM) 5.1
  • Microsoft Macro Assembler (MASM) 5.1a
This article was previously published under Q59567

SYMPTOMS

Microsoft Macro Assembler (MASM) may generate the following warning
A4100: Impure memory reference
on a line where data is being accessed from the code segment, not where it is being stored.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 5.0, 5.1, and 5.1a. This problem was corrected in MASM version 6.0.

MORE INFORMATION

Writing data to the code segment is acceptable in real mode; however, it may cause problems in protected (priviledged) mode. The /P option tells MASM 5.0, 5.1, and 5.1a to generate warning messages if an attempt is made to store data in the code segment when operating in protected mode. This is called an impure memory reference.

Note that in MASM 6.0, checking for impure memory references is no longer enabled by a command-line option, but by OPTION READONLY. The equivalent warning message in MASM 6.0 would be A4000: cannot modify READONLY segment.

Sample Code

; Assemble options needed: none

.MODEL small
.286p

.CODE
    mov al, cs:[bp]
    mul word ptr cs:[bp+2]  ; flagged as an error
END
Modification Type:MajorLast Reviewed:10/17/2003
Keywords:kbfix KB59567
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