Microsoft KB Archive/80116

= FIX: Illegal Instruction from Structure Member Offset =

Article ID: 80116

Article Last Modified on 10/16/2003

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APPLIES TO


 * Microsoft Macro Assembler 5.1 Standard Edition
 * Microsoft Macro Assembler 5.1a

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This article was previously published under Q80116



SYMPTOMS
In the Microsoft Macro Assembler (MASM) versions 5.1 and 5.1a, if the offset of a structure template member is used as a 32-bit operand, the code generated may be incorrect. This will cause the program to hang or generate an illegal instruction message in CodeView.



CAUSE
The problem occurs when the offset of a structure template member is being used as a 32-bit immediate value, and the member's offset is greater than 100h. If the offset is greater than 100h, the code generated will not contain the 66h prefix byte, which makes the instruction a 32-bit operation.



RESOLUTION
Embed the 66h byte by using the DB directive, as shown in the example below.



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



MORE INFORMATION
The following sample code can be used to demonstrate the problem.

Sample Code
; Assemble options needed: none

.MODEL small .386

strctr STRUC data1 DB 100h DUP (?) data2 DB 100h DUP (?) strctr ENDS

.CODE start: mov esi, strctr.data1 ;This works since data1 < 100h. ;DB 66h              ;Uncomment to fix next instruction. mov esi, strctr.data2 ;This doesn't work since data2 > FFh.

add esi, strctr.data1 ;This works since data1 < 100h. ;DB 66h              ;Uncomment to fix next instruction. add esi, strctr.data2 ;This doesn't work since data2 > FFh.

END start

Additional query words: 5.10 5.10a buglist5.10 buglist5.10a fixlist6.00

Keywords: kbfix KB80116

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