MSM9201005: Illegal Instruction from Structure Member Offset

ID Number: Q80116

5.10 5.10a | 5.10 5.10a

MS-DOS | OS/2

buglist5.10 buglist5.10a fixlist6.00

Summary:

PROBLEM ID: MSM9201005

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:

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 reference words: 5.10 5.10a 6.00