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