FIX: Illegal Instruction from Structure Member Offset

MS-DOS     | OS/2

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 5.1 and 5.1a
• Microsoft Macro Assembler for OS/2, versions 5.1 and 5.1a

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 (?)

.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.