FIX: Prefix Bytes Are Order-Dependent in CodeView 4.0x

ID: Q103721

The information in this article applies to:

Microsoft CodeView for MS-DOS, versions 4.0, 4.01

SYMPTOMS

The disassembler, which is used to display mixed source and assembly or assembly in CodeView, is sensitive to the order in which the 66h and 67h prefix bytes occur in 32-bit instructions. If the order is 67h 66h, then the instruction is disassembled as:

67668B03 MOV AX,WORD PTR [EBX]

CodeView incorrectly shows AX as the destination operand. However, if the order is 66h 67h, then the instruction is disassembled as:

66678B03 MOV EAX,DWORD PTR [EBX]

In this case, CodeView correctly shows EAX. The MASM version 6.x program shown below demonstrates the problem.

STATUS

Microsoft has confirmed this to be a problem in CodeView versions 4.0 and 4.01. The problem was corrected in CodeView version 4.1.

Sample Code


; Assemble options needed: /Zi

.MODEL small
.386

.STACK 4096

.CODE
    .startup
loc1:
    mov eax, [ebx]
loc2:
    ORG loc1
    DB 66h, 67h    ; This forces the prefix bytes to 66h 67h
    ORG loc2
loc3:
    mov eax, [ebx]
loc4:
    ORG loc3
    DB 67h, 66h    ; This forces the prefix bytes to 67h 66h
    ORG loc4
    .exit 0
END

Additional query words: reverse 4.00

Keywords : kb16bitonly
Version : :4.0,4.01
Platform :
Issue type :