FIX: DUP Behavior Different with Span-Dependent Value

• Microsoft Macro Assembler for MS-DOS and OS/2, versions 6.0, 6.0a, 6.0b

SYMPTOMS

In Microsoft Macro Assembler (MASM) versions 6.0, 6.0a, and 6.0b, if the size of a DUP directive depends on the difference between the values of two labels (a span-dependent value), the assembled code differs from that produced by previous MASM versions.

CAUSE

The size of the DUP depends on label values that are not yet determined in the first assembly pass.

RESOLUTION

Modify the code to remove span-dependent values in a DUP directive in code assembled with MASM versions 6.0, 6.0a, or 6.0b.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, and 6.0b for MS-DOS and OS/2. This problem was corrected in MASM version 6.1 for MS-DOS.

MORE INFORMATION

This problem usually occurs when a forward reference occurs between the two labels. The assembler adds padding bytes for the forward reference and eliminates these bytes during a subsequent assembly pass. When this occurs, the second label has a larger value during the first assembly pass than it does in subsequent passes.

The sample code below demonstrates this behavior.

Sample Code

; Assemble options needed: none

tst1 SEGMENT para public
ASSUME cs:tst1
start:
        jmp SHORT forward
forward:
        mov ax, 4C00h
        int 21h
tst1 ENDS

count = offset forward - offset start

tst2 SEGMENT para public
        DB count DUP (2)  ; This assembles as DUP 10 in MASM versions 6.0,
                          ; 6.0a, and 6.0b. It assembles as DUP 2 in MASM
                          ; versions 5.1 and 6.1.
xxx     DB 1
tst2 ENDS

END start 

Additional query words: 6.00 6.00a 6.00b buglist6.00 buglist6.00a buglist6.00b fixlist6.10

Keywords :
Version : MS-DOS:6.0,6.0a,6.0b
Platform : MS-DOS
Issue type :