FIX: DUP Behavior Different with Span-Dependent Value

MS-DOS           | OS/2

The information in this article applies to:

• 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

        mov ax, 4C00h
        int 21h

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

END start