BUG: ORG in STRUCT Affects Current Segment Size with -Sf

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11

SYMPTOMS

A STRUCT declaration containing an ORG directive will change the current segment size when the -Sf switch is used and a listing file is produced with MASM 6.x.

CAUSE

When adding a first-pass listing (the -Sf switch) to the listing file, MASM's internal location counter is affected when the ORG directive is encountered within a STRUCT declaration. This location counter is ultimately used to determine segment size.

RESOLUTION

When declaring a STRUCT that contains an ORG directive, place the declaration outside of any open segments. Because this is a structure declaration rather than an actual instance, the structure does not have to be within a segment and can be placed outside all open segments where the location counter will not be adversely affected.

STATUS

Microsoft has confirmed this to be a problem in the Microsoft Macro Assembler (MASM) versions 6.x. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The following sample code illustrates this problem. When the structure is declared within the open data segment, the size of the data segment is increased. This can be viewed in the map file for the sample, as well as in the listing file. Move the structure declaration before the .DATA and .STACK directives to obtain the expected behavior.

An ORG directive is permitted in a structure declaration beginning with MASM version 6.0. This enables the generation of a template that can ease references to the internals of data structures that are predefined, for instance by the operating system, the operating platform, or a high-level language compiler.

For additional information about this MASM construct as well as sample code demonstrating its use, please see the following article(s) in the Microsoft Knowledge Base:

   ARTICLE-ID: Q111474
   TITLE     : PRB: Usage of an ORG Directive in a STRUCT Declaration

Sample Code

; Assemble options needed: -Fl -Sf -Fm .MODEL SMALL,C .386

NUMLINES     EQU 25
NUMCOLS      EQU 80

.DATA

status STRUCT                          ; Move STRUCT declaration
    org (NUMLINES - 2) * NUMCOLS * 2   ;  before .DATA and .STACK
    line1   db ?                       ;  to work around problem.
    attrib1 db ?
    org (NUMLINES - 1) * NUMCOLS * 2
    line2   db ?
    attrib2 db ?
status ENDS                            ; end STRUCT declaration

    msg db "Sample code", 13, 10, "$"

    mov dx, OFFSET msg
    mov ah, 9
    int 21h                               ; output string

END