Q49246: Using OFFSET with a Group of Data Segments

Article: Q49246
Product(s): Microsoft Macro Assembler
Version(s): 5.0,5.1,5.1a
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 5.1, 5.1a 
- Microsoft Macro Assembler (MASM), version 5.0 
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SUMMARY
=======

When using the OFFSET directive to refer to a variable defined in a SEGMENT that
is part of a GROUP, the offset generated is from the beginning of the SEGMENT if
full segment declarations are used. Normally when using the OFFSET directive,
the offset generated is from the beginning of the GROUP when simplified segment
declarations are used.

This behavior occurs in Microsoft Macro Assembler (MASM) versions 5.0, 5.1, and
5.1a and in QuickAssembler versions 2.01 and 2.51. Starting with MASM 6.0 you
can use the OPTION OFFSET directive to control whether the offset generated is
from the beginning of the SEGMENT or from the beginning of the GROUP.

MORE INFORMATION
================

The code below will demonstrate this behavior. The OFFSET directive generates an
offset relative to the beginning of the SEGMENT instead of the beginning of the
GROUP. When a segment override is used to specify the segment to take the
address relative to, the expected offset is generated. Also, the LEA instruction
generates the offset relative to the segment the DS register is assumed to.

Sample Code
-----------

  ; Assemble options needed: none

  dgroup GROUP data1,data2

  data1 SEGMENT PARA PUBLIC 'DATA'
  var1 DW 12 DUP (1)
  data1 ENDS

  data2 SEGMENT PARA PUBLIC 'DATA'
  var2 DW 0
  data2 ENDS

  code SEGMENT PARA PUBLIC 'CODE'
  ASSUME cs:code,ds:dgroup
  start:
      mov bx, OFFSET var2        ; <--offset relative to segment
      mov bx, OFFSET dgroup:var2 ; <--offset relative to dgroup
      lea bx, var2               ; <--offset relative to ds
      mov ax, 4C00h
      int 21h
  code ENDS

  END start

Additional query words: kbinf 5.00 5.10 5.10a

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Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM500 kbMASM510a
Version           : :5.0,5.1,5.1a

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