Article ID: 49246
Article Last Modified on 10/17/2003
APPLIES TO
- Microsoft Macro Assembler 5.1 Standard Edition
- Microsoft Macro Assembler 5.1a
- Microsoft Macro Assembler 5.0
This article was previously published under Q49246
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
Keywords: KB49246
