Using OFFSET with a Group of Data Segments

• Microsoft Macro Assembler for MS-DOS, versions 5.0, 5.1 and 5.1a
• Microsoft Macro Assembler for OS/2, versions 5.1 and 5.1a

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

END start