OFFSET Operator Is Segment Relative (30517)

SUMMARY

In Microsoft Macro Assembler version 5.10 The OFFSET operator is segment relative. The GROUP operator must be used to make the OFFSET operator group relative. In Microsoft Macro Assembler version 6.00 The OFFSET operator is group relative by default. This default can be over ridden with the M510 and OFFSET: options.

MORE INFORMATION

The following is an example of using the OFFSET operator:

group_1 GROUP seg_1,seg_2,seg_3

seg_1 SEGMENT word public 'data'
assume ds:group_1
        dw  0
item1   dw  20 dup(0)

seg_1 ends

seg_2 SEGMENT word public 'code'
assume cs:group_1

func1   PROC    NEAR
        mov ax, offset item1
        lea ax, item1
        mov ax, offset group_1:item1
        mov ax, offset item2
        lea ax, item2
        mov ax, offset group_1:item2

func1   ENDP

seg_2  ENDS

seg_3  SEGMENT  word  public  'data2'
assume ds:group_1
item2   dw  0
seg_3  ENDS

In MASM 5.1 the following will occur: In the first move instruction, item1 will have the offset of 0002 relative to the start of seg_1 segment. The next move instruction is using the group name to force the evaluation of item1 relative to the start of the group. The offset for item1 will be 0002 because seg_1 is the first segment in the group.

The third move instruction containing item2 will have an offset 0002 relative to the start of the seg_3 segment. Due to the alignment of the seg_3 segment, which is word aligned, the offset for item2 is not 0000. The fourth move instruction forces the item2 offset to be calculated from the beginning of the group. The seg_3 segment is not the first segment in the group but the third segment. The offset for item2 will be 0042.

In MASM 6.0, the first two OFFSET calculations will the same (group relative) and the second two OFFSET calulations will be tha same (OFFSET item2 == OFFSET group_1:item2). If OPTION M510 is used, the code will behave as specified for MASM 5.10.