Segment Relative Versus Group Relative

ID: Q30500

The information in this article applies to:

Microsoft Macro Assembler for MS-DOS, version 5.1

SUMMARY

The following sample code will be used to explain segment relative and group relative:


   dgroup  GROUP   aseg,cseg
   ASSUME  cs:dgroup,ds:dgroup
   aseg    SEGMENT public  byte    'CODE'
           org 100h
           start:
           data_pointer    dw  the_data
   aseg    ends
   cseg    SEGMENT public  byte    'CODE'
           the_data    label   byte
   cseg    ends
           end start

When defining data, fixups are relative to the start of the segment. The variable data_pointer will have an address relative to the start of the aseg segment. If you want the variable to have an address relative to the start of the group, you can explicitly state the offset relative to dgroup as follows.


   data_pointer    dw  offset  dgroup:the_data

The exception is when you are using simplified segment directives. In that case, group relative fixups always are used rather than segment relative.

In code segments, fixups are relative to either segment or group depending on the ASSUME statements. In other words, the ASSUME STATEMENT is checked before the fixup is generated. The exception here is the same as above. When using simplified segment directives, group relative fixups are the default.

The OFFSET operator is segment relative and is discussed further in the following article in the Microsoft Knowledge Base:

Q30517 OFFSET Operator Is Segment Relative

Additional query words: 5.10

Keywords :
Version : :5.1
Platform :
Issue type :