ID Number: Q30500
5.10
MS-DOS
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.
More Information:
The OFFSET operator is segment relative and is discussed further in
a separate article.