A group is a collection of segments totalling not more than 64K in 16-bit mode. Each code or data item in the group can be addressed relative to the beginning of the group through DS or SS.
Summary: Segments within a group can be treated as if they shared the same segment address.
A group lets you develop separate segments for different kinds of data and then combine these into one segment (a group) for all the data. Using a group can save you from having to continually reload segment registers to access different segments. As a result, the program uses fewer instructions and runs faster.
The most common example of a group is the specially named group for near data, DGROUP. In the Microsoft segment model, several segments (_DATA, _BSS, CONST, and STACK) are combined into a single group called DGROUP. Microsoft high-level languages place all near data segments in this group. (By default, the stack is placed here, too.) The .MODEL directive automatically defines DGROUP. The DS register normally points to the beginning of the group, giving you relatively fast access to all data in DGROUP.
The syntax of the group directive is
name GROUP segment[[,segment]]...
The name labels the group. It can refer to a group that was previously defined. This feature lets you add segments to a group one at a time. For example, if MYGROUP was previously defined to include ASEG and BSEG, then the statement
MYGROUP GROUP CSEG
is perfectly legal. It simply adds CSEG to the group MYGROUP; ASEG and BSEG are not removed.
Each segment can be any valid segment name (including a segment defined later in source code), with one restriction: a segment cannot belong to more than one group.
The GROUP directive does not affect the order in which segments of a group are loaded. You can place any number of 16-bit segments in a group as long as the total size does not exceed 65,536 bytes. If the processor is in 32-bit mode, the maximum size is four gigabytes. You need to make sure that non-grouped segments do not get placed between grouped segments in such a way that the size of the group exceeds 64K or 4 gigabytes. Neither can you place a 16-bit and a 32-bit segment in the same group.