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The "zu" option relaxes the restriction that the SS register contains the
base address of the default data segment, "DGROUP".  Normally, all data
items are placed into the group "DGROUP" and the SS register contains the
base address of this group.  When the "zu" option is selected, the SS
register is volatile (assumed to point to another segment) and any global
data references require loading a segment register such as DS with the base
address of "DGROUP".
(16-bit only) This option is useful when compiling routines that are to be
placed in a Dynamic Link Library (DLL) since the SS register points to the
stack segment of the calling application upon entry to the function.

The macro __SW_ZU will be predefined if "zu" is selected.