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The "zdf" option allows the code generator to use the DS register to point
to other segments besides "DGROUP" This is the default in the 16-bit
compact, large, and huge memory models (except for 16-bit Windows
applications).
The "zdp" option informs the code generator that the DS register must always
point to "DGROUP" This is the default in the 16-bit small and medium memory
models, all of the 16-bit Windows memory models, and the 32-bit small and
flat memory models.  The macro __SW_ZDF will be predefined if "zdf" is
selected.  The macro __SW_ZDP will be predefined if "zdp" is selected.