ZiLOG Z80-RIO Operating System User's Manual page 74

DEFINE
DEFINE
DEFINE
*
Restores all units to the defaults established at
system initialization (after bootstrap). All
defined units are closed prior to being redefined
as their defaults.
DEFINE 12 $YOUR.DOS/YOURFILE NO
Links unit 12 to the device YOUR. DOS (which must
be active). An Assign I/O request is then generated
with YOURFILE as the filename, but no Open request.
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