This tells the driver to return its input status; in particular, to
indicate whether there are characters in the device input buffer ready for
immediate retrieval.

DevReq06Rec
Offset Size Contents
                                                                          
+0      1  bLen         0dH (length of this request packet)
+1      1  bUnitNo      (not used)
+2      1  bCmd         06H (command code)
+3      2  rStatus      OUTPUT: Device Status Word must fill before exit
+5H     8  res          (reserved)
13               size of this structure

rStatus  On exit, the driver must place a DevStatusWord here.  Always
set the Done bit (bit 8).

If no characters are ready, set the Busy bit (bit 9).
Otherwise, clear bit 9 to indicate that a call to DvRq 04H
will be satisfied without waiting.

Notes:   This is the device driver "catch-point" for DOS fn 0bH (get
input status).

It is also called by DOS to avoid waiting in an input loop.

See Also: Device Requests
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Device Request 06H: Input Status