Bitfields for SMARTDRV status:
Bit(s)	Description	)
 7	not cached (no read-caching)
 6	write-through (not write-cached)
 0-5	real drive number (0=A, 1=B...)
Notes:	the "real" drive number is always set to 00h for RAM disks and to 3Fh
	  for CD-ROMs; unfortunately, the latter means that an uncached CD-ROM
	  has status FFh, just as a nonexistent drive does
	also supported by Novell DOS 7 - Caldera/Lineo DR-DOS 7.03 NWCACHE