Int 2F Fn 4A10  - Smartdrv V4.00+ - Status                                 [c]

   AX = 4A10h
   BX = 0003h
   BP = drive number (0=A, 1=B, etc.)
   DL = subfunction
       00h only get information
       01h turn on read cache
       02h turn off read cache
       03h turn on write cache
       04h turn off write cache
       ---NWCACHE---
       05h ???
       06h ???

Return: AX = BABEh if OK
   DL = status (see #2487)
   DL = FFh if drive does not exist

Notes: If the read cache is off, reads will not be cached, but writes will
     continue to be cached if the write-cache is enabled.
   this function is also supported by PC-Cache v8.0.

See Also: AX=4A10h/BX=0000h

Bitfields for SMARTDRV status:
Bit(s) Description (Table 2487)
 7 not cached
 6 write-through (not write-cached)
 0-5   real drive number (0=A, 1=B...)

Note:  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