INT 2F - SMARTDRV v4.00+ - STATUS
        AX = 4A10h
        BX = 0003h
        BP = drive # (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
Return: AX = BABEh if OK
        DL = status
            bit 7  not cached
            bit 6  write-through (not write-cached)
            bit 5  ???
            bits 0-4 drive # (0=A, 1=B...)
        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.
SeeAlso: AX=4A10h/BX=0000h

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