INT 11 - WD7000 SDLP interface - EXECUTE GENERIC SCSI COMMAND
        AH = FFh
        SI = 6A6Ah
        AL = SCSI Addressing
            bits 2-0 SCSI Target LUN (logical unit number)
            bits 5-3 SCSI Target ID
            bit 7    write flag, set for write operations, clear otherwise
        CX = bytes of data to be transmitted (max FFF0h)
        DH = 00h
        DL = length of SCSI Command Descriptor Block
        DS:DI -> SCSI Command Descriptor Block
        ES:BX -> data buffer
Return: CF set on error
            AL = error code
        CF clear if successful
Note:   because of busmaster operations with WD7000FASST avoid accessing
          video memory directly; check 386 memory manager for VDS support.
          The WD7000XTAT works with programmed IO and does not have this
          limitation.