CDFSDriver
~~~~~~~~~~
  
  SWI CD_Register (SWI &41260) / CD_Unregister (SWI &41261):
  
    Bit 21 of the information word may be set to indicate that CDFSDriver
    should call SWI SCSI_Op to implement SWI CD_SCSIUserOp, instead of
    calling the soft-loadable driver.
  
  SWI CD_SCSIUserOp (SWI &4126B):
  
    On entry:
      R0  bits 24-25 = 0 => no data transfer
                     = 1 => read
                     = 2 => write
                     = 3 reserved
      R1 = length of control block
      R2 -> control block
      R3 -> buffer for data transfer
      R4 = length of transfer
      R5 = timeout in cs (0 = default)
      R7 -> CDFS control block
    On exit:
      R3,R4 updated
      R5-R9 preserved
      NZC flags corrupted
      If V set,
        R0-R2 are as for SWI SCSI_Op
        or R0 -> error not in SCSI range and R1,R2 corrupt
      Else R0-R2 preserved
    Must not be called from background (for compatibility with ATAPI drives)



"Soft-loadable" drivers
~~~~~~~~~~~~~~~~~~~~~~~
  
  Entry #36 ("SCSIUserOp"):
  
    On entry:
      R0  bits 24-25 = 0 => no data transfer
                     = 1 => read
                     = 2 => write
                     = 3 reserved
      R1 = length of control block
      R2 -> control block
      R3 -> buffer for data transfer
      R4 = length of transfer
      R5 = timeout in cs (0 = default)
      R7 -> CDFS control block
      R11 = reason code (36)
      R12 = workspace
    On exit:
      R3,R4 updated
      R5-R12 may be corrupted
      NZC flags may be corrupted
      If V set,
        R0-R2 are as for SWI SCSI_Op
        or R0 -> error not in SCSI range and R1,R2 may be corrupted
      Else R0-R2 may be corrupted
