52,53



     is a one-byte location used to hold the eight bits which will be
     transmitted one bit at a time (serially) to or from the device. The
     computer takes the eight bits for processing when the register is
     full or replaces another byte in it when empty after a
     transmission.

52,53          34,35          BFENLO/HI
     Next byte past the end of the SIO and DCB data buffer described
     above.

54             36             CRETRY
     Number of command frame retries. Default is 13 ($0D). This is the
     number of times a device will attempt to carry out a command
     such as read a sector or format a disk.

55             37             DRETRY
     Number of device retries. The default is one.

56             38             BUFRFL
     Data buffer full flag (255; $FF equals full).

57             39             RECVDN
     Receive done flag (255; $FF equals done).

58             3A             XMTDON
     Transmission done flag (255; $FF equals done).

59             3B             CHKSNT
     Checksum sent flag (255; $FF equals sent).

60             3C             NOCKSM
     Flag for "no checksum follows data." Not zero means no
     checksum follows; zero equals checksum follows transmission
     data.

61             3D             BPTR
     Cassette buffer pointer: record data index into the portion of data
     being read or written. Ranges from zero to the current value at
     location 650 ($28A). When these values are equal, the buffer at
     1021 ($3FD) is empty if reading or full if writing. Initialized to 128
     ($80).

62             3E             FTYPE
     Inter-record gap type between cassette records, copied from
     location 43 ($2B; ICAX2Z) in the ZIOCB, stored there from
     DAUX2 (779; $30B) by the user. Normal gaps are a non-zero
     positive number; continuous gaps are zero (negative number).

63             3F             FEOF
     Cassette end of file flag. If the value is zero, an end of file (EOF)
     has not been reached. Any other number means it has been