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Register C

Register C holds drive's Status and operational Mode, according to the following bit-map:

  Bits    Meaning
  ----------------------------
  0-3     Mode
  4       BOT (Begin Of Tape)
  5       EOT (End Of Tape)
  6       On-Line
  7       Alarm
  ----------------------------

Where Mode, in turn, is bit-mapped as following:

  Bits    Meaning
  ----------------------------
  0-1     Transport Status:
            00   STOP
            01   Rewinding
            10   Fast Forwarding
            11   Moving at nominal speed (Read, Write, or Making GAP)

  2-3     Operation:
            00  (Illegal)
            01   READ
            10   WRITE
            11   Making GAP
  ----------------------------

The register can be read or written though the EXT-BUS but the Drive's electronics must prevent bits 4-7 from being overwritten. Software running at the LC-81 can then read the entire register but only write the least significant nibble (Mode); the most significant nibble is simply ignored and can be filled with zeros. This results in the following:

  Mode    Meaning
  ----------------------------
  00H     STOP
  01H     RWD
  02H     FF
  07H     READ
  0bH     WRITE
  0fH     Make GAP
  ----------------------------