64



     detected. An EOF record has been reached when the command
     byte of a data record equals 254 ($FE). See location 1021 ($3FD).

64             40             FREQ
     Beep count retain register. Counts the number of beeps required
     by the cassette handler during the OPEN command for play or
     record operations; one beep for play, two for record.

65             41             SOUNDR
     Noisy I/O flag used by SIO to signal the beeping heard during
     disk and cassette I/O. POKE here with zero for blessed silence
     during these operations. Other numbers return the beep. In-
     itialized to three. The hardware solution to this problem is to turn
     your speaker volume down. This can also be used to silence the
     digital track when playing synchronized voice/data tapes. See
     location 54018.

66             42             CRITIC
     Critical I/O region flag; defines the current operation as a time-
     critical section when the value here is non-zero. Checked at the
     NMI process after the stage one VBLANK has been processed.
     POKEing any number other than zero here will disable the repeat
     action of the keys and change the sound of the CTRL-2 buzzer.

     Zero is normal; setting CRITIC to a non-zero value suspends a
     number of OS processes including system software timer coun-
     ting (timers two, three, four and five; see locations 536 to 558;
     $218 to $22E). It is suggested that you do not set CRITIC for any
     length of time. When one timer is being set, CRITIC stops the
     other timers to do so, causing a tiny amount of time to be "lost."
     When CRITIC is zero, both stage one and stage two VBLANK
     procedures will be executed. When non-zero, only the stage one
     VBLANK will be processed.

67-73          43-49          FMZSPG
     Disk file manager system (FMS) page zero registers (seven
     bytes).

67,68          43,44          ZBUFP
     Page zero buffer pointer to the user filename for disk I/O.

69,70          45,46          ZDRVA
     Page zero drive pointer. Copied to here from DBUFAL and
     DBUFAH; 4905 and 4913 ($1329, $1331). Also used in FMS "free
     sector," setup and "get sector" routines.

71,72          47,48          ZSBA
     Zero page sector buffer pointer.

73             49             ERRNO
     Disk I/O error number. Initialized to 159 ($9F) by FMS.