771



     All of the above are disk device commands, except write and
     status, which are also printer commands (with no verify).

771            303            DSTATS
     The status code upon return to user. Also used to set the data
     direction; whether the device is to send or receive a data frame.
     This byte is used by the device handler to indicate to SIO what to
     do after the command frame is sent and acknowledged. Prior to
     the SIO call, the handler examines BIT 6 (one equals receive
     data) and BIT 7 (one equals send data). If both bits are zero, then
     no data transfer is associated with the operation. Both bits set to
     one is invalid. SIO uses it to indicate to the handler the status of
     the requested operation after the SIO call.

772,773        304,305        DBUFLO/HI
     Data buffer address of the source or destination of the data to be
     transferred or the device status information (or the disk sector
     data). Set by the user, it need not be set if there is no data
     transferred, as in a status request.

774            306            DTIMLO
     The time-out value for the handler in one-second units, supplied
     by the handler for use by SIO. The cassette time-out value is 35,
     just over 37 seconds. The timer values are 64 seconds per 60 units
     of measurement. Initialized to 31.

775            307            DUNUSE
     Unused byte.

776,777        308,309        DBYTLO/HI
     The number of bytes transferred to or from the data buffer (or the
     disk) as a result of the most recent operation, set by the handler.
     Also used for the count of bad sector data. There is a small bug in
     SIO which causes incorrect system actions when the last byte in a
     buffer is in a memory location ending with $FF, such as $A0FF.

778,779        30A,30B        DAUX1/2
     Used for device specific information such as the disk sector
     number for the read or write operation. Loaded down to locations
     572, 573 ($23C, $23D) by SIO.

     There are only five commands supported by the disk handler:
     GET sector (82; $52), PUT sector (80; $50), PUT sector with
     VERIFY (87; $57), STATUS request (83; $53) and FORMAT entire
     disk (33; $21). There is no command to FORMAT a portion of the
     disk; this is done by the INS 1771-1 formatter/controller chip in
     the drive itself and isn't user-accessible. There is a new disk drive
     ROM to replace the current "C" version. It is the "E" ROM. Not
     only is it faster than the older ROMs, but it also allows for
     selective formatting of disk sectors. Atari has not announced yet