792



     decremented during the I/O operation. If it reaches zero (after
     two seconds) before the first byte of the cassette record is read,
     the operation will be aborted.

792            318            STACKP
     SIO stack pointer register. Points to a byte in the stack being
     used in the current operation (locations 256 to 511; $100 to $1FF).

793            319            TSTAT
     Temporary status holder for location 48 ($30).

794-831        31A-33F        HATABS
     Handler Address Table. Thirty-eight bytes are reserved for up to
     12 entries of three bytes per handler, the last two bytes being set
     to zero. On powerup, the HATABS table is copied from ROM.
     Devices to be booted, such as the disk drive, add their handler
     information to the end of the table. Each entry has the character
     device name (C,D,E,K,P,S,R) in ATASCII code and the handler
     address (LSB/MSB). Unused bytes are all set to zero. FMS
     searches HATABS from the top for a device "D:" entry, and when
     it doesn't find it, it then sets the device vector at the end of the
     table to point to the FMS vector at 1995 ($7CB). CIO searches for
     a handler character from the bottom up. This allows new handlers
     to take precedence over the old. Pressing RESET clears HATABS
     of all but the resident handler entries!

     794   31A   Printer device ID (P:), initialized to 58416 ($E430).
     797   31D   Cassette device ID (C:), initialized to 58432 ($E440).
     800   320   Display editor ID (E:), initialized to 58368 ($E400).
     803   323   Screen handler ID (S:), initialized to 58384 ($E410).
     806   326   Keyboard handler ID (K:), initialized to 58400
                 ($E420).

     HATABS unused entry points:
     809 ($329), 812 ($32C), 815 ($32F), 818 ($332), 821 ($335), 824
     ($338), 827 ($33B), and 830 ($33E). These are numbered
     sequentially from one to eight. There are only two bytes in the last
     entry (unused), both of which are set to zero. When DOS is
     present, it adds an entry to the table with the ATASCII code for
     the letter "D" and a vector to address 1995 ($7CB).

     The format for the HATABS table is:

     Device name
     Handler vector table address
     More entries
     Zero fill to the end of the table

     The device handler address table entry above for the specific
     handler points to the first byte (low byte/high byte) of the vector