53774



     a one-byte unit before transmission.

                              SERIN
     (R) Serial port input. Reads the one-byte parallel holding register
     that is loaded when a full byte of serial input data has been
     received. As above, this holding register is used to hold the bits as
     they are received one bit at a time until a full byte is received. This
     byte is then taken by the computer for processing. Also used to
     verify the checksum value at location 49 ($31).

     The serial bus is the port on the Atari into which you plug your
     cassette or disk cable. For the pin values of this port, see the OS
     User's Manual, p. 133, and the Hardware Manual.

53774          D20E           IRQEN
     (W) Interrupt request enable. Zero turns off all interrupt requests
     such as the BREAK key; to disable or re-enable interrupts, POKE
     with the values according to the following chart (setting a bit to one
     -- i.e., true -- enables that interrupt; decimal values are also
     shown for each bit):

     Bit Decimal     Interrupt                         Vector
     0       1       Timer 1 (counted down to zero)    VTIMR1
                                                       (528; $210)
     1       2       Timer 2 (counted down to zero)    VTIMR2
                                                       (530; $212)
     2       4       Timer 4 (counted down to zero)    VTIMR4
                                                       (532; $214), OS
                                                       "B" ROMs only)
     3       8       Serial output transmission done   VSEROC (526;
                                                       $20E)
     4      16       Serial output data needed         VSEROR
                                                       (524; $20C)
     5      32       Serial input data ready           VSERIN
                                                       (522; $20A)
     6      64       Other key pressed                 VKEYBD
                                                       (520; $208)
     7     128       BREAK key pressed                 see below

     Here is the procedure for the BREAK key interrupt: clear the
     interrupt register. Set BRKKEY (17; $11) to zero; clear the
     start/stop flag SSFLAG at 767 ($2FF); clear the cursor inhibit flag
     CRSINH at 752 ($2F0); clear the attract mode flag at 77 ($4D), and
     return from the interrupt after restoring the 6502 A register. (There
     is now (in the OS "B" ROMs) a proper vector for BREAK key
     interrupts at 566, 567 ($236, $237) which is initialized to point to
     59220 ($E754).) If the interrupt was due to a serial I/O bus proceed
     line interrupt, then vector through VPRCED at 514 ($202). If due to
     a serial I/O bus interrupt line interrupt, then vector through