APPENDIX TWELVE



49152-52223    C000-CBFF      Interrupt handlers
     Os ROM. In the 400/800, the block between 49152 and 53247
     was unused; now the area holds many of the interrupt handlers
     (vectored here from page two). Some 400/800 software checks
     for certain values in these locations and won't run if the value is
     not found. Use the Translator disk in that case (with the 400/800
     OS installed; the area between $C000 and $CEFF becomes user-
     accessible RAM). The area between 52069 ($CB65) and 52223
     ($CBFF) is empty (all zeros).

     A lot of interrupts are set to jump to 49357 or 49358 ($C0CD or
     $C0CE). The former contains a PLA statement followed by an
     RTI. The net result is a simple return back into the program
     without any other activity taking place.

     Bytes 49152-49163 ($C000-$C00B) are used to identify the com-
     puter and the ROM in the $C000-$DFFF block:

     Byte              Use
     49152-3/C000-1    Checksum (LSB/MSB) of all the bytes
                       in ROM except the checksum bytes
                       themselves.
     49154/C002        Revision date, stored in the form
                       DDMMYY. This is DD, day, usually $10.
     49155/C003        Revision date, month; usually $05.
     49156/C004        Revision date, year; usually $83.
     49157/C005        Reserved option byte; reads zero for
                       the 1200, 800XL, and 130XE.
     49158/C006        Part number in the form AANNNNNN;
                       AA is an ASCII character and
                       NNNNNN is a four-bit BCD digit. This is
                       byte A1.
     49159-62/C007-A   Part number, bytes A2, N1-N6 (each
                       byte has two N values of four bits
                       each).
     49163/C00B        Revision number. My 800XL and
                       130XE say 2.
     49164/C00C        Interrupt handler initialization
     49176/C018        NMI intitialization

     Interrupt handlers and other routines in the $C000 block:

     Entry        Handler or Use
     49196/C02C   IRQ processor
     49298/C092   BREAK key IRQ
     49312/C0A0   Continue IRQ processing
     49359/C0CF   Table of IRQ types and offsets (16 bytes)
     49378/C0E2   Immediate VBLANK NMI processing
     49743/C24F   Process countdown timer 1 expiration