APPENDIX TWELVE



1001           3E9            CKEY
     Moved from 74 ($4A) in the 400/800.

1002           3EA            CASSBT
     Moved from 75 ($4B) in the 400/800.

1003           3EB            CARTCK
     Cartridge checksum. A checksum of page one of the cartridge.
     The checksum is recalculated each VBLANK and checked
     against this register. If not the same, the OS assumes the car-
     tridge isn't there any more (was pulled out) and does a cold
     start; 1200XL only.

1004           3EC            DERRF
     Screen open error flag; if zero, then no error, if nonzero, then OS
     can't initialize the screen editor.

1005-1015      3ED-3F7        ACMVAR
     Reserved for OS variables; on power-up or cold start, all vari-
     ables between 1005 and 1023 ($3ED-$3FF), inclusive, are set to
     zero, but are left unchanged on warm start.

1016           3F8            BASICF
     Shadow of current status of BASIC. Zero means ROM BASIC is
     enabled; nonzero means it's not. Must be in sync with disabling
     of ROM BASIC. To disable BASIC, set BASICF to nonzero, then do
     a warm start (press RESET); DOS will load and tell you there is
     no cartridge present when you try to return to BASIC.

1017           3F9            MINTLK
     Unused.

1018           3FA            GINTLK
     Cartridge interlock register; the complement of BASICF, above.
     It reads 1 when an external cartridge is installed, 0 when not
     (or ROM BASIC is in use). The value of TRIG3 (53267; $D103) is
     loaded here by the OS initialization routine. If at any time, the
     external cartridge is pulled, the system will crash.

1019,1020      3FB,3FC        CHLINK
     Relocatable handler chain use; allows chaining of portions of
     handler routines.

1792-7419      700-1CFB       ....
     Used by DOS when loaded; otherwise available as free RAM.