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Interrupt Scheduler

The Interrupt Scheduler handles system timers and counters.

Interrupt Scheduler Routines

Name	Description
ACTINT	Activate automatic interrupt service (timers, music, blackout)
CTIMER	Call custom timer
DECCTS	Decrement counters/timers under mask (used by ACTINT and DECCTS)
STIMER	Track and decrement game timers

ACTIVE INTERRUPTS

Calling Sequence:	SYSTEM ACTINT or SYSSUK ACTINT
Arguments:	None
Outputs:	None
Description:	ACTINT provides automatic interrupt service and runs the seconds timer, the game timer, the music processor, and blackout timers, plus CT0–CT3.
Notes:	Timers function every 1/60^th of a second.

Once ACTINT is called, it sets:

ACTINT also calls TIMEX and TIMEY which decrement counters/timers.

CUSTOM TIMER

Calling Sequence:	CALL CTIMER
Input:	HL = address of custom time base B = value to load into time base 1 to 0 transition C = counter mask as in DECCTS
Description:	HL is loaded and decremented. If HL != 0, then a return is executed, otherwise, HL is loaded with B and DECCTS is called. Registers HL, DE, BC, and AF are undefined upon exit.

DECREMENT COUNTERS/TIMERS

Calling Sequence:	SYSTEM DECCTS or SYSSUK DECCTS DB (mask)
Arguments:	C = mask for which counters to decrement
Outputs:	SENTRY will notify the program
Description:	Decrements counter if they are non-zero. If any counters go from 1 to 0, SENTRY is notified.

DECREMENT TIMERS

Calling Sequence:	PUSH AF PUSH BC PUSH DE PUSH HL CALL STIMER POP HL POP DE POP BC POP AF
Input:	None
Description:	STIMER keeps track of game time. If it hits 0, then GSBEND bit in the game status byte (GAMSTB) is set.
Uses:	AF, BC, DE, HL
Calls:	Music processor on note (duration) expiration
Note:	Sets bit 7 of KEYSEX to 1 on every second