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Interrupt Scheduler

The Interrupt Scheduler handles systems dependent upon timing, i.e., counters, timers, music, and interrupts.

Interrupt Scheduler Routines

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

Interrupt Scheduler Routine Descriptions

ACTINT

ACTIVE INTERRUPTS

Calling Sequence: 
SYSTEM    ACTINT

or 

SYSSUK    ACTINT
Arguments: None
Description: ACTINT provides automatic interrupt service and runs the seconds timer, the game timer, the music processor, and blackout timers, plus CT0–CT3. Functions as 60th of a second timers.

Once ACTINT is called, it sets:

  • Z80 Interrupt mode 2 (IM 2)
  • INLIN = 200 (line 100)
  • I register and Interrupt Feedback (INFBK)
  • Enable interrupts (EI)

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

CTIMER

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.

BMUSIC

BEGIN PLAYING MUSIC

Calling Sequence: 
SYSTEM    BMUSIC

or 

SYSSUK    BMUSIC
DW        (music stack)
DB        (voices byte)
DW        (score)
Arguments: A = voices to start with
HL = music program counter (score)
IX = music stack pointer
Description: Quiets any previous music, then interprets specified “score.”

DECCTS

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.

EMUSIC

STOP MUSIC

Calling Sequence: 
SYSTEM    EMUSIC

or 

SYSSUK    EMUSIC
Arguments: None
Description: Outputs 0 to volume ports and halt music processor.

STIMER

DECREMENT TIMERS

Calling Sequence: 
PUSH      AF
PUSH      BC
PUSH      DE
PUSH      HL
PUSH      IX
PUSH      IY
CALL      STIMER
POP       IY
POP       IX
POP       HL
POP       DE
POP       BC
POP       AF
Input: None
Description: STIMER keeps track of game time. If it hits 0, then bit 7 (GSBEND, i.e., game end) 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 (KEYS-EX tracking byte) to 1 on every second