iigs-game-engine/src/Timer.s
2021-10-07 16:38:20 -05:00

216 lines
7.0 KiB
ArmAsm

; Timer implementation
;
; The engire provides four timer slot that can be used by one-shot or
; recurring timers. Each timer is given an initial tick count, a
; reset tick count (0 = one-shot), and an action to perform.
;
; The timers handle overflow, so if a recurring timer has a tick count of 3
; and 7 VBL ticks have passed, then the timer will be fired twice and
; a tick count of 2 will be set.
;
; As such, the timers are appropriate to drive physical and other game
; behaviors at a frame-independent rate.
;
; A collection of 4 timers that are triggered when their countdown
; goes below zero. Each timer takes up 16 bytes
;
; A timer can fire multiple times during a singular evaluation. For example, if the
; timer delay is set to 1 and 3 VBL ticks happen, then the timer delta is -2, will fire,
; have the delay added and get -1, fire again, increment to zero, first again and then
; finally reset to 1.
;
; +0 counter decremented by the number of ticks since last run
; +2 reset copied into counter when triggered. 0 turns off the timer.
; +4 addr long address of timer routine
; +8 user 8 bytes of user data space for timer state
MAX_TIMERS equ 4
TIMER_REC_SIZE equ 16
lastTick ds 2
Timers ds TIMER_REC_SIZE*MAX_TIMERS
GetVBLTicks ENT
jsr _GetVBLTicks
rtl
_GetVBLTicks
PushLong #0
_GetTick
pla
plx
rts
; Initialize the timers
InitTimers
jsr _GetVBLTicks
sta lastTick
lda #0
ldx #{TIMER_REC_SIZE*MAX_TIMERS}-2
:loop sta Timers,x
dex
dex
bne :loop
rts
; Add a new timer
;
; Input
; A = low word of timer callback function
; X = high word of timer callback function
; Y = number of ticks for the timer to delay
; C = set to 1 if the timer is a one-shot timer
;
; Return
; C = 0 if success, 1 if no timer slots are available
; A = timer slot ID if C = 0
AddTimer ENT
phb
php ; Save the input parameters
phx
pha
phy
phk
plb
ldx #0
:loop lda Timers,x ; If the counter is zero, timer is free
beq :freeslot
txa ; Advance to the next timer record
clc
adc #TIMER_REC_SIZE
tax
cpx #{TIMER_REC_SIZE*MAX_TIMERS}
bcc :loop
bra :notimers
:freeslot pla
sta Timers+0,x ; set the counter and
stz Timers+2,x ; default to a zero reset value
pla
sta Timers+4,x ; set the callback address
pla
sta Timers+6,x
stz Timers+8,x ; Clear the user data space
stz Timers+10,x ; Clear the user data space
stz Timers+12,x ; Clear the user data space
stz Timers+14,x ; Clear the user data space
plp
bcs :oneshot
lda Timers+0,x ; if not a one-shot, put the counter
sta Timers+2,x ; value into the reset field
:oneshot plb
txa ; return the slot ID and a success status
clc
rtl
:notimers ply
pla
plx
plp
plb
sec ; Return an error status
lda #0
rtl
; Small function to remove a timer
;
; A = Timer ID
RemoveTimer ENT
phb
phk
plb
cmp #{TIMER_REC_SIZE*{MAX_TIMERS-1}}+1
bcs :exit
tax
stz Timers,x
stz Timers+2,x
stz Timers+4,x
stz Timers+6,x
:exit
plb
rtl
; Execute the timer functions
DoTimers ENT
phb
phk
plb
jsr _GetVBLTicks
cmp lastTick ; Throttle to 60 fps
beq :exit
tax ; Calculate the increment
sec
sbc lastTick
stx lastTick
; We don't want times to fire excessively. If the timer has nt been evaluated for over
; one second, then just skip processing and wait for the next call.
cmp #60
bcs :exit
jsr _DoTimers
:exit plb
rtl
; Countdown the timers
;
; A = number of elapsed ticks
_DoTimers
pha
ldx #0
:loop
lda Timers,x ; A zero means do not fire
beq :skip
sec
sbc 1,s ; subtract the number of ticks
sta Timers,x
:retry beq :fire ; getting <= zero triggers
bpl :skip
:fire pha ; Save the counter
phx ; Save our index
lda Timers+4,x ; execute the timer callback
sta :dispatch+1
lda Timers+5,x
sta :dispatch+2
:dispatch jsl $000000
plx
pla
lda Timers+2,x ; Load the reset value, if it's zero
beq :oneshot ; then this was a one-shot timer
clc
adc Timers,x ; Add to the current count and store
sta Timers,x
bra :retry ; See if we have >0 ticks to wait until the next trigger
:oneshot stz Timers,x
:skip txa
clc
adc #TIMER_REC_SIZE
tax
cpx #{TIMER_REC_SIZE*MAX_TIMERS}
bcc :loop
pla
rts