emailler/client/drivers/c64timer.s

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; timer routines
;
; the timer should be a 16-bit counter that's incremented by about
; 1000 units per second. it doesn't have to be particularly accurate.
; this C64 implementation requires the routine timer_vbl_handler be called 60 times per second
.include "../inc/common.i"
.export timer_init
.export timer_read
.export timer_vbl_handler
.code
.bss
current_time_value: .res 2
.code
;reset timer to 0
;inputs: none
;outputs: none
timer_init:
ldax #0
stax current_time_value
rts
;read the current timer value
; inputs: none
; outputs: AX = current timer value (roughly equal to number of milliseconds since the last call to 'timer_init')
timer_read:
ldax current_time_value
rts
; tick over the current timer value - should be called 60 times per second
; inputs: none
; outputs: none (all registers preserved, by carry flag can be modified)
timer_vbl_handler:
pha
lda #$11
adc current_time_value
sta current_time_value
bcc :+
inc current_time_value+1
:
pla
rts