emailler/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,
; if you're working with e.g. a 60 Hz VBLANK IRQ, adding 17 to the
; counter every frame would be just fine.
.include "../inc/common.i"
.export timer_init
.export timer_read
.code
; initialize timers
timer_init:
lda #$80 ; stop timers
sta $dd0e
sta $dd0f
ldax #999 ; timer A to 1000 cycles
stax $dd04
ldax #$ffff ; timer B to max cycles
stax $dd06
lda #$81 ; timer A in continuous mode
sta $dd0e
lda #$c1 ; timer B to count timer A underflows
sta $dd0f
rts
; return the current value
timer_read:
lda $dd07 ; cia counts backwards, return inverted value
eor #$ff
tax
lda $dd06
eor #$ff
rts