emailler/drivers/c64timer.s

; 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.

.include "../inc/common.i"

.export timer_init
.export timer_read
.export timer_seconds           ; this should return a single BCD byte (00..59) which is a count of seconds


.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

  lda #0
  sta $dc08
  sta $dc09
  rts

timer_seconds:
  lda $dc09
  rts

; return the current value
timer_read:
  lda $dd07                     ; cia counts backwards, return inverted value
  eor #$ff
  tax
  lda $dd06
  eor #$ff
  rts



; -- LICENSE FOR c64timer.s --
; The contents of this file are subject to the Mozilla Public License
; Version 1.1 (the "License"); you may not use this file except in
; compliance with the License. You may obtain a copy of the License at
; http://www.mozilla.org/MPL/
;
; Software distributed under the License is distributed on an "AS IS"
; basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
; License for the specific language governing rights and limitations
; under the License.
;
; The Original Code is ip65.
;
; The Initial Developer of the Original Code is Jonno Downes,
; jonno@jamtronix.com.
; Portions created by the Initial Developer are Copyright (C) 2009
; Jonno Downes. All Rights Reserved.
; -- LICENSE END --