emailler/drivers/c64timer.s
2018-02-23 16:36:05 +01:00

71 lines
1.7 KiB
ArmAsm

; 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.inc"
.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 --