emailler/drivers/atrtimer.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.
.include "atari.inc"
.include "../inc/common.inc"
.export timer_init
.export timer_exit
.export timer_read
.export timer_seconds
.bss
current_time_value: .res 2
current_seconds: .res 1
current_jiffies: .res 1
timer_freq: .res 1 ; VBLANK frequency: 50 - PAL, 60 - NTSC
timer_freq_reciproc:.res 1 ; reciprocal for timer_freq in ms: 20 - PAL, ~17 - NTSC
.data
vbichain: .word 0
.code
; reset timer to 0 and install handler
; inputs: none
; outputs: none
timer_init:
lda vbichain+1
bne @done
lda PAL ; hardware register describing TV system of GTIA
and #$0e ; mask out irrelevant bits
bne @ntsc
; PAL system
lda #50
sta timer_freq
lda #20
sta timer_freq_reciproc
bne @system_set
@ntsc:
; NTSC system
lda #60
sta timer_freq
lda #17
sta timer_freq_reciproc
@system_set:
lda #0 ; initialize time variables
sta current_time_value
sta current_time_value+1
sta current_seconds
sta current_jiffies
ldax VVBLKI ; IMMEDIATE VERTICAL BLANK NMI VECTOR
stax vbichain ; save old immediate vector
ldy #<timer_vbl_handler
ldx #>timer_vbl_handler
lda #6 ; STAGE 1 VBI
jsr SETVBV ; vector to set VBLANK parameters
@done:
rts
timer_exit:
lda vbichain+1
beq @handler_not_installed
ldy vbichain
ldx vbichain+1
lda #6 ; STAGE 1 VBI
jsr SETVBV ; vector to set VBLANK parameters
lda #0
sta vbichain
sta vbichain+1
@handler_not_installed:
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 50 or 60 times per second, depending on the TV system
; inputs: none
; outputs: none (all registers preserved, but carry flag can be modified)
timer_vbl_handler:
pha
lda timer_freq_reciproc ; ms per tick
clc
adc current_time_value
sta current_time_value
bcc :+
inc current_time_value+1
: inc current_jiffies
lda current_jiffies
cmp timer_freq ; full second?
bne @done ; no, we're done
lda #0 ; yes, increment "seconds" counter
sta current_jiffies
sed
clc
lda #$01
adc current_seconds
cld
cmp #$60
bne :+
lda #$00
: sta current_seconds
@done:
pla
jmp SYSVBV ; vector to process immediate VBLANK
timer_seconds:
lda current_seconds
rts
;-- LICENSE FOR atrtimer.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 --