emailler/drivers/vic20timer.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 VIC20 implementation requires the routine timer_vbl_handler be called 60 times per second
.include "../inc/common.inc"
.export timer_init
.export timer_read
.export timer_seconds
IRQ_VECTOR = $314
.bss
current_time_value: .res 2
current_seconds: .res 1
current_jiffies: .res 1
.data
jmp_old_handler:
.byte $4c ; JMP
old_handler:
.word $00
.code
; reset timer to 0
; inputs: none
; outputs: none
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timer_init:
lda old_handler+1
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bne @handler_installed
ldax IRQ_VECTOR
stax old_handler
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ldax #timer_vbl_handler
stax IRQ_VECTOR
@handler_installed:
lda #0
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sta current_time_value
sta current_time_value+1
sta current_seconds
sta current_jiffies
rts
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; read the current timer value
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; 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
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rts
; tick over the current timer value - should be called 60 times per second
; inputs: none
; outputs: none (all registers preserved, but carry flag can be modified)
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timer_vbl_handler:
pha
lda #17 ; 60 HZ =~ 17 ms per 'tick'
clc
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adc current_time_value
sta current_time_value
bcc :+
inc current_time_value+1
: inc current_jiffies
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lda current_jiffies
cmp #60 ; full second?
bne @done ; no, we're done
lda #0 ; yes, increment "seconds" counter
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sta current_jiffies
sed
clc
lda #$01
adc current_seconds
cld
cmp #$60
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bne :+
lda #$00
: sta current_seconds
@done:
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pla
jmp jmp_old_handler
timer_seconds:
lda current_seconds
rts
; -- LICENSE FOR c64timer_nb65.s --
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; 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/
;
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; 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.
;
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; The Original Code is ip65.
;
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; 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.
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; -- LICENSE END --