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112 lines
2.5 KiB
ArmAsm
112 lines
2.5 KiB
ArmAsm
; timer routines
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;
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; the timer should be a 16-bit counter that's incremented by about
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; 1000 units per second. it doesn't have to be particularly accurate.
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; this VIC20 implementation requires the routine timer_vbl_handler be called 60 times per second
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.include "../inc/common.inc"
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.export timer_init
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.export timer_read
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.export timer_seconds
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IRQ_VECTOR = $314
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.bss
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current_time_value: .res 2
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current_seconds: .res 1
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current_jiffies: .res 1
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.data
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jmp_old_handler:
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.byte $4c ; JMP
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old_handler:
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.word $00
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.code
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; reset timer to 0
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; inputs: none
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; outputs: none
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timer_init:
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lda old_handler+1
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bne @handler_installed
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ldax IRQ_VECTOR
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stax old_handler
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ldax #timer_vbl_handler
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stax IRQ_VECTOR
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@handler_installed:
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lda #0
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sta current_time_value
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sta current_time_value+1
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sta current_seconds
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sta current_jiffies
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rts
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; read the current timer value
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; inputs: none
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; outputs: AX = current timer value (roughly equal to number of milliseconds since the last call to 'timer_init')
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timer_read:
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ldax current_time_value
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rts
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; tick over the current timer value - should be called 60 times per second
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; inputs: none
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; outputs: none (all registers preserved, but carry flag can be modified)
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timer_vbl_handler:
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pha
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lda #17 ; 60 HZ =~ 17 ms per 'tick'
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clc
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adc current_time_value
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sta current_time_value
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bcc :+
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inc current_time_value+1
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: inc current_jiffies
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lda current_jiffies
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cmp #60 ; full second?
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bne @done ; no, we're done
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lda #0 ; yes, increment "seconds" counter
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sta current_jiffies
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sed
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clc
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lda #$01
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adc current_seconds
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cld
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cmp #$60
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bne :+
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lda #$00
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: sta current_seconds
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@done:
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pla
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jmp jmp_old_handler
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timer_seconds:
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lda current_seconds
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rts
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; -- LICENSE FOR c64timer_nb65.s --
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; The contents of this file are subject to the Mozilla Public License
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; Version 1.1 (the "License"); you may not use this file except in
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; compliance with the License. You may obtain a copy of the License at
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; http://www.mozilla.org/MPL/
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;
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; Software distributed under the License is distributed on an "AS IS"
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; basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
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; License for the specific language governing rights and limitations
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; under the License.
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;
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; The Original Code is ip65.
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;
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; The Initial Developer of the Original Code is Jonno Downes,
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; jonno@jamtronix.com.
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; Portions created by the Initial Developer are Copyright (C) 2009
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; Jonno Downes. All Rights Reserved.
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; -- LICENSE END --
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