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git-svn-id: http://svn.code.sf.net/p/netboot65/code@207 93682198-c243-4bdb-bd91-e943c89aac3b
77 lines
2.2 KiB
ArmAsm
77 lines
2.2 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 C64 implementation requires the routine timer_vbl_handler be called 60 times per second
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; this is for use in the NB65 ROM, where we also install an IRQ handler
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; for normal userland programs where it is known what else is going on with the timer ,
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; use "c64timer.s" routines instead
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.include "../inc/common.i"
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.export timer_init
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.export timer_read
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.export timer_vbl_handler
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.bss
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current_time_value: .res 2
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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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ldax #0
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stax current_time_value
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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, by carry flag can be modified)
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timer_vbl_handler:
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pha
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lda $02A6 ;PAL/NTSC flag (0 = NTSC, 1 = PAL),
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beq @was_ntsc
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lda #$14 ;PAL = 50 HZ =~ 20 ms per 'tick'
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bne :+
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@was_ntsc:
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lda #$11 ;NTSC = 60 HZ =~ 17 ms per 'tick'
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:
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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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:
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pla
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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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