emailler/drivers/vic20timer.s
Oliver Schmidt 8960e0bebb Improved VBL handler installation check.
Check hibyte of old handler in case it happens to be placed on a page boundary (+ minor beautifications).
2015-08-28 14:42:29 +02:00

125 lines
2.6 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.
; this VIC20 implementation requires the routine timer_vbl_handler be called 60 times per second
.include "../inc/common.i"
.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
timer_init:
lda old_handler+1
bne @handler_installed
ldax IRQ_VECTOR
stax old_handler
ldax #timer_vbl_handler
stax IRQ_VECTOR
@handler_installed:
lda #0
sta current_time_value
sta current_time_value+1
sta current_seconds
sta current_jiffies
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 60 times per second
; inputs: none
; outputs: none (all registers preserved, but carry flag can be modified)
timer_vbl_handler:
pha
lda #17 ; 60 HZ =~ 17 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 #60
bne @done
lda #0
sta current_jiffies
inc current_seconds
; we don't want to mess around with decimal mode in an IRQ handler
lda current_seconds
cmp #$0a
bne :+
lda #$10
: cmp #$1a
bne :+
lda #$20
: cmp #$2a
bne :+
lda #$30
: cmp #$3a
bne :+
lda #$40
: cmp #$4a
bne :+
lda #$50
: cmp #$5a
bne :+
lda #$00
: sta current_seconds
@done:
pla
jmp jmp_old_handler
timer_seconds:
lda current_seconds
rts
; -- LICENSE FOR c64timer_nb65.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 --