emailler/drivers/clk_timer.s

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; timer routines
; this implementation is for Atari and C64.
; it uses parts of the C runtime library
;
; 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 "../inc/common.inc"
.export timer_init
.export timer_read
.export timer_seconds
.import __clocks_per_sec, _clock
.import tosumodeax, tosudiveax
.import pusheax, incsp4
.importzp sp, sreg
.bss
timer_freq: .res 1 ; VBLANK frequency: 50 - PAL, 60 - NTSC (Atari); always 60 (C64)
mult_temp: .res 2 ; temp var for multiplication routines
mult_temp_x:.res 1 ; another temp var for multiplication routines
.data
mult: .byte $4C ; JMP opcode
.res 2 ; address
.code
; get timer frequency and patch multiplication routine pointer
; inputs: none
; outputs: none
timer_init:
jsr __clocks_per_sec
sta timer_freq
cmp #50
beq @timer50
lda #<mult17
ldx #>mult17
bne @timerset ; jmp always
@timer50:
lda #<mult20
ldx #>mult20
@timerset:
sta mult+1
stx mult+2
rts
; read the current timer value
; inputs: none
; outputs: AX = current timer value in milliseconds
timer_read = mult
; get current seconds clock hand
; inputs: none
; outputs: A = seconds hand (in BCD, range $00..$59)
timer_seconds:
jsr _clock ; return current tick count in sreg:AX (high:low 16bits)
jsr pusheax ; push tick count onto stack
ldx #0
stx sreg
stx sreg+1
lda timer_freq
jsr tosudiveax ; dividend on stack, divisor in sreg:AX
jsr pusheax
ldx #0
stx sreg
stx sreg+1
lda #60
jsr tosumodeax ; result modulo 60
; convert to BCD, a poor man's conversion here....
cmp #50
bcs @rs_50
cmp #40
bcs @rs_40
cmp #30
bcs @rs_30
cmp #20
bcs @rs_20
cmp #10
bcs @rs_10
rts
@rs_10:
sbc #10
ora #$10
rts
@rs_20:
sbc #20
ora #$20
rts
@rs_30:
sbc #30
ora #$30
rts
@rs_40:
sbc #40
ora #$40
rts
@rs_50:
sbc #50
ora #$50
rts
; get the current tick count, multiply it by 20, and return the lower 16 bits
; x*20 = x*16 + x*4
; inputs: none
; outputs: AX - tick count times 20 ('milliseconds')
mult20:
jsr _clock ; return current tick count in sreg:AX (high:low 16bits)
stx mult_temp_x ; remember high byte of lower 16bits
asl a
rol mult_temp_x
asl a
rol mult_temp_x
sta mult_temp
ldx mult_temp_x
stx mult_temp+1 ; mult_temp = ticks * 4
asl a
rol mult_temp_x
asl a
rol mult_temp_x ; mult_temp_x:A = 'ticks * 16'
clc ; AX - tick count * 16, mult_temp - tick count * 4
adc mult_temp
sta mult_temp
lda mult_temp+1
adc mult_temp_x
tax
lda mult_temp
rts
; get the current tick count, multiply it by 17, and return the lower 16 bits
; x*17 = x*16 + x
; inputs: none
; outputs: AX - tick count times 17 ('milliseconds')
mult17:
jsr _clock ; return current tick count in sreg:AX (high:low 16bits)
sta mult_temp
stx mult_temp+1
stx mult_temp_x
.repeat 4
asl a
rol mult_temp_x
.endrepeat ; mult_temp_x:A = 'ticks * 16'
clc
adc mult_temp
sta mult_temp
lda mult_temp+1
adc mult_temp_x
tax
lda mult_temp
rts
;-- LICENSE FOR clk_timer.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.
; -- LICENSE END --