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Adapt Atari timer code to PAL systems.

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The vertical blank interrupt (which is used by the timer) runs with
60Hz on NTSC systems and 50Hz on PAL systems.
This commit also clears 'vbichain' in 'timer_exit', so a subsequent
'timer_init' would do the right thing.
This commit is contained in:
Christian Groessler 2017-09-12 14:14:14 +02:00
parent e87fbb6111
commit 2685d01fe5
1 changed files with 23 additions and 4 deletions
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27
drivers/atrtimer.s
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@ -2,7 +2,6 @@
;
; 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 Atari implementation requires the routine timer_vbl_handler be called 60 times per second
.include "atari.inc"
.include "../inc/common.i"
@ -18,7 +17,8 @@
current_time_value: .res 2
current_seconds: .res 1
current_jiffies: .res 1
timer_freq: .res 1 ; VBLANK frequency: 50 - PAL, 60 - NTSC
timer_freq_reciproc:.res 1 ; reciprocal for timer_freq in ms: 20 - PAL, ~17 - NTSC
.data
@ -40,6 +40,22 @@ timer_init:
lda #6 ; STAGE 1 VBI
jsr SETVBV ; vector to set VBLANK parameters
@handler_installed:
lda PAL ; hardware register describing TV system of GTIA
and #$0e ; mask out irrelevant bits
bne @ntsc
; PAL system
lda #50
sta timer_freq
lda #20
sta timer_freq_reciproc
bne @system_set
@ntsc:
; NTSC system
lda #60
sta timer_freq
lda #17
sta timer_freq_reciproc
@system_set:
lda #0
sta current_time_value
sta current_time_value+1
@ -54,6 +70,9 @@ timer_exit:
ldx vbichain+1
lda #6 ; STAGE 1 VBI
jsr SETVBV ; vector to set VBLANK parameters
lda #0
sta vbichain
sta vbichain+1
@handler_not_installed:
rts
@ -69,7 +88,7 @@ timer_read:
; outputs: none (all registers preserved, but carry flag can be modified)
timer_vbl_handler:
pha
lda #17 ; 60 HZ =~ 17 ms per 'tick'
lda timer_freq_reciproc ; ms per tick
clc
adc current_time_value
sta current_time_value
@ -77,7 +96,7 @@ timer_vbl_handler:
inc current_time_value+1
: inc current_jiffies
lda current_jiffies
cmp #60
cmp timer_freq ; full second?
bne @done
lda #0
sta current_jiffies