Sparse Invaders Soruce Timer.txt - simondotm/stardot-wiki GitHub Wiki
The following Timer functionality handles the vertical blank triggering, it triggers at the bottom of visiable screen setting a flag once reached. This code is quite well documented, however please refer to the advanced BBC B manual as to the workings of the timers.
;-------------------------------------------------------------------------------
; Timer Interrupt
; Written by Neil Beresford.
;
; Copyright 2008,2009 Neil Beresford
;
; This file is part of Sparse Invaders.
; Sparse Invaders is free software: you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 3 of the License, or
; (at your option) any later version.
;
; Sparse Invaders is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
; GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with Sparse Invaders. If not, see <http://www.gnu.org/licenses/>.
;
;
; Notes:
;
; The timer times out at $4730, this corrasponds to the bottom of the screen
; give or take a scan line. Once timed, this inc TIMER_FLAG.
; On the VBlank interrupt, this activates the timer again.
;
; Functionality...
;
; External - Setup
; Timer_Init - starts the interrupt off, activating the timer
;
; Internal -
; Timer_IRQCheck - checks to see if VB or timer2 then acts on it...
; TI_VSync - restarts user timer2
; TI_Timer - inc timer flag
;
;-------------------------------------------------------------------------------
.require "Constands"
.require "Macros"
;-------------------------------------------------------------------------------
;-------------------------------------------------------------------------------
; Timer_Init -
;
; Activates the interrupt handler that on the next VBlank - will
; start the timer, so allowing monitoring when the scan line reaches the
; bottom edge of the screen
;
;-------------------------------------------------------------------------------
Timer_Init:
; reset the timer flag
LDA #0
STA TIMER_FLAG
; Stop interrupts as we change the interrupt handler ...
SEI
; store the old interrupt handler
LDA $204
STA OldInt
LDA $205
STA OldInt+1
; patch in our interrupt handler
LDA #<Timer_IRQCheck
STA $204
LDA #>`Timer_IRQCheck`
STA $205
; sets user timer 2 mode to countdown
LDA #0
STA $FE6B
CLI
RTS
;-------------------------------------------------------------------------------
OldInt: ; Store for old handler ...
.byte 0,0
;-------------------------------------------------------------------------------
;-------------------------------------------------------------------------------
; Timer_IRQCheck -
;
; Chceks for timer2 and VBlank.. Timer2 inc timer_flag, VBlank re-activates
; timer2
;
;-------------------------------------------------------------------------------
Timer_IRQCheck:
; check the interrupt to see if we need to act on it...
LDA $FE4D ; check and jump to VBlank...
AND #2
BNE TI_VSync
LDA $FE6D ; check and jump to timer 2...
AND #$20
BNE TI_Timer
; jmp to the old interrupt handler...
jmp (OldInt) ; let system handle others
TI_VSync:
; reset the timer to trigger at bottom of screen...
LDA #$30
STA $FE68
LDA #$47
STA $FE69
; enable user timer 2 interrupts
LDA #$A0
STA $FE6E
; jmp to the old interrupt handler...
jmp (OldInt)
TI_Timer:
; handle the interrupt
STA $FE6D
STA $FE6E
; check standard delay and dec if needed...
LDA DELAY_COUNT
BEQ TI_Timercont
DEC DELAY_COUNT
TI_Timercont:
; and set the timer flag ...
INC TIMER_FLAG
; update the counters for various game updates
LDA SCREEN_UPDATE
BEQ TIT_skipSU
DEC SCREEN_UPDATE
TIT_skipSU:
; jmp to the old interrupt handler...
jmp (OldInt)
;-------------------------------------------------------------------------------
; End of Timer
;-------------------------------------------------------------------------------