Every once in a while there comes someone to ask if I could give just a basic example of how to handle line interrupts.
So... Here you can find BASIC line interrupt handler with documentation:
10 ' This example program shows how to use line interrupts 20 ' Made By : NYYRIKKI 2008 30 ' 40 DEFINT A-Z 50 ' This program requires X-BASIC 60 _TURBO ON 70 GOTO 90 ' Little jump to avoid RENUM problem 80 GOTO 410 ' This line will be copied to interrupt hook 90 VDP(9)=10 ' In this example we don't need sprites, so we disable them. 100 ' Let's draw something to screen to see the line interrupt effect 110 SCREEN 8:FORY=0TO255:FORX=0TO255:VPOKEX+Y*256,XXORY:NEXTX,Y 120 ' 130 ' Let's disable interrups until we are ready (DI) 140 '#I 243 150 ' We will use general interrupt hook in address &hFD9A (-614) 160 ' This address is called every time when interrupt occurs. 170 ' To use it, we have to copy line 80 to the hook. 180 ' First we need to know in what memory address line 80 is: 190 ' AD = LINE 80 (NOTE: RENUM does not work in this case!!!) 200 '#I 33,@80,34,AD 210 ' ... and then just copy... 220 FOR I=0 TO 4:POKE -614+I,PEEK(AD+I):NEXT I 230 ' We want to have line interrupts, so let's enable them. 240 VDP(0)=VDP(0) OR 16 250 ' Let's set the interrupt to happen on line 100 260 VDP(20)=100 270 ' Now we are ready and we can enable interrupts (EI) 280 '#I 251 290 ' Do what ever you want to do here in main program 300 ' In this example we make some noise... 310 SOUND 8,15 320 SOUND 1,RND(1)*8:SOUND 0,RND(1)*255 330 IF INKEY$="" THEN 320 340 ' Before we can exit the program we have to disable line interrupts 350 VDP(0)=VDP(0) AND 239 360 ' ... and release the interrupt hook (put RETurn to it) 370 POKE -614,201 380 ' Now it is safe to exit 390 BEEP:END 400 ' 410 ' This is interrupt routine 420 ' Here we make sure, that the example interrupt handler does not end up 430 ' to infinite loop in case of nested interrupts 440 IF IN=0 THEN IN=1:GOSUB 470:IN=0:T=0 ELSE T=T+1:IF T=100 THEN T=0:IN=0 450 RETURN 460 ' 470 ' Example interrupt handler: 480 IF (VDP(-1)AND1)=1 THEN 530 ' Is this line interrupt? 490 ' This was not line interrupt, so it's propably VBLANK 500 ' VBLANK happens when screen has been drawn. 510 VDP(24)=0 ' Upper part of screen shows still picture 520 RETURN 530 ' Here we handle line interrupt 540 ' Lower part of screen jumps 550 VDP(24)=P:P=ABS(SIN(R/20)*100):R=R+1 560 RETURN
