Subject: Re: [stella] PIA timer questions From: emooney@xxxxxxxxxxxxxxxx (Erik Mooney) Date: Wed, 18 Aug 1999 23:30:03 GMT |
>According to the Stella manual, it says that the hex address determines >how many clock counts it takes for things to count down. But Bensema's >playfield document uses TIM64T and sets it for 44. If the number >automatically decrements every machine cycle, why not write 37 to >address $294 (TIM1T) to count down the number of scanlines? Or if I'm >thinking of the wrong cycles it needs to synch up with, write 111 >(37*3) to address $294? What am I missing? Or is Bensema not using >the timer for counting the scanlines? What you're missing is that the timer decrements every *64* machine cycles, when you start it by writing to TIM64T. 64 cycles * 44 = 2816 cycles, which is very close to the number of cycles during vertical blank, 76 * 37 = 2812. You start the timer at the beginning of vertical blank, then it expires 37 scanlines later, so you know when to start drawing the screen. >The timer is read at $284. How does it know which timer of the four >addresses to read? There's only one timer; how fast it counts depends on which register you write to. >The way the computer works, wouldn't the numbers in >all four addresses start cycling down from whatever is in them at that >random moment when the Atari starts up? The number is only read at one address, but yeah, it would start cycling down right at powerup. >"It holds 0 for one interval, then flips to $FF and decrements once >each clock cycle...to allow the programmer to determine how long ago >the timer zeroed out...." What happens if it hits zero again? It >starts at $FF again or it freezes at 0? It would keep going at $FF again. It wouldn't have any memory that this is the second or later time it had hit zero. -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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