>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.
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