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APU x $4015 reads?
http://forums.nesdev.com/viewtopic.php?f=3&t=14553
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Author:  Zepper [ Mon Jul 18, 2016 7:18 am ]
Post subject:  APU x $4015 reads?

Let's take square wave 1 active-time-left (atl) set at $4003. It's decremented twice per frame.
On a lda $4015 and the atl goes 1->0 right before the effective $4015 read, does $4015 return 1 or 0?

Author:  Disch [ Wed Jul 20, 2016 7:36 pm ]
Post subject:  Re: APU x $4015 reads?

I'm not aware of any delay. So if the decrement happens before the read, you would read 0.

Author:  Zepper [ Thu Jul 21, 2016 6:49 pm ]
Post subject:  Re: APU x $4015 reads?

Disch wrote:
I'm not aware of any delay. So if the decrement happens before the read, you would read 0.

It bugs my mind. Actually, the order for running the units is PPU->APU->CPU. If I change it to CPU->PPU->APU, on $4015 reads, the atl=1, not zero as I'm doing. Got it?

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