Sprite evaluation timing

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zzzz898
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Sprite evaluation timing

Post by zzzz898 » Tue Sep 11, 2018 9:03 am

I'm trying to figure out a good way of programming the PPU sprite evaluation state machine, but the nesdev page is a little unclear to me.
n even cycles, data is written to secondary OAM (unless secondary OAM is full, in which case it will read the value in secondary OAM instead)
1. Starting at n = 0, read a sprite's Y-coordinate (OAM[n][0], copying it to the next open slot in secondary OAM (unless 8 sprites have been found, in which case the write is ignored).

1a. If Y-coordinate is in range, copy remaining bytes of sprite data (OAM[n][1] thru OAM[n][3]) into secondary OAM.

2. Increment n

2a. If n has overflowed back to zero (all 64 sprites evaluated), go to 4
2b. If less than 8 sprites have been found, go to 1
2c. If exactly 8 sprites have been found, disable writes to secondary OAM because it is full. This causes sprites in back to drop out.
Does 1 and 2 happen on the same cycle ?

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Zepper
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Re: Sprite evaluation timing

Post by Zepper » Tue Sep 11, 2018 7:36 pm

I'm still rewriting my sprite evaluation logic, but I have a few notes. Hope it helps.

1. The sprite evaluation starts at PPU cycle 65. On odd cycles, data is read from primary OAM (sprite ram). On even cycles, the buffered data is written to the secondary OAM (32 bytes long).
2. Steps 1 and 2 refer to the normal evaluation. Step 3 is the weird evaluation, and step 4 is when less than 8 sprites are found in the current scanline.
3. 'n' scans through the primary OAM. If 'n'=64, then 'n' warps to 0. Go to step 4. It may be less than 8 sprites in the OAM secondary buffer though. No weird evaluation happens here.
4. If the secondary OAM is full, the weird evaluation starts! If 'n'=64, then 'n' warps to 0. Step 4 now takes place.
zzzz898 wrote:Does 1 and 2 happen on the same cycle ?
Nope. Data must be read, then written to the secondary OAM. Next, it's checked if the buffer's full (8 sprites found).

zzzz898
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Re: Sprite evaluation timing

Post by zzzz898 » Thu Oct 04, 2018 12:09 am

Well, now I'm confused. The way I had it setup didn't work (step 1 and step 2 happen on different cycles):
1. fetch data from OAM
2. step 1 - sprite not in range
3. fetch data from OAM
3. step 2 - go back to step 1

There is no way you can scan through 64 different sprites, if 1 of them takes 4 cycles, right ?

The way I have it now works better, it looks like this:
1. fetch data from OAM
2. step 1 - sprite not in range, decide to go to step 1 (step 1 and 2 happen on the same cycle, this is what I meant <<)
3. fetch data from OAM
4. and so on...

tepples
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Re: Sprite evaluation timing

Post by tepples » Thu Oct 04, 2018 5:49 am

1-2. read Y, calculate LY - Y, and compare to the current sprite size
3-8. if LY - Y in range, read tile, attributes, X

64 times of 1-2 (128 dots) and 8 times of 3-8 (48 dots) will fit into the 192 dots of picture between secondary OAM clearing (1-64) and hblank (257+).

zzzz898
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Re: Sprite evaluation timing

Post by zzzz898 » Sun Oct 07, 2018 9:37 am

Yes, that's what I meant, thanks :)

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