What are ROM Nametables useful for?

Discuss technical or other issues relating to programming the Nintendo Entertainment System, Famicom, or compatible systems.

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scrimpeh
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What are ROM Nametables useful for?

Post by scrimpeh » Tue Nov 24, 2020 2:37 pm

This is something I've been wondering about for a while and can't find an answer for.

Certain mappers like 19, 24 and 209 can map CHR ROM into Nametable RAM, but I never understood why this feature exists. The only reason I can think of would be to quickly show static screens, but that doesn't seem worth the extra hardware when you can easily write the nametable yourself.

What's a good use for it. Anyone have an idea?

NewRisingSun
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Re: What are ROM Nametables useful for?

Post by NewRisingSun » Tue Nov 24, 2020 2:48 pm

It's used by games with a 3-D perspective such as Final Lap and After Burner II.

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Quietust
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Re: What are ROM Nametables useful for?

Post by Quietust » Tue Nov 24, 2020 2:56 pm

scrimpeh wrote:
Tue Nov 24, 2020 2:37 pm
This is something I've been wondering about for a while and can't find an answer for.

Certain mappers like 19, 24 and 209 can map CHR ROM into Nametable RAM, but I never understood why this feature exists. The only reason I can think of would be to quickly show static screens, but that doesn't seem worth the extra hardware when you can easily write the nametable yourself.

What's a good use for it. Anyone have an idea?
If you want to display a full-screen animation that's reasonably short (e.g. less than a dozen or so frames), then ROM nametables will allow you to run that animation as fast as you want - if your nametables were located in RAM, then you'd be limited to around 10-12 frames per second on NTSC because there's only enough video bandwidth to update about 150-200 bytes in a single VBLANK period (and a single nametable is 1024 bytes).
Quietust, QMT Productions
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aquasnake
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Re: What are ROM Nametables useful for?

Post by aquasnake » Tue Nov 24, 2020 5:40 pm

The significance of mapping ntrom to ciram is that it can display graphics quickly without waiting to write to ciram

After all, the hardware remapping is much faster than the software operation of $2006-$2007 loops.

This technique is usually in junction with switching the 8K pattern of CROM to improve the display frame rate

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scrimpeh
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Re: What are ROM Nametables useful for?

Post by scrimpeh » Wed Nov 25, 2020 10:18 pm

Thanks for the responses, everyone!

After Burner II especially is a really good use case that I hadn't considered. It's a very interesting game.

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