What are the fetches on the pre-render scanline?

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Drag
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What are the fetches on the pre-render scanline?

Post by Drag »

I noticed a question on the PPU Rendering page on the wiki, back from 2015, asking what the fetches on the prerender scanline are, since it makes the same fetches a visible scanline would.

My guess is that the fetches depend on what the current value of the PPU's V register is at the time the prerender scanline starts. So for example, if the game's vblank routine finishes by setting the scroll registers, V will be the same as T, and scanline -1 will make the same fetches scanline 0 will. If the PPU isn't touched at all, then scanline -1 will make fetches like it's rendering a hypothetical "scanline 240" (i.e., where V left off when the previous frame finished).

Could anyone confirm this before I add anything to the wiki? :P
lidnariq
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Re: What are the fetches on the pre-render scanline?

Post by lidnariq »

It does just start rendering with the contents of V before it reloads V from T.

This can be verified by looking at my hardware capture of the SMB3 title in this post
Drag
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Re: What are the fetches on the pre-render scanline?

Post by Drag »

Thank you, I've added this and the sprite fetches to the wiki.

As a sidebar, it's mind blowing what a difference 12 years of knowledge makes; back then, I'd have no clue, but today, it was just "oh yeah it's probably just whatever V's set to, easy peasy". :P (Still wanted to be sure though)
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