PPU scanlines & frame

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PPU scanlines & frame
by Jaeger on 2007-02-27 (#22189)
There are 262 scanlines in each frame, of which 20 are vblank and 240 are visible. Where do the other two scanlines fit in? Are they both between vblank and the start of the next frame? End of frame and vblank?
Re: PPU scanlines & frame
by tepples on 2007-02-27 (#22193)
Jaeger wrote:
Where do the other two scanlines fit in? Are they both between vblank and the start of the next frame? End of frame and vblank?

Both.

The picture is scanlines 0 through 239, and vertical blanking is scanlines 241 through 260 (PAL 310) inclusive. On scanlines 240 and 261 (PAL 311), the PPU goes through the motions of VRAM fetching but renders nothing, in order to get the prefetch buffers into a known state for scanline 0. I'm guessing that like the other inefficiencies of the NES PPU, this was done to save gates.

by WedNESday on 2007-02-28 (#22196)
Is this what you mean?

20/70 - NTSC/PAL VBlank Scanlines
1 - Dummy Scanline (for pre-fetching tile and sprite data)
240 - Render Scanlines
1 - Dead Scanline (PPU does nothing at all)

by tepples on 2007-02-28 (#22198)
WedNESday wrote:
1 - Dead Scanline (PPU does nothing at all)

"Does nothing at all"? Then why does it still hog the VRAM bus at this point?

by Disch on 2007-02-28 (#22208)
tepples wrote:
"Does nothing at all"? Then why does it still hog the VRAM bus at this point?


News to me. I was under the impression it was idle for that whole scanline. BT's doc gives that impression, anyway... this is the first time I've heard otherwise.