bottom dots

[evildragon]

I have always wondered what these "dots" actually are, the ones at the bottom of the Genesis VDP display.

When I used composite, it looked like a single multi-colored flickering line. But when I got a Component adapter for the Genesis, I noticed they were actually "dots"..

But what are they? They seem to reflect the color palette. For example, at the Sega CD BIOS screen, those dots are cycling the rainbow color like the "SEGA CD" logo.

[TmEE co.(TM)]

Recently I've wondered the same thing, and I think that this happens when you DMA palettes.

[Fonzie]

Those dots are the representation of all the colors written in Color RAM during the INACTIVE display period.

I think that the megadrive vdp just send a group "dead background colors lines" to the tv during the whole inactive period in order to get sync for next period?
For exemple, my C code to refresh palette is quite slow, so there is big spaces between those dots... And if I refresh 3 palettes in a row, I get 3 lines of dots.

Btw, if the game is refreshing colors during the ACTIVE display period, you get exactly same kind of dots, but in top of the picture (raster effect).

[LocalH]

I think it's basically a data bus collision - you're writing to the VDP RAM at the same time the VDP is trying to read from it, so the VDP essentially reads what you are writing instead of what's actually in the RAM itself.

[Fonzie]

Yeah, color ram (and possibly scroll ram?).

The VRAM itself isn't affected by the collide (there is never glitching when writting to vram).
I wonder if later models like genesis3 or some GOAC have the same issues...

[evildragon]

Yeah, color ram (and possibly scroll ram?).

The VRAM itself isn't affected by the collide (there is never glitching when writting to vram).
I wonder if later models like genesis3 or some GOAC have the same issues...

i think they all have the problem... even the Sega CDX has it..