V9990 split...

Prophet (3494)

06-01-2018, 18:55

Does the V9990 support raster split?
palette split
screen mode split
some other raster split?
or there are some limitations?

are there any limits when writing to registers? (meant: some register cannot be accessed while in active area or some other similar caveats)

By Grauw

Ascended (8612)

06-01-2018, 19:48

You’ve read before that changing the palette on the fly can cause visible artefacts. More information would be welcome… like if it matters if you do it during horizontal blank.

You missed the scroll split and page split, I think those are the most common. I assume those work.

By PingPong

Prophet (3494)

06-01-2018, 19:58

Grauw wrote:

You’ve read before that changing the palette on the fly can cause visible artefacts. More information would be welcome… like if it matters if you do it during horizontal blank.

You missed the scroll split and page split, I think those are the most common. I assume those work.

Yes, i remember something about splits, but honestly i did not remember in what situations problems arise, nor if it was related to which kind of split.
I've not found the post you referring to in my previous search.
That's because i've re-asked about it.

By Grauw

Ascended (8612)

06-01-2018, 20:38

Palette is the only one I’ve heard of so far. But I guess few people have done much V9990 trickery.

By Mumbly

Resident (49)

06-01-2018, 21:43

(On real hardware) Indeed palette split doesn’t really works , scroll split works, sprite display split not, display enable split not.

By GhostwriterP

Hero (527)

06-01-2018, 21:59

Well what I can share for P1 mode (which may also be applicable for B1 modes, I am not sure) is that during display period:
-hi part of y scroll register cannot be updated, low part only does work
+ x scroll registers work just fine
-display cannot be disabled... but who wants to (I guess the same for the sprite disable, screen mode or whatever)
-updating colors within a pallet may cause some white noise on that display line (unless perfectly timed in v/h blank)
+layer color pallet offsets can be changed (i.e. selecting 1 set out of the 4; which works also in 16 color B mode)

did no try anything else sofar

By Grauw

Ascended (8612)

06-01-2018, 22:21

Nice info. Though it sure seems fickle, what works and what doesn’t. On V9938 kinda just everything works.

Disabling display is nice for hiding a line if the split needs to do more than fits within a hblank period… shame that doesn’t work.

As for layer palette offset, if you change a palette entry which is not currently selected on the layer, do you still get the noise? Otherwise you could double buffer palette swaps, which would actually be kinda nice for seamless splits.

By msd

Paragon (1375)

07-01-2018, 06:56

You only don’t get noise if the v9990 is not accessing the palette.