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VDP register 3 in mode 4
Post Posted: Thu Jan 15, 2009 3:48 am
Enri have been working on figuring out the side-effects of clearing VDP register 3 bits in mode 4:
http://smspower.org/forums/viewtopic.php?t=11582
It's in Japanese. I haven't bothered carefully reading/translating this yet, but it suggest that some tiles ends overlapping due to the effect of those bits on internal addressing.

Currently they are described as is in Charles' documentation:
 Register $03 - Color Table Base Address

 D7 - No effect [...] D0 - No effect

 For the SMS VDP only, all bits should be set. Otherwise the VDP will fetch
 pattern data and name table data incorrectly.

 Register $04 - Background Pattern Generator Base Address

 D7 - No effect [...]  D0 - No effect

 For the SMS VDP only, bits 2-0 should be set. Otherwise the VDP will fetch
 pattern data and name table data incorrectly.
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Charles
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Post Posted: Fri Jan 16, 2009 1:44 am
Bock wrote
Enri have been working on figuring out the side-effects of clearing VDP register 3 bits in mode 4:


This sounds about right. I noticed that it alters graphics in rectangular areas, as if bits of the register were masked against the tile row and column values as the VDP scans the name table.

There seemed to be a lot of garbage in the display, even when I carefully set up VRAM. So in the end it seemed unusable. But I am sure there could be applications if more details were known about how the VDP operates when this register is changed. I am glad Enri is investigating it.
 
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Post Posted: Fri Jan 16, 2009 5:03 am
(Charles do you have problems to login lately?)
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hap
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Post Posted: Fri Jan 16, 2009 11:08 am
Sounds similar to mode 3 colour/pattern address table masking Sean documented in tms9918a.txt, a handful of games on MSX use this to their advantage. Does the SMS have this feature in TMS mode btw?
 
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Post Posted: Fri Jan 16, 2009 1:07 pm
Doesn't it just mask the tiles and tilemap addresses?
Maybe I should do a bit of testing on this...
@ hap: Do you know any these MSX games which use mode3 with address masking?
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Post Posted: Fri Jan 16, 2009 1:30 pm
FluBBa wrote
Doesn't it just mask the tiles and tilemap addresses?
Maybe I should do a bit of testing on this...


That would be logical, but it doesn't quite seem to be as straightforward as that. It would be very cool to see the results of your tests!

I could have done something improper in my test, but even if I restricted tile data to bitplane 0 only changing this register caused data to be shown in the other bitplanes. I remember seeing background colors that shouldn't have shown up based on the tile data in VRAM.

Maybe the color table register is only used for one of several VRAM accesses, say fetching bitplanes 0,1 and not 2,3. This would allow a name table value at one location to reference bitmap data at two distinct addresses, if some of the bitplane reads were masked and others were not.

Bock wrote
(Charles do you have problems to login lately?)


I think I have it solved now. :)
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hap
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Post Posted: Fri Jan 16, 2009 4:33 pm
FluBBa: Unfortunately, I haven't documented the affected games, here's a few from the top of my head:
- Can of Worms (not in all subgames)
- Dr. Archie
- Ice King
 
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