Screen 8 is offically screen9, what happend to 8?

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By inchl

Expert (83)

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20-12-2022, 09:31

On my vg8235/00 I once had a screen 8 in 32x212 resolution. Each 8 pixels had the color of the first of 8. I will try to replicate this again.

By gdx

Enlighted (6215)

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20-12-2022, 10:13

Briqunullus wrote:

Fascinating. Has nobody played around with this before? Almost can't be. Also very interesting to see what will happen on a 9958.

Probably it is but few people share their findings. There has also been a lot of information lost over the years.

By Accumulator

Champion (339)

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20-12-2022, 12:37

@gdx, you are correct. In demoscene you do not really share tricks. Some info is not shared, so some info is lost.

I will check all possibilities related to M1 to M5, and unused or noted as 0 bits in the mode registers, as well as vram select.
In basic I understand when the screen goes white, the mode is invalid. In this case I do not trust Basic, will test in assembly.
Note: V9958 only, but expect V9938 behaves identical.

Get back with results, it will take some time... So don't stay up for results..

By Accumulator

Champion (339)

Accumulator さんの画像

18-01-2023, 21:24

Some small update...

After analyzing the different screen modes I came to the following preliminary conclusion.
The screen modes of TMS99x8/9A and V99x8 are linked by pairs.
The ratio is always 1:4, what does this mean:

M1 & M2 (Screen 0 and Screen 3) are linked:
M1 set can can have 2 colors on X direction and Y direction (font & background). (1)
M2 has blocks of 4 x 4, 2 colors on 8 pixels, X and Y. (4)

The following modes are also linked:

M5 M4 M3
0 0 0 Screen 1 Relation not checked, TBA
0 0 1 Screen 2 Relation not checked, TBA

0 1 0 Screen 4 (2 colors in 8 pixels, 256px) (1)
0 1 1 Screen 5 (8 colors in 8 pixels, 256px) (4)

1 0 0 Screen 6 (4 colors, M5 set, M4 unset = 512) (1)
1 0 1 Screen7 (16 colors M5 set, M4 unset = 512) (4)

1 1 0 Undocumented original screen 8,
(expected 256px, 64 colors, but 256 cannot find mode to organize color bits, maybe reason to disable) (1)
1 1 1 Screen 9 (implemented as 8) 256px, 256 colors (4)

Finished routines to check possibilities I will post at: My VDP Github page
Currently only 1, to determine the amount of VRAM of the machine, MSX1+ (not V9990), can be used to determine TMS9918/28/29

Working on detecting the white screen of invalid and checking configurations of VRAM select (MSX1 and MSX2 and up), VDP R#1, R#2, R#4, R#8, R#9

Will inform updates..

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