At 11:30 PM 9/16/98 +0000, you wrote:
>-For most screens, there is a relationship between the exact VRAM
>address, and the VRAM page/pixel-coordinates that is about what you
>would expect. In some screen modes (I think Graphic 5 & 6 modes, that
>is, SCREEN 6 & 7), this relationship is different, so the contents of
>for instance 2 SCREEN 5-pages is not visible in one SCREEN 7-page,
>but 're-arranged' somehow (does anyone know how exactly?). In SCREEN
>8, this is 'back to normal'.
I thought VRAM arrangements were different in SCREEN 7 & 8.
The reason is in hardware. In SCREEN 7 & 8, more pixels per second are
needed to generate the image. Because fast RAM is expensive, instead they
used 2 slower RAM ICs as VRAM. In SCREEN 7 & 8, of every byte the
odd-numbered bits come from one RAM IC, and the even-numbered bits come
from the other RAM IC. In all other modes, all bits come from the same RAM IC.
This is also the reason why the V9938 databook says that SCREEN 7 & 8 are
only available in 128K configurations. In theory, you could have SCREEN 7 &
8 with just 64K VRAM, because one page takes 54K. But 64K VRAM means just
one RAM IC, and that's why SCREEN 7 & 8 won't work on 64K VRAM.
Note: I'm not sure I got every detail right, but this is the general idea.
By the way, there are some bits in a VDP register controlling "VRAM
config". Have you tried them? Any effect?
Bye,
Maarten
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