:-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'.
When you put a screen 7-picture in page 1, and then go to screen 5 and take
a look at pages 21-3, then the screen-7 picture will be visible just fine
exept for that each 1st pixel (1st half of the byte) will be displayed on
page 1 and every 2nd pixel (2nd nibble) on page 3.
It seems very unlogic to me but it's quite useful for converting screen
5-screen 7 and vice versa.
All the screens have some kind of (strange...) characteristics like these.
But now the question arises: Why? For god's sake, why??? This should only
lower the speed of the VDP, in my opinion. In Dutch we call this "Waarom
makkelijk doen als het ook moeilijk kan"... I can't imagine they implemented
this just to make conversion easier...
The thing to do when you want to put a pixel for screen 5 in the screen
7-modus:
Page is the page in screen 5 you want to put a pixel on, X is the X-axis and
Y is the Y-axis.
:setpage PageMOD2,PageMOD2
:pset (Xand510+Page\2,Y),Color
So the bit 1 of the Page-number indicates the highest VRAM-adressbit (the
17th bit in r#14),
and bit 0 of the page-number indicates if it uses the first nibble or the
second nibble of all bytes.
> Is there any relation between the _SINGLE_ bits and the vdp-behaviour?
>
> f.e. bit 1 set: maximum 256 colors (unset: 16 colors)
> bit 2 set: display 128 bytes per line (unset: 256 bytes)
> ...
:Hey, I guess there must be, because it's all implemented in hardware
:somehow...
:V9938 databook doesn't tell, so this is 'undefined'. Only way to find
:out is to investigate a real V9938, and no guarantees it will work
:the same in for instance a V9958 (or other compatible?)
If there were possibilities for new screen-modes, it would already have been
known to anyone. I have also tried it but it was a failure.
~Grauw
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