Re: Commodore 8296GD

On Sat 29 Sep 2012 at 22:02:17 +0200, Micha? Pleban wrote:
> 
> Rhialto wrote:
> 
> > Well, as the board seems simpler than the HSG board... (somebody wrote
> > that the chip on that was rather complicated to emulate)
> > if you find out enough details about this, I'd be happy to write an
> > emulation for it to include in VICE.
> 
> Sure, I'll try to figure it out. As I can see, it just uses the original
> CRTC chip by "faking" the character set somehow as hires graphics.

Sounds much like what the DWW board is doing. I posted about that in
this very mailing list. It has 8 banks of 1KB. It (most likely) used the
offset into character cell memory, as output by the CRTC, as index into
a 1 KB block of bitmap memory, combining it with the vertical offset
into the character (0-7) (also as output by the CRTC) to select which bank.
(Except that the DWW was designed before the CRTC was used, but I'm sure
the same counters were present in the design anyway)

> The
> bitmap to display most probably comes from the extra RAM.

Yes, that is what I read elsewhere too. The "extra 32K" aka the
"inaccessible 32K". Not that they're totally inaccessible: those 32 K
are the second part of the non-expansion RAM (the other 64 KB of the 128
KB are expansion RAM), and therefore would map from $8000-$FFFF if there
were not ROM and I/O there.

And in fact you can access parts of it: $8xxx is normal screen
(character cell) memory. $9xxx and $Axxxx can be easily jumpered to be
RAM as well (if you don't have EPROMs installed in the sockets of the
same address). And iirc there is also a 'jumper' option (but not so
easily accessible) to connect a user port line to the NOROM line and
make it possible to map out all ROMs, thereby uncovering even more of
that RAM).

Given this, the interesting issue gets to be: Where is the bitmap, and
how is it accessed by the CPU?

Given the resolution you mention (512 x 256) it would need 16 KB of
memory, or half of that 32 KB space. If normal text output would remain
working, and perhaps they thought it was nice if the $9xxx and $Axxx
would keep working as it was, it should be at $C000-$FFFF. Somehow.

I wonder why that resolution was chosen, and not 640 x 200 (which is 8 x
(80 x 25)). That would make it much more compatible with the text
screen.

> So far I was
> able to write a simple bitmap displayer in BASIC ;-)
> 
> https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-snc6/222187_10151041489882026_2036796824_n.jpg

Looks good!

> Regards,
> Michau.
> 
> 
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