Re: 6809 in a C64

Den Wed, 23 May 2018 16:28:20 +0200 skrev Anders Carlsson
<anders.carlsson@sfks.se>:
> Mia Magnusson wrote:
> 
> > Maybe they used hires mode to be able to freely select foreground
> > and background colors?
> >
> Bingo!
> 
> I just downloaded Teledata.CRT and remembered that the title screen
> has double height text which of course requires either a full set of
> custom characters or working in hires mode.

Oh yes, I forgot about that. So both double height chars and free
choice of foreground/background combinations are the reasons for hires
mode.

Btw did you somehow see if it would produce the Swedish national chars
ÅÄÖ even without those chars in charrom?

> According to a register dump in VICE, $D011 is set to $3B = hires
> mode and $D018 = $29. The latter register appears to be set all the
> time, which kind of suggests there are two buffers of hires data so
> the program can switch between those. Probably that it loads and
> generates a videotex page in the buffer not currently displayed and
> switches to that one when it is done.

It would be almost trivial to test how it works with real modem data.
As you dial up with your phone and not your computer, it would just
recieve data using 1200bps and display it without any working transmit
connection. Thus you could just hook up an RS232 interface and a null
modem to any other computer and just send Prestel coded data in 1200
baud and just ignoring the 75 baud back channel.

A qualified guess is that they might have used two hires screens to
make scrolling less jerky, and also to make a screen clear to appear
instant.

Or I might remember things badly, were there even scrolling on Prestel
or did you just have everything in one page?

Anyway afaik Teledata did support downloading a few pages and keeping
them in memory, maybe running them as a slide show, so they probably
used two hires screens to be able to flip screens without jerkyness.
When in online mode they might have used two screens just to have an
already cleared screen to flip to as soon as a clear screen command is
received, and then slowly clearing the other screen when there are
cycles left over.

(Never had any manual for the software back in the days, and those
features aren't intuitive so I never figured them out IIRC).

> Whether generating a hires screen for every page loaded had been
> faster with a 6809 in the cartridge to offload the 6510 inside the
> computer is beyond my knowledge, but I understand that the BTX
> cartridges might use the 680x chips more due to the (existing?)
> hardware decoding solution was based on those CPUs than for speed
> purposes.

But the 68xx and 65xx buses are rather similar, so at least most
peripherals can be used with either CPU. Maybe that knowledge weren't
passed along to such extent back in the days?

Or maybe there were just example code written for 68xx which they used
for most computers?

For example I've read that there were a peripheral for Atari which
contained a modem or serial interface, some video generation and some
CPU, for Prestel. Expensive. Afaik there were also something similar
available for Spectravideo.

Even though the C64 were in the top of the price range on home
computers back in the days, it must have been one of the cheapest
solutions for Prestel.

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