[Coco] [Color Computer] Re: Japanese PC Game in CoCo Style?

[James Diffendaffer]

--- In ColorComputer at yahoogroups.com, "James Tamer" <emucompboy at ...>
wrote:

> 
> > As far as CoCo 1/2 VS TREK... I think the TREK had some additional
> > graphics capabilities by switching color mode with hardware on the 
> fly
> > so you could get more colors on screen at once but I've never been
> > able to find docs about it.
> 
> This was implemented as a RAM array of 512 attribute bytes;  it was 
> read in parallel with the screen RAM to provide mode information for 
> each screen byte read.  The big purple-blue book that came with your 
> NEC Trek talks about this.

Sorry, it was boxed but didn't have a manual and nobody has put a
scanned one online yet.  At least not that I have found.


> You can mix and match modes on the
> screen, with the caveat that putting incompatible things on the same
> scanline causes undesirable effects, and the border getting its color
> from whatever happens to be last on the scanline. The array wraps,
> so it's read several times in graphics modes. The NEC Trek's normal
> mode for BASIC is the extended characters, for which it has a ROM.
> One downside is that the screen DMA cycle-steals or stretches, so the
> computer runs at about 59% the speed you'd expect. There's a bit to
> turn off the screen DMA, and then it runs full speed with the screen
> blanked.

Yeah, I figured that was the case.  To do everything they were trying
without impacting the CPU would have required doubling the memory
speed and alternating cycles between the GFX hardware and CPU.  Even
if RAM fast enough to do that had been available I'm not sure the CPU
or GFX hardware could have handled it.  It might be an interesting
project for an FPGA implementation though.