[Coco] Cocovga 64 column not 80?

Joel Rees joel.rees at gmail.com
Wed May 1 22:33:46 EDT 2019

Okay, I'm beginning to get the picture ...

2019年5月2日(木) 10:06 Brendan Donahe <brendan at polylith.com>:

> Joel,
> There were a number of reasons for this CoCoVGA FPGA design decision:
> 1.) I could conserve space in the FPGA by reusing the character ROMs Steve
> Spiller and I re-implemented in the design, as opposed to creating another
> new character set.
> 2.) By default in text mode or RG6/PMODE4, each CoCo video pixel consumes a
> 2x2 pixel of the VGA 640x480.  This means that resolution could be doubled
> in each direction without exceeding the current VGA video format or doing
> away with the border around the green field.

So does 640 wide include the border area, which means 512 pixels of useable

Or is there a mode which enables 640 useable pixel width?

3.) Doubling the 32x16 = 512 byte text and semigraphics display
> horizontally and vertically still enables nice power-of-2 (64x32 = 2048)
> counters and sampling into the FPGA's video buffers.

Nice numbers indeed, should work well when editing a traditional Forth
fixed-width "screen".

4.) Sticking to a 8x12 character enables the software-uploadable custom
> character sets to be used in both 32-column and 64-column modes.
> I agree it was a bit of a compromise.  I would have preferred 80 columns,
> but not at the expense to the design size and complexity.

[thumbs-up /]

Note that all of this logic has been re-implemented from the 6847.  In
> other words, the VDG and CoCoVGA FPGA each have their own character
> generator.

Mimicking the internal to keep it simple. Gotcha.

> On Wed, May 1, 2019 at 7:02 PM Joel Rees <joel.rees at gmail.com> wrote:
> > So, obnoxious question that it is, is it 64 column because it is using
> the
> > internal character generator?

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