[Coco] RGB to XGA (Bruce Borer)

[Robert Emery]

I recommend a book called Tandy's Little Wonder by Frank Swygert...
available on the Color Computer Archive.

Tons of useful info in a fairly succinct  but informative guide to CoCo
foundations.

Cheers,
Bob

On Wed, Dec 24, 2025, 7:42 AM Bruce Borer via Coco <coco at maltedmedia.com>
wrote:

> Hi Andrew,
>     Thanks for all the useful information.  Something to look into in the
> new year.
> Bruce
>
> On Wed, Dec 24, 2025 at 4:41 AM Andrew Ayers <keeper63 at cox.net> wrote:
>
> > Probably the "best" that I know of (if anyone knows of a better one, I'm
> > all ears) is Chris Lomont's memory map.
> >
> > https://www.lomont.org/software/misc/coco/Lomont_CoCoHardware.pdf
> >
> > There's also this version - I'm not sure which version it is, though:
> >
> > http://lomont.org/software/misc/coco/Lomont_CoCoHardware_2.pdf
> >
> > As you can tell by either - there is a ton of "TODOs" and missing
> > information. Some is stuff that just needs to be filled in and checked
> > from other sources (?). Others may need deeper investigation. But the
> > majority of useful stuff is all there.
> >
> > Also check out the various "Unravelled" books - while these are more
> > about the ROM contents and such, they can be helpful when used with the
> > memory maps:
> >
> >
> https://colorcomputerarchive.com/repo/Documents/Books/Unravelled%20Series/
> >
> > Note that if you want to use them, you'll need all of them; also, the
> > XLS (Excel) file is an update in some fashion, but I haven't delved into
> > it much - I'm not sure how it is structured.
> >
> > You need all of those books at hand because each (in each ROM
> > disassembly) reference "labels" - and those labels could be in "earlier"
> > (or "lower level") ROMs. For instance, Disk BASIC will reference stuff
> > in Extended BASIC, which in turn will reference stuff in Color BASIC.
> >
> > Each book is mainly interested in the BASIC ROM it has disassembled, and
> > doesn't include those other ROMs. Also note that Super Extended BASIC
> > can be tricky to understand in areas, due to how SECB does it's overlay
> > and patching in the "All-RAM" mode that is native to the CoCo 3 (I know
> > the CoCo 2 could be put into "All-RAM" mode, and I think the CoCo 1
> > could as well - but the CoCo 3 boots into that mode; I don't recall
> > off-hand, but I think there is actually a way to disable the mode on the
> > CoCo 3 which probably clobbers things).
> >
> > You might find the Color BASIC and Extended BASIC books useful for
> > learning more about how the CoCo VDG stuff works, which the GIME in the
> > CoCo 3 partially "emulates" (it doesn't handle all of the SG modes
> > mainly that the CoCo 1/2 does.
> >
> > I think the GIME supports everything else, but it also allows the
> > changing of the palette for the SG and PMODEs - which is how, IIRC,
> > those "composite emulation" mods for RGB monitors work; the mode is set
> > for PMODE 3 (128x192x4-color mode) and the colors are changed to match
> up).
> >
> > If that doesn't make sense, the explanation is that PMODE 4 is actually
> > a monochrome mode (black and white or black and green); on a proper
> > composite monitor with enough bandwidth, you only get the monochrome
> > representation.
> >
> > Color on TVs and lower-bandwidth composite monitors are achieved by
> > putting black and white vertical stripes next to each other 1 pixel
> > wide, with "even/odd" selection (so - black/white vs white/black); the
> > pixel color transitions between "white" and "black" (two different
> > voltage levels) "bleeds" over (due to lack of bandwidth) and shows
> > different colors...depending on where the clock is in its phase for the
> > chip, which is "random" at startup (for the 6847; it is -fixed- on the
> > CoCo 3 and GIME!). Which is why PMODE 4 games say "press reset until
> > screen is red" or whatever. This is why it is also called "artifact
> > coloring" - because the colors are the result of an artifact of the
> > hardware and NTSC color system being...well, crappy in a way (but was
> > done that way to support and allow for b&w broadcasting to continue and
> > be viewed on color TV sets, and for color broadcasting to be seen as
> > black and white on those sets - a clever hack, I guess.
> >
> > Anyhow - because of that "even/odd" spacing thing, horizontal resolution
> > is effectively considered halved, so on the CoCo 3, switching the mode
> > to PMODE 3 and changing the palette of the 4 colors in that mode can be
> > done to simulate/emulate PMODE 4 colors on an RGB monitor.
> >
> > With that said - what you see in PMODE 4 on an RGB monitor without that
> > trick is actually how the mode is -meant- to be seen, at least according
> > to the Motorola 6847 datasheet. Which is kinda a pity, because there
> > were games on other systems that had similar limitations to their
> > graphics, which used a different method (or simply because it wasn't
> > NTSC) - and they couldn't use artifact coloring. Which led to a
> > different way of doing things to give detail and such to graphics for
> > games (dithering of black/white pixels, patterns, etc). It also led to
> > (for the CoCo; not sure about the Dragon) software which was meant to
> > use artifacting looking weird on (say) PAL televisions. Strangely
> > though, there are some titles for the CoCo that were made to look decent
> > on either video standard (IIRC, the one Walt Disney title was that way).
> > Most though, either had strange "colors" (not at all what was meant) or
> > were just monochrome (with colors showing as vertical stripes).
> >
> > ...and with all that, I will probably be ripped a new one by AllenH,
> > because my descriptions and such are mostly wrong in a technical sense,
> > because I'm not an engineer who worked with such systems intimately like
> > he did (I don't mind, Allen - I know what I'm talking about isn't 100%
> > right, but hopefully I do convey the intention and reasoning - so
> > correct away if you can or must - when your arm is feeling better,
> > perhaps!).
> >
> > Andrew L. Ayers
> > Glendale, Arizona
> > phoenixgarage.org
> > github.com/andrew-ayers
> >
> > On 12/24/25 12:57 AM, Bruce Borer wrote:
> > > Hi Andrew,
> > >      After reading your comments about 225 lines I searched and found
> > > this site
> > > BASIC225 <https://nickmarentes.com/ProjectArchive/basic225.html>
> > > This will help me support 192, 200 and 225 lines.  But that will be in
> > > the  new year.
> > >
> > > Bruce
> >
>
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