[Coco] Coco 3 Missing Mystery Color Mode

[Darren A]

On 4/26/10, Steve Ostrom wrote:
> This may seem like a very foolish question to those of you on the list who
> are Coco hardware experts, but I had to ask.
>
> I was reading a 1989 Coco-Link magazine from Australia this weekend and came
> across this bit of information I had never heard of before.  They stated
> that the PIA outputs, which control the display modes of the video display
> chip (6847) on the Coco2, are not connected to anything on the Coco3.  They
> went on to show a hardware project that could use these unused PIA outputs.
>
> I'm not sure that the above information is even true, but if true, could our
> missing high color video mode be a result of this "non-connection"?  (OK, go
> tell me to crawl back in my hole.)
>
> -- Steve --
>

--

PIA 1 in the CoCo 3 does not control video in the same manner as on
the CoCo 1 and 2, but writing to the PIA is noticed by the GIME.
Below is a paragraph from "Super Color Basic Unravelled" which
disusses this topic:

In the old CoCos the graphics display was taken care of by the Video
Display Generator (VDG). Controls were passed from the CPU to the VDG
by way of Peripheral Interface Adapter 1 (PIA1). The graphics display
of the CoCo 3 is handled entirely by the GIME chip, which has
eliminated the need to pass controls through PIA1. However, in order
to maintain compatibility with the older CoCos, a register has been
built into the GIME chip which will retain any information written to
the old VDG control bits of $FF22. This internal GIME chip register is
not accessible by the user and any data returned by reading $FF22 will
come from PIA1, not from the GIME chip. The PIA1 bits, which provided
control to the VDG in the older CoCos, are not used in the CoCo 3. Bit
2 of $FF22 (RAMSZ) is also not used - there is no hardware flag in the
CoCo 3 to tell the user if the system contains 128K or 512K. The
existence of the GIME chip’s internal $FF22 register has allowed the
addition of some extra features to the CoCo 3's CoCo compatible mode
(32 column). Bits 4 (upper/lower case) and 5 (invert) can be used to
invert the foreground and background colors of the text screen or to
allow true lower case characters. If bit 4 = 0, the ASCII codes from
0-31 will be the inverse video representations of the codes from
64-95. If bit 4 = 1, the ASCII codes from 0-31 will contain lower case
characters. Appendix I contains a complete chart of theses codes. If
bit 5 = 0, the text screen will be black characters on a green
background. If bit 5 = 1, the text screen will be green characters on
a black background.