[Coco] Coco 3 Missing Mystery Color Mode

[jdaggett at gate.net]

Steve

The GIME chip mirrors two addresses of PIA1, $FF22 and $FF23. This is a write only 
register and can not be read.  

According to the service manual there is an implication that any writes to bits 3 - 7 of $FF22 
will affect performance of the 1986 (TC1014) version of the GIME chip and not the 
1987(TC1014A) version. 

All reads of the above two addresses come from the PIA port pins. 

james
 
On 1 May 2010 at 21:53, Steve Ostrom wrote:

> 
> ----- Original Message ----- 
> From: "brian palmer" <tigers2roar at yahoo.com.au>
> To: <coco at maltedmedia.com>
> Sent: Tuesday, April 27, 2010 12:47 AM
> Subject: [Coco] Coco 3 Missing Mystery Color Mode
> 
> 
> > Hi Steve.
> >
> > Can you tell us what 1989 issue of Coco-Link this article was in. And what 
> > Page it is on thanks. I have 4 issues of 89 coco-link but none seem to 
> > have anything about this article you mentioned.
> >
> > laters
> >
> > Briza
> >
> 
> Briza, this article, titled Coco Real Time Clock by Ken Wagnitz, was in 
> Coco-Link vol. 2, No.3 from June 1989 and starts on page 25.  Here are some 
> of the more relevent quotes:
> 
>      Browsing through the circuit diagram for the Coco3, I discovered that 
> the PIA outputs which control the display modes of the Video Display chip 
> (6847) of the Coco2, are not connected to anything in the Coco3!  Being of a 
> suspicious nature, I even checked the board in my Coco3 to confirm this 
> terrific fact.
>      It is true.  There are unused outputs there for us hackers.  So I wired 
> up my clock circuit directly to that PIA, using the `unused` PIA port pins. 
> It worked fine.
> ...
>      He reported that my Basic clock-read program mucked up the screen on 
> his Coco3.  On further questioning, I find out that the same happens when he 
> boots OS9.
> ...
>      It turns out that although the PIA outputs are not connected to the 
> GIME chip on the Coco3 (which handles the display), the GIME responds to 
> writes to the PIA as if it was connected!
> 
> -- Steve --
> 
> 
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