[Coco] Re: Re: [Color Computer] Re: Adapter from Coo3 to SVGA

[jdaggett at gate.net]

Kevin

Yes the Coco does use an incomplete address decode for the PIA 
chips. Poor design. Each set of 4 registers of the PIA can be 
accesses on a four byte boundary anywhere in the 32 byte range of 
$FF00 - $FF1F for PAI1 and $FF20 - $FF3F for PIA2.

The GIME mirrored register of $FF22 uses all 16 address lines to do 
a complete address decode.When the GIME chip decodes any 
address between $FF00 and $FF3F, it will output on the S-Buss a 
"010". This will force pin #13 of the 74LS138 to go low and that in 
turn goes to the CS0* pin of the two PIA chips. 

james



On 6 Apr 2005 at 10:57, Kevin Diggs wrote:

Date sent:      	Wed, 06 Apr 2005 10:57:45 -0700
From:           	Kevin Diggs <kevdig at hypersurf.com>
To:             	CoCoList for Color Computer Enthusiasts 
<coco at maltedmedia.com>
Subject:        	Re: [Coco] Re: Re: [Color Computer] Re: 
Adapter from Coo3 to SVGA
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> Hi,
> 
> jdaggett at gate.net wrote:
> > Rodney
> > 
> > Here is a better explanation.
> > 
> > When you write to $FF22 you will write to the GIME chip i nternal
> > register at $FF22. Internal to the GIME this register is a write
> > only register. So that the old Coco1/2 bits for mode control of the
> > 6847 will be maintained. A read of that address address will
> > actually read from the PIA chip. 
> > 
> > Thus PB3 to PB7 of the PIA chip can actually be used as general
> > purpose inputs only. I was in error when I said that they could be
> > used as general purpose I/O (input and/or output).
> > 
> > james
> > 
> 
>  Didn't the PIAs in the unis and deuces use sloppy decoding. An entire
>  
> 32 byte block was assigned when only 4 are needed. Didn't they just
> repeat? i.e. $ff24-$ff27 == $ff20-$ff23? Does the GIME also do this?
> If not could we just use $ff24-$ff27 for output?
> 
>      kevin
> 
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