[Coco] RAM SIZE INPUT mystery
cyouse at serialtechnologies.com
Sat Sep 27 10:43:10 EDT 2008
On Sat, 2008-09-27 at 18:50 +1000, Nickolas Marentes wrote:
> In the CoCo3 Service manual on page 10 it shows Bit 2 of $FF22 as "RAM
> SIZE INPUT".
> I find it strange that such a pin would be provided and documented as
> something it is not.
Two words: backwards compatibility. The designers of the Coco 3 bent
over backwards to maintain compatibility with the Coco/Coco 2 software
base, and that meant maintaining the PIA pin functions. Does it do
anything on the Coco 3? Nah, other than convincing Coco 2 programs that
they've got 64K to play with. Or something like that.
> And why is it connected to the keyboard? Why or
> when is it necessary to possibly hold a key on this keyboard column
> send an input to the RAM SIZE INPUT pin of the PIA?
Nah, the functions are unrelated. It's not atypical to share pins in
this way - even in new designs I/O pins are often in short supply!
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