[Coco] RAM SIZE INPUT mystery

[Chuck Youse]

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
> to 
> 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!

C.