[Coco] RAM SIZE INPUT mystery

[Nickolas Marentes]

Can someone shed some light on this...

In the CoCo3 Service manual on page 10 it shows Bit 2 of $FF22 as "RAM 
SIZE INPUT".

Then, on page 9 it shows bit 6 of $FF02 as "KEYBOARD COLUMN 7 / RAM SIZE 
OUTPUT".

I had a look at the CoCo3 schematic diagram on page 103 and investigated 
these PIA addresses. It shows Pin 12 (PB2) of IC4 (68B21) as being 
connected via a 4.7K resistor to 5V. This indicates that the "RAM SIZE 
INPUT" is set to high (1).

It also has a link that takes it down to pin 16 if IC5 (LDC81001) which 
is indeed column 7 of the keyboard matrix. This matrix includes the F2 key.

I cannot see how this arrangement determines RAM size. The CoCo 1 and 2 
had a similar pin but it actually was wired to a jumper which you set to 
High or Low to select 16K/32K of RAM. This one in the CoCo3 is connected 
to the keyboard and would appear that a key (F2?) be held down to send 
the "RAM SIZE INPUT" low.

In the Super Extended Color Basic Unravelled book, it states that this 
RAM SIZE INPUT pin is unused.

I find it strange that such a pin would be provided and documented as 
something it is not. 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?

Can any shed some light into the logic behind this?

Nick Marentes