[Coco] embeddedcoco

[peak at mail.polarcomm.com]

There seems to be some more interest in an embedded coco, 
especially additional memory. Is 8MB enough? How about 64MB?
How can we do that?
If we use the 6809's A15 pin when low that givesd us the 
lower 32kb. Now we can interface a 1 Byte "output port" to 
the machine. Really just a "register" that can be used to 
expand memory in a couple of ways. Using the bits out of the 
port as enable pins would let us enable 8 blocks of that 
lower 32k.(256Kb) That does not seem to be very much. The 
other way to use those bit is to use them as if they were 
address lines. Then we would have A0-A14 plus "Pseudo A15-
A22" thats 23 address lines giving 8Mega bytes.
Now using one 8-bit "port" as address lines and another 8-
bit "port" as enable pins gives 8MB x 8(64MB).

Using this kind of"bank switching" does have some 
disadvantages. Because this circuitry is NOT trully an MMU 
and does NOT have task registers like the gime chip, the 
computer will Not keep track of this memory. That means that 
YOU will have to keep track of what active bits in 
the "pseudo" address line register, and what bits in 
the "bank enable" register correspond to what part of this 
memory!

If you wanted OS-9 to recognize the additional memory that is 
a problem. It does not mean that you cant use os-9, it just 
means that os-9 wont know about and use the memory.

You could use a Table in memory to keep track of the memory 
which would assist programming somewhat. You could even use a 
kind of FAT(file access table) and use the memory as a 
Ramdrive. Also it should be possible to have a Driver and 
Descriptor so that OS-9 or annother operating system could 
operate the memory as if it were a drive.

Eric