[Coco] Coco 3, Expanded Memory and Assembler

theother_bob theother_bob at yahoo.com
Mon Feb 25 21:38:57 EST 2008

Hey Charles, if you're looking for suggestions, you could download the source/data files for the old Scott Adams Adventures, (which have been made freely available from the original author; search Google) and create an 80 column CoCo3 version. The original CoCo 1/2 cassette versions won't work on a CC3, unless there is some workaround that I don't know.

I can tell you that the CoCo3 uses 8K blocks of memory and there are two sets of registers in the GIME chip that are used to select eight 8K blocks at a time. You can switch between banks and do other tricks, even within Basic if you're careful. I'll leave the details to those more expert on the subject. This may not be 100% accurate, but a good general overview.

Read Super Extended Basic Unravelled for lots of good info on how the GIME works.

Good luck!

----- Original Message ----
From: Charles Shrader <charles at shrader.com>
To: coco at maltedmedia.com
Sent: Monday, February 25, 2008 6:22:43 PM
Subject: [Coco] Coco 3, Expanded Memory and Assembler

Greetings all!

I'm still toying around with 6809 Assembler on the Coco and have written
a few basic programs to accept user input and repeat it to the screen.
I'm toying with the idea of writing a text adventure completely in
assembler but that's still in the "dream" stage.

If the 6809 can only access 65,535 memory locations, how would one
access the additional RAM on a Coco3 that has 512k RAM memory?  On my 8
bit Commodores I believe this is done using something called "banking".
Is there a similar methodology for the Coco?

For some reason I find assembler delightfully enjoyable to write for the
Coco.  I don't recall similar programming for the 6502 or the Z-80 chips
being nearly as fun.  Especially the Z-80; it seemed to be cantankerous
when trying to run compiled programs on disks that are accessed on
drives other than the current drive.  Or it could just be that CP/M is
cantankerous.  Or maybe it's the programmer.  ;-)

I really enjoy being part of and reading your posts to this list.  Being
fairly new to the Coco crowd, it's been a great experience thus far.


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