[Coco] CoCo bus interface project

[Bill Pierce]

Matthew, A good place to start is the several 32k rompak to disk saving programs that are in the archives. Some of them were written is basic (I think) and may give insight to how the banks were switched.

Bill Pierce
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-----Original Message-----
From: Matthew D Stock <stock at csgeeks.org>
To: coco <coco at maltedmedia.com>
Sent: Wed, Apr 17, 2013 9:12 pm
Subject: Re: [Coco] CoCo bus interface project


On 4/17/2013 8:57 PM, Darren A wrote:
> The CoCo 3 has a 32K external ROM mode that is activated by some bits 
> in the GIME. This mode activates CTS assertions for the upper 32K of 
> the address space. The CoCo 1 and 2 have no such mode. For those you 
> would need to decode the addresses yourself and assert SLENB to 
> prevent the internal ROMs from being selected. Some commercial ROM 
> Paks included their own custom circuit for bank switching. Darren -
Thanks Darren.  That's both good and bad.  It means that I'm on the 
right track, but it also means that I need to know if the ROM in 
question supported the CoCo 3 to know which method to use.  How did the 
"old style" Paks behave when they were plugged into a CoCo 3?  I may be 
able to assert SLENB and use A15 (and probably a few others to avoid the 
high registers) instead of CTS when I know I've got a 32K ROM, even on a 
CoCo 3.

For the custom bank switching, I'm curious how they were able to effect 
those bank changes given the constraints on CTS and RW.  Did they send 
instructions to a special memory byte in the $FF40 using SCS?  It will 
be tricky to implement a generic method if there were different bank 
implementations.

Thanks for the tip... I'm going to start poking around with some of the 
other control signals and see what I can learn.
   -Matt


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