[Coco] Cartridge memory - mapping in and out

[Phill Harvey-Smith]

Quoting Willard Goosey <goosey at virgo.sdc.org>:

>> Date: Tue, 30 Oct 2007 16:05:12 +1100
>> From: Mark McDougall <msmcdoug at iinet.net.au>
>> Mike Pepe wrote:
>>> 'fraid so. The designers of the SAM and GIME apparently never saw that
>>> anyone would have a need to write to a ROM.

Well IIRC the first CoCo 1s and pretty much all of the Dragon 32s  
didn't disable CTS when writing, but they only ever operated in map  
type 0. However in map type 1 (all ram), you have to prevent the cart  
rom being activated when writing to the RAM in the cart area, as  
previously noted this will really bork things.

>> Hmmm, so there's no way to 'trick' the GIME into asserting CTS# during a
>> write cycle?
>
> May 1988 Rainbow had a project like this, except using SRAM instead of
> flash.  You have to create your own *CTS from the address lines.

You could do this with some logic or a CPLD chip for example, but you  
would also have to ensure that by default this behavior was off, and  
just enabled when needed, say by a write to a location in the SCS  
$FF40-$FF5F region.

You would need to do this to again prevent trouble when the machine is  
in all ram mode to stop your cart and the RAM being activated at the  
same time.

This is pretty much the reason when I built my multi-cart that I  
decided to have it only programable by a device reader and not  
directly on the CoCo/Dragon.

Cheers.

Phill.


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