[Coco] [Color Computer] Coco3 GIME Chip Question

Robert Gault robert.gault at att.net
Mon Mar 14 07:50:42 EDT 2011

news at wzydhek.com wrote:
> Question regarding operation of the GIME Chip in the Coco3.
> If $FF90 bit 6 (MMUEN) is cleared, I understand that the MMU Registers are
> disabled, and ram is the logical address space from $70000 to $7FFFF.
> In this case, what values are present in the MMU Registers $FFA0-$FFAF? Do
> they retain their previous values? Or will they hold the values of $38-$3F
> cooresponding to the $70000-$7FFFF logical address space? If this is the
> case, will they revert back when MMUEN is set again?
> -Walt Zydhek

This is easy enough to test on a real Coco3 under Disk Basic as long as you realize that 
you must change the master image of $FF90 at $E032 rather than $FF90 directly. Here is 
what happens on my system.

I placed data in the region $2000-$3FFF (using a 32 character screen). I changed $FFA1 to 
a different value and memory changed. Then $E032 (used with a 32 character screen) was 
changed from $CC to $8C and the original memory returned.
So, it looks like the effective memory blocks with the MMUs deactivated are $38-$3F 
regardless of the values in $FFA0-$FFAF.

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