[Coco] a not very important drivewire question

[William Astle]

On 14-07-18 02:16 PM, Juan Castro wrote:
> Maybe even find a way to use stuff from the "unusable" parts of the ROM by
> creative use of bank switching. Are the MMU registers readable, or only
> writable?

The MMU registers are readable though they behave less than helpfully 
for machines with >512K installed. (The upper two bits are meaningless 
on a read.)

Also, the ROM stuff doesn't bank switch properly. In ROM mode, the ROMs 
appear at $78000...$7FFFF (physical addresses), or in blocks $3C-$3F. 
However, what you get with $3C-$3F depends where you put the block in 
the MMU. It seems rather complex but when you get down to it, the upshot 
is the following:

ROM area $8000...$9FFF can only appear at $0000...$1FFF or $8000...$9FFF
ROM area $A000...$BFFF can only appear at $2000...$3FFF or $A000...$BFFF
ROM area $C000...$DFFF can only appear at $4000...$5FFF or $C000...$DFFF
ROM area $E000...$FFFF can only appear at $6000...$7FFF or $E000...$FFFF

A note on the last one: the upper 512 bytes are mostly inaccessible in 
the $E000...$FFFF range but those upper 512 bytes are accessible when 
mapped in the $6000...$7FFF range.

Furthermore, when in ROM mode, you cannot access RAM blocks $3C..$3F.