[Coco] CoCo 3 MMU test for all

Robert Gault robert.gault at worldnet.att.net
Sun Jan 20 21:53:01 EST 2008

Roger Taylor wrote:

> You say $0-$f is for a 128k CoCo, so why are blocks 48-63 used within 
> BASIC?   56-63 are stored in the TASK 0 64k map.

That is done so the ROM works with both 128K and 512K in a transparent 
fashion. With less memory (128K) only bits values $0-$F are active.

You could save that with 128K memory starts at $00000 and ends at 
$20000. But then with 512K, the ROMs would "relocate". Things look more 
consistent if you start 128K at $60000 even if that isn't physically 

If you think in $hex or better yet %binary, things might make more sense.
$38-$3F are the default values but in binary are
%00111000, %00111001, ... $00111111
With 128K you actually use $0-$F.

Why not use $0-$F in the ROM tables? If that were done, then any memory 
used for the high res text or graphic screens would not be in the same 
location relative to the ROMs going from 128K to 512K. Would it matter? 
Probably not except from an aesthetic view point.
Yes Roger, my hair is indeed getting thin. :)

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