[Coco] CoCo 3 MMU test for all

Roger Taylor operator at coco3.com
Sun Jan 20 20:06:24 EST 2008

At 03:49 PM 1/20/2008, you wrote:
>I haven't been following this that close because it is something I 
>do everyday.
>As james has indicated, basic CoCo understanding of the CoCo's 
>internal registers. A stock MMU is only 6 bits wide. 'AND' off the 
>upper two bits, bit6 & bit7.
>Go to a 1 meg machine and bit0-6 will be used.
>Go to a 2meg machine and bit0-7 will be used.
>256 blocks of 8k sized blocks = 2megs.

At least one of my comments were "sleep deprived" comments.  ;)  I've 
written programs like P-3 that take advantage of the full 512k memory 
map in a tricky way but using apparent blocks 0-63.  If 64 is added 
by the GIME for 128-512k CoCo's then I suppose it really means blocks 
64-127 for 512k, and 112-127 for 128k even if you poke 0-63, while 1 
meg upgrades *may* set the highest bit 7 by default to give the same 
behavior to push the blocks upward in the same fashion?

If so, what is now apparent for the actual hardware block #'s 
(regardless of what a PEEK reveals at the prompt for an MMU register 
for 128k-512k CoCo's):

128k: 112-127
512k: 64-127
1 meg: 64-255
2 meg: 0-255

Somebody will correct me if I'm wrong, but this is what I'm 
gathering.  For a stock CoCo 3, blocks are referenced as 48-63, though.

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