4 Sep
2006
4 Sep
'06
2:17 p.m.
On Monday 04 September 2006 04:39 pm, Jim Brain wrote:
Just to clarify, if we're talking about the 128, there was both the 8721 PLA
in there _and_ the 8722 MMU (I hope I got those numbers right).
I also have
the 128 manual on hand as well, but never really dug into
the workings of that MMU chip. As a service center and wanting to have a
complete set of spares on hand I did get a hold of some of those, and
some of the PLA chips that were used in the 128, as well as a number of
the parts peculiar to the 1571, and ended up never using any of them. I
don't know if they'd be useful in anything else offhand.
Well, technically, I'm talking about the ROMH, ROML, EXROM, etc. lines
of the 64, which would change the memory map. The IC controlling them
is the PLA. Calling it an MMU (given the current usage of the term) is
probably misleading. The C128 has a better attempt at an MMU, where memory
can be
programmatically remapped.
Ok, this makes it look like you were referring to the 64 above, which I was
not. When I said "MMU" I was talking about the 128 only.
IN essence, the EXROM, ROMH, ROML were only intended
to be changed upon
powerup, but on the 64, they could be changed at any time to affect the
memory map. Some carts would use this to an advantage. The C128 had issues
with this usage of the lines.
Gotcha.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin