Date   : Fri, 12 Aug 2011 22:10:48 +0100
From   : theom+news@... (Theo Markettos)
Subject: Risc PC (Was 'Minitel in France')

In article <1313152887.6733.151.camel@...> you wrote:
> On Thu, 2011-08-11 at 19:10 +0000, J.G.Harston wrote:
> > That really took me some time to get my head around when I was
> > first trying to understand it, as I was so used to (and it was more
> > intuitive with) logical-to-physical mapping (eg the Z80
> > 1MbMem.gif), not physical-to-logical mapping.
> 
> Yeah, I don't quite know why Acorn did it the way that they did in MEMC.
> I guess it seemed like a good idea at the time for some reason but it's
> hard to imagine what that reason might have been.

One possible reason is that a machine might have 1MB of physical RAM and
32MB of logical address space.

AIUI the L2P tables are direct indexed from the physical side (ie page 47 in
physical RAM stores its mapping in entry 47 in the L2P table, the page size
varying with the amount of physical RAM fitted).  If you had the same
structure the other way around (ie one mapping entry for each logical page)
most of the entries would be wasted as there would be no physical RAM there. 
So with 128 direct indexed entries pages would be 256KB, which would be
rather painful.  Particularly on 512KB machines!

Theo