Date   : Thu, 11 Aug 2011 08:14:35 +0100
From   : info@... (Sprow)
Subject: Risc PC (Was 'Minitel in France')

In article <4E42F255.1070501@...>,
   Rick Murray <rick@...> wrote:
> On 10/08/2011 16:42, J.G.Harston wrote:
> > Theo Markettos wrote:
> >> Furthermore, the 26 bit memory map limits the
> >> amount of physical RAM to 16MB.
>
> Or 
> does the MEMC 'hide' physical locations from the processor?

The MMU deals with logical to physical mappings, so once it's enabled the
processor can only see a logical memory map. 

For convenience, the OS may choose to set up the page tables so that part of
the logical memory map is set aside with a 1:1 mapping to the physical
memory map, but that's by no means mandatory. Especially if the number of
entries in the page table is fixed as it would be on simple microcontrollers.

On the Risc PC the logical address space starts at &00000000 but there's a
copy of physical I/O space at &80000000. With MEMC I think logical RAM is at
&00000000 and physical copy at &02000000, so you could be perverse and write
to the physical copy of DRAM and then read it back from its logical address
space. 

You could triple or quadruple map stuff if you wanted. To get back to the
BBC angle here, it might have been nice to have two views of the video
memory: one character cell based for fast text printing, and one flat bitmap
for fast sprite and line plotting,
Sprow.