Date   : Wed, 10 Aug 2011 23:04:21 +0200
From   : rick@... (Rick Murray)
Subject: Risc PC (Was 'Minitel in France')

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.

> The CPU's address bus doesn't limit the amount of physical memory,
> only the amount of memory the CPU can see at one time.

Ah, but I think his point was that you have a 26 bit addressing 
capability (64MiB) going into a fairly simplistic memory controller that 
splits this into 16MiB for physical memory, and 16MiB for I/O memory 
(overkill much?!? do ROMs count as I/O?), with 32MiB capacity for 
logical RAM.

This seems to imply, to me, that a given memory location may exist in 
two different places - the physical address and the logical address. Or 
does the MEMC 'hide' physical locations from the processor?


> The 6502 only has a 16-bit address bus, but the BBC has 20 bits of
> memory (16 sideways banks at&8000-&BFFF).

But, like the Z80, it can only flat-space address 64KiB.


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...