[Gllug] 4G Memory Restriction

[Alain Williams]

On Mon, Apr 10, 2006 at 11:21:44AM +0100, Steve Nelson wrote:
> On 4/10/06, Anthony Newman <anthony.newman at uk.clara.net> wrote:
> > Steve Nelson wrote:
> > >
> > > """4GB is the maximum any OS can address in 32-bits.
> >
> > 4GB is the maximum a processor with a non-multiplexed address bus with a
> > width of 32 bits can address. The 32-bitness of the processor or OS is
> > related to the width of the data bus. The two are not related.
> 
> Sorry which two are not related? The OS and the Processor - sorry if
> I;m being slow here!
> 
> The width of the data bus restricts the addressable memory size, as I
> understand it.  Is that correct?  Therefore regardless of OS, if you
> have a 32 bit machine, you'll only ever be able to address 4G of
> memory at a time?

OK, it's more complicated than that. How many bits a processor has
depends on how you look at it.

Eg, the motorola 68000 series was a 32 bit processor.
This meant that you wrote your programs using 32 bit integers and
(more importantly for this discussion) 32 bit pointers (memory addresses).
That is one way of looking at it.

The other way is from the hardware designer. The original 68000 IIRC was
a 16 bit processor from the point of view of external data bus & the such.
the 68020 was fully 32 bit data bus.

The point is that CPUs can appear differently.

With recent machines you can have > 32 bits external memory addressing,
the OS needs to be aware of this but it makes it look (to the program)
as if it were on a straight 32 bit machine.

The 68000 series would have been a far better choice for the IBM PC than
the intel chip where you had to work around the monstrous issues of a paged
architecture. Unfortunately IBM was into producing something cheap.

> What about the other way round?  Suppose I have a 32 bit userland
> running on a 64 bit processor?  Is the restriction there still because
> the userland is coded only to handle 32 bit words?

-- 
Alain Williams
Parliament Hill Computers Ltd.
Linux Consultant - Mail systems, Web sites, Networking, Programmer, IT Lecturer.
+44 (0) 787 668 0256  http://www.phcomp.co.uk/

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