Is 4GB the max on a 32 bit OS?

[Gavin McCullagh]

Hi,

On Thu, 18 Jan 2007, Scott Ledyard wrote:

> Can the x86 versions of Ubuntu (or any OS) employ more than 4GB of RAM?

Probably not without a custom kernel.  Linux kernels can indeed do this,
but I've been told that there is a considerable performance cost in doing
so.
	http://kerneltrap.org/node/2450

I've never used the >4GB stuff myself but I believe it can be done.  one
would want to have good reason to bother I suspect.

> I read with interest how to size an LTSP server and noticed that people have
> recommended lots of RAM. But, I've also heard that a 32 bit processor cannot
> physically address more than 4GB of memory. True?

In practice yes, for most people.  

Another thing to bear in mind is that if you do put 4GB RAM in a 32-bit
machine you probably won't even be able to address the full 4GB.  The PCI
devices are apparently mapped into the 4GB address space too.  So, there is
less address space available for RAM.

For example, this dell poweredge has 4x 1GB ram sticks in it:

gavinmc at moodle:~$ cat /proc/meminfo 
MemTotal:      3370992 kB
...

I guess in practice it would make no difference to put 3.5GB of ram in it
as it seemingly can't address more than 3.4GB.  The amount used seems to
vary from machine to machine, presumably depending on the nature and number
of pci devices.  I'm not sure if disabling devices in the BIOS might get
you a little more RAM.

Gavin