4 Gig in new Prec 490 showing as 3 Gig?

[V. Alex Brennen]

On Thu, 2007-05-03 at 11:10 -0400, Derek Atkins wrote:
> Jerry Feldman <gaf at blu.org> writes:
> 
> > As mentioned previously, to see more than 3GB you need to have PAE
> > enabled. Linux will be able to access the 4GB (or in this case 3.5GB),
> > but on a 32-bit system your applications are limited to 3GB. 
> > Here is a pretty good detailed explanation:
> > http://kerneltrap.org/node/2450
> 
> Okay... So how does one enable PAE?  (I've got 4GB in my new t60
> but it seems to only see 3GB of it)

The Linux Kernel will enable it automatically if it is necessary.

To access your 4GB of RAM, you'll just need to compile your kernel with
CONFIG_HIGHMEM which is under the "menuconfig":

  Processor type and features  ---> High Memory Support

My understanding of PAE is that it isn't necessisary for 4GB and your
system will not enable it.  A Linux Kernel with the CONFIG_HIGHMEM
option enabled will simply create an additional layer of abstraction
between the primary kernel memory map and the physical memory addresses.
With PAE, a third layer of abstraction is used, rather than the two
layers (High Mem and Normal Memory), because the 64GB map is not able to
easily fit in the High Mem allocation.

The help page for the High Memory Support option in "menuconfig" or the
kernel's text file documentation explains all of this in great detail. 
I have a number of intel based servers with 4GB of RAM and a few with
12GB.  I had to spend some time reading through all that info a few
months ago. Because, I initially incorrectly assumed that I would need
CONFIG_HIGHMEM only for the 12GB systems and not for the 4GB installs.


   - VAB

> -derek
> -- 
>        Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
>        Member, MIT Student Information Processing Board  (SIPB)
>        URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
>        warlord at MIT.EDU                        PGP key available
>