Using all of 4GB RAM... questions and Vista versus Linux...

[Roger Heflin]

Linuxguy123 wrote:
> I have a new HP hdx laptop with a Core Duo T8100 processor and 4 GB of
> RAM.
> 
> $ uname -a
> Linux localhost.localdomain 2.6.26.3-14.fc8 #1 SMP Wed Sep 3 03:40:05
> EDT 2008 i686 i686 i386 GNU/Linux
> 
> Even though I have 4GB of RAM installed, Linux appears to only be using
> 3GB of it.
> 
> $ free -t
>              total       used       free     shared    buffers
> cached
> Mem:       3106944     777056    2329888          0      60608
> 419608
> -/+ buffers/cache:     296840    2810104
> Swap:      2040244          0    2040244
> Total:     5147188     777056    4370132
> 
> I've read in other posts to this group that the cause of this is that
> the BIOS remaps the some (1GB) of memory to serve as address space for
> PCI devices, thus creating a memory "hole".   
> 
> I understand that some BIOSes allow one to remap those devices elsewhere
> in the map.  My BIOS does NOT allow that option.

There is really no other place to put the PCI devices except in the lower 4GB 
since anywhere else would result in 32-bit OSes being unable to access them.
> 
> Questions:
> 
> a) On machines that do not allow PCI remapping, is the processor
> physically disallowed from accessing that 4GB of RAM ?  Ie have the
> address lines from the processor been disconnected from that RAM due to
> being connected to the PCI devices ?

The chipset is not capable of remapping under certain conditions, or the bios 
won't do it because it would cause lower memory availability with windows.  Some 
bios can only remap entire dimms, and that would result in less memory for a 
32-bit only OS so they don't do it even though it would result in more memory 
for a 64-bit/PAE os.

> 
> b) How do XP and Vista handle this ? Are they limited to 3GB of RAM
> too ?

In the case of a bios limitation they would also be limited to only 3GB.

> 
> c) I am running the 32 bit version of Linux.  Would it make any
> difference to my RAM access if I ran the 64 bit version ?

Some bioses will remap memory to above 4GB, but often only if a certain option 
is set, and often these options are only available on server class MB's, though 
these options may now be showing up on desktops since desktops can now support 
more memory.

You would have to boot a PAE or 64bit kernel and mess around with bios options 
to see if you bios can do it, it may or may not be able to do it in a useful 
way.   And of course the bios options are poorly documented.

                        Roger
> 
> Thanks
> 
>