memory remapping, 4gb memory on 945gt

memory remapping, 4gb memory on 945gt

[felix-linuxkernel]

I recently put 4 GB of memory in my Acer Travelmate 8210 series
notebook.  The BIOS only detects 3 GB.

I googled around a little.  It appears to be a chipset limitation of the
945gt, which uses the fourth gig for devices.

Now I can understand this explanation for 32-bit mode, but I'm running
in 64-bit mode.  There should be a way to use the fourth gig under
Linux.  Is there?

Has anyone else solved this problem?  What happens if you just use the
mem= option to tell the kernel you have 4 GB?  I'd just try it but I
don't want to corrupt my buffer cache and get crap writte over my data
on disk.

Any advice?

Felix

Re: memory remapping, 4gb memory on 945gt

[Andi Kleen]

felix-linuxkernel@fefe.de writes:

> I recently put 4 GB of memory in my Acer Travelmate 8210 series
> notebook.  The BIOS only detects 3 GB.

Actually it will detect 4GB, but put the PCI hole over the last GB.
One way to get more usable memory is to configure the BIOS to use a smaller
PCI hole (e.g. use smaller frame buffer/aperture etc.)

> I googled around a little.  It appears to be a chipset limitation of the
> 945gt, which uses the fourth gig for devices.
>
> Now I can understand this explanation for 32-bit mode, but I'm running
> in 64-bit mode.  There should be a way to use the fourth gig under
> Linux.  Is there?

The chipset limitation applies to 64bit mode as well as to 32bit mode
(which actually does not have a 4GB limitation with PAE)

-Andi

Re: memory remapping, 4gb memory on 945gt

[Dr. David Alan Gilbert]

* Andi Kleen (andi@firstfloor.org) wrote:
> felix-linuxkernel@fefe.de writes:
> 
> > I recently put 4 GB of memory in my Acer Travelmate 8210 series
> > notebook.  The BIOS only detects 3 GB.
> 
> Actually it will detect 4GB, but put the PCI hole over the last GB.
> One way to get more usable memory is to configure the BIOS to use a smaller
> PCI hole (e.g. use smaller frame buffer/aperture etc.)

Unfortunately there are certainly some that don't make many options
for configuration and also use a vast amount; my Toshiba A100-306
uses an entire 1GB for the PCI space and has very little in the
way of configuration.

> > I googled around a little.  It appears to be a chipset limitation of the
> > 945gt, which uses the fourth gig for devices.
> >
> > Now I can understand this explanation for 32-bit mode, but I'm running
> > in 64-bit mode.  There should be a way to use the fourth gig under
> > Linux.  Is there?
> 
> The chipset limitation applies to 64bit mode as well as to 32bit mode
> (which actually does not have a 4GB limitation with PAE)

Note that some system and memory vendors have incorrect statements
stating that the 3.xGB limit is a 32bit OS issue on these machines
and it'll all just work fine on 64bit OSs.

Dave
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    | Running GNU/Linux on Alpha,68K| Happy  \ 
\ gro.gilbert @ treblig.org | MIPS,x86,ARM,SPARC,PPC & HPPA | In Hex /
 \ _________________________|_____ http://www.treblig.org   |_______/

Re: memory remapping, 4gb memory on 945gt

[H. Peter Anvin]

Dr. David Alan Gilbert wrote:
>>>
>>> Now I can understand this explanation for 32-bit mode, but I'm running
>>> in 64-bit mode.  There should be a way to use the fourth gig under
>>> Linux.  Is there?
>> The chipset limitation applies to 64bit mode as well as to 32bit mode
>> (which actually does not have a 4GB limitation with PAE)
> 
> Note that some system and memory vendors have incorrect statements
> stating that the 3.xGB limit is a 32bit OS issue on these machines
> and it'll all just work fine on 64bit OSs.
> 

Supposedly WinXP-32 doesn't use memory over 4 GB even if it is available 
(a market-segmentation decision of the part of Microsoft, to force 
people to buy WinServer 2003; WinXP SP2 does PAE so there is no 
technical reason.)  This probably has disincentivized hardware vendors 
from providing support for remapping the memory from the aperture above 
the 4 GB mark.  The logic isn't all that trivial, especially if the 
aperture size is configurable, so I can understand why they would punt.

	-hpa

Re: memory remapping, 4gb memory on 945gt

[Parag Warudkar]

H. Peter Anvin <hpa <at> zytor.com> writes:

> 
> Supposedly WinXP-32 doesn't use memory over 4 GB even if it is available 
> (a market-segmentation decision of the part of Microsoft, to force 
> people to buy WinServer 2003; WinXP SP2 does PAE so there is no 
> technical reason.)  This probably has disincentivized hardware vendors 
> from providing support for remapping the memory from the aperture above 
> the 4 GB mark.  The logic isn't all that trivial, especially if the 
> aperture size is configurable, so I can understand why they would punt.
> 

The reason according to Microsoft is that enabling PAE induced driver
incompatibilities - to me it sounds quite plausible given the quality of drivers
for consumer versions of XP/Vista and the fact that 4GB RAM in consumer class
machine is a newer thing.

There is a KB document detailing the reasons for disabling >4Gb RAM in XP SP2
and later 32-bit OSes - http://support.microsoft.com/kb/888137 .

Parag

Re: memory remapping, 4gb memory on 945gt

[Udo A. Steinberg]

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On Fri, 04 Jan 2008 17:22:57 +0100 Andi Kleen (AK) wrote:

AK> felix-linuxkernel@fefe.de writes:
AK> 
AK> > I recently put 4 GB of memory in my Acer Travelmate 8210 series
AK> > notebook.  The BIOS only detects 3 GB.
AK> 
AK> Actually it will detect 4GB, but put the PCI hole over the last GB.
AK> One way to get more usable memory is to configure the BIOS to use a
AK> smaller PCI hole (e.g. use smaller frame buffer/aperture etc.)
AK> 
AK> > I googled around a little.  It appears to be a chipset limitation of
AK> > the 945gt, which uses the fourth gig for devices.
AK> >
AK> > Now I can understand this explanation for 32-bit mode, but I'm running
AK> > in 64-bit mode.  There should be a way to use the fourth gig under
AK> > Linux.  Is there?
AK> 
AK> The chipset limitation applies to 64bit mode as well as to 32bit mode
AK> (which actually does not have a 4GB limitation with PAE)

Using a 64-bit or PAE kernel won't give access to more than 4GB of RAM
because that only increases the linear address space of the CPU to 48 or
36 bits respectively. The limitation with 945GT is that the chipset only
supports 32-bit physical addresses. As Andi already pointed out, parts of
that 4GB physical address space are used by devices and their apertures,
so you can only try to shrink those devices ranges, if configurable, but
you won't be able to use the full 4GB RAM.

Cheers,

	- Udo

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