Re: Three out of Four Isn't Bad

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On Wednesday 16 April 2008, Roger Heflin wrote:
> Dennis Gilmore wrote:
> > On Wednesday 16 April 2008, Bill Davidsen wrote:
> >> I just popped another 2GB in my development machine, 2GB to 4GB RAM, and
> >> only 3GB shows. Yes I looked at the 820 output, yes I tried PAE kernels,
> >> yes I checked the BIOS to see the the hole options were tried, other
> >> thoughts.
> >>
> >> FC6 running 2.6.22.14-72.fc6PAE, also 2.6.24.4 kernel.org build.
> >>
> >> dmesg hopefully attached if it doesn't get stripped again.
> >
> > I have a Dell D820 and while its a 64 bit system,  it has a 32 bit chip
> > in there somewhere so i can only use 3.25gb of thr 4gb that is installed
> > could very well be a hardware limitation somewhere.
> >
> > Dennis
>
> Actually, since most OSes are 32bit (read Windows) none of the bios makers
> set things up  so that the missing ram can be remapped over 4GB since that
> would be useless under windows.    So they just waste the memory generally,
> though sometimes there is a bios option to set things to non-windows OS, or
> change the memory mapping that will allow more to be used, but generally
> that is only with the higher end machines and even there (where a large
> number of the machines are used with 64bit OSes) the options don't always
> exist.
>
> On a normal desktop system generally you are out of luck and will at best
> get maybe 3.2-3.5 GB of ram out of 4GB, and sometimes less depending on how
> well be bios was setup.    The older the motherboard is the worse it gets.
>
> My 3 year old intel board lets me use 2.8GB out of 3GB, I am not sure how
> bad it would get if I added another GB, I would expect to at best get maybe
> .5 GB more.
>
> Do a "cat /proc/mtrr" to get a better idea of what the bios is telling
> linux to do.
>
> Mine looks like this:
>
> reg00: base=0x00000000 (   0MB), size=4096MB: write-back, count=1
> reg01: base=0xb0000000 (2816MB), size= 256MB: uncachable, count=1
> reg02: base=0xc0000000 (3072MB), size=1024MB: uncachable, count=1
>
> So 4GB total could be useable, but 256MB has been labeled uncacheable for
> something and 1024MB more is also labeled uncacheable, so with an actual
> 4GB I may not have any more memory than with 3GB of ram installed with this
> MB.
>
>                                     Roger

So i get 
[root@bratac ~]# cat /proc/mtrr
reg00: base=0x00000000 (   0MB), size=2048MB: write-back, count=1
reg01: base=0x80000000 (2048MB), size=1024MB: write-back, count=1
reg02: base=0xc0000000 (3072MB), size= 256MB: write-back, count=1
reg03: base=0xcf800000 (3320MB), size=   8MB: uncachable, count=1
reg04: base=0xcf700000 (3319MB), size=   1MB: uncachable, count=1
reg05: base=0xd0000000 (3328MB), size= 256MB: write-combining, count=1
there is 4gb installed

the bios doesnt show the other 768MB ram
http://www.interact-sw.co.uk/iangblog/2005/08/05/is3gbenough
somewhat describes the problem.  
The extra memory is being mapped for i/o devices

Dennis


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