Three out of Four Isn't Bad

[Roger Heflin]

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