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
