Re: Accessing more than 2GB of memory with a 32 bit kernel

[JA MagallÃn]

On 06/12/2013 10:10 PM, Rik van Riel wrote:
> On 06/12/2013 12:54 PM, Simon Brown wrote:
> > Hello,
> >
> > For the sake of an old prototype peripheral I'm using a non PAE 32 bit
> > x86 kernel and I'm having trouble accessing memory above 2 GB. The
> > system has 4GB installed and all is well with a PAE kernel.
> >
> > I'm obviously expecting to lose some memory due to memory mapped devices
> > but I wasn't expecting to lose 2GB. Instead I'm suspecting a BIOS bug.
> > The system reports:
> > free -m
> >               total       used       free     shared    buffers
> >               cached
> > Mem:          2012        491       1521          0         40
> > 277
> >
> > The mtrr table looked odd so I enabled sanitisation:
> > [    0.000000] original variable MTRRs
> > [    0.000000] reg 0, base: 2GB, range: 2GB, type UC
> > [    0.000000] reg 1, base: 0GB, range: 4GB, type WB
> > [    0.000000] reg 2, base: 4GB, range: 2GB, type WB
> > [    0.000000] total RAM covered: 4096M
> > [    0.000000] Found optimal setting for mtrr clean up
> > [    0.000000]  gran_size: 64K  chunk_size: 64K         num_reg: 2
> > lose cover RAM: 0G
> > [    0.000000] New variable MTRRs
> > [    0.000000] reg 0, base: 0GB, range: 2GB, type WB
> > [    0.000000] reg 1, base: 4GB, range: 2GB, type WB
> >
> > I don't understand the gap in the new table.
>
> Check the e820 table. Chances are the BIOS is reserving 2GB to
> map various devices (especially video cards) below the 4GB limit.

Acording to manual, that mobo has an option to "Memory remap feature"
in BIOS that looks like that...

--
J.A. Magallon <jamagallon()ono!com>        \               Winter is coming...
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