Support for more than 4GB on x86 platform

[Klaasjan Brand]

On 8/15/05, Rick Wagner <wagnerric at condor.cxo.cpqcorp.net> wrote:
> Hi,
> 
> I have seen reference to kernel support for up to 32GB on 32 bit x86
> platforms, with supporting hardware (some versions of Xeon?).

Yes, the kernel can use up to 64Gb.
 
> Perhaps I used the wrong search terms, but I could not locate much detailed
> information on the subject with Google.  Specifically, I was told that the
> kernel could map the memory virtually, but the I/O sub-system could not
> handle physical I/O to address over 32 bits, thus I/O to high memory was done
> via bounce buffers (i.e., DMA to low memory, then CPU copy to high).

I'm no hardware guru, but I believe it also has to do with devices on
the PCI bus not being able to access the high memory.
 
> Can anyone point to some documentation on the large memory implementation I
> can use to confirm or refute what I was told?

Search for "linux high memory"; first hit is quite extensive:
http://kerneltrap.org/node/2450

Klaasjan