Support for more than 4GB on x86 platform

Rick Wagner wagnerric at condor.cxo.cpqcorp.net
Tue Aug 16 17:22:25 UTC 2005


On Tuesday 16 August 2005 7:17 am, Klaasjan Brand wrote:
> 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


Thanks Klaasjan.  This was very helpful. 

It mentions the PCI bus limitation, but I'm wondering about PCI-X and PCI-E, 
which I think go beyond 32 bit addresses.  I'll keep digging, and see if 
there are more up to date articles; this one was based on 2.4, with mention 
of 2.6.

Thanks again,
	--rick




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