[THIN] Re: Quick PAE question
- From: "Rick Mack" <ulrich.mack@xxxxxxxxx>
- To: thin@xxxxxxxxxxxxx
- Date: Sat, 13 Jan 2007 15:18:38 +1000
Hi Michael,
It depends on the hardware and bios version.
By convention the PCI address space is put on top of the 32 bit address
space, so the CPU can "talk" directly to the PCI registers and memory
arrays. Now we've also got PCI Express which sits above the PCI address
space. Multiple PCI busses can extend this usage as well.
If you've got hardware designed (or bios, see below) for 32 bit/4 GB, the
PCI/PCI express address space is above 4 GB and is basically invisible
because it's not in your normal 4GB RAM address space.
Where you've X64 hardware designed for more than 4 GB of RAM, then the
convention is to drop the PCI/PCI Express memory space down to between
3.5-4GB (generally PCI at
3.5 to 3.75, PCI express at 3.75 to 4 GB). The RAM you bought is all there
but you've got a distribution like RAM [0-3.5 GB], the PCI/PCIE hole
[3.5-4GB] and then the rest of your RAM [
4-4.5 GB]. Since the "normal" o.s. (not PAE or enterprise) can only see
physical RAM to 4 GB, it now appears as a memory hole.
If you've got PAE then the o.s. can use RAM above 4 GB so it can "see" the
extra 500 MB above 4 GB and you've got your 4 GB back.
When the 64-bit capable systems first started coming out, a lot of people
complained because they were losing at least 500 MB of useable memory. So
manufacturers either "fixed" the bios to move the memory hole back up "out
of sight", at least for Intel CPUs. I believe AMD CPUs have a fixed hole at
3.5-4 GB so there's no bios fix, just PAE.
So you've got at least 3 variables, motherboard design, bios revision and
cpu type that can affect what you see without PAE. With PAE it doesn't
matter.
So is /PAE a good thing? I'd have to say yes since it gives you at least
another 500 MB of useable RAM on your 4GB RAM systems.
regards,
Rick
On 1/13/07, Michael Pardee <pardeemp.list@xxxxxxxxx> wrote:
I don't remember the final answer, but was /PAE a good thing or a bad
thing?
We have a couple hundred IBM HS20 Blades, and on some Blades that OS sees
all 4GB of memory and on other it may see 3.2GB or 3.5GB of memory. Same
BIOS version on most. Adding /PAE we see it all but for some reason I
remember this being a bad thing instead of a good thing to do.
Windows2003 Standard Edition with SP1.
Thanks in advance.
--
Michael Pardee
www.blindsquirrel.org
--
Ulrich Mack
Commander Australia
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