Myth: PAE increases the virtual address space beyond 4GB

re: Myth: PAE increases the virtual address space beyond 4GB

neli — Wed, 18 Aug 2004 14:51:00 GMT

LOL, there is 1TB memory available, but there is no addressing space left to map it into :).

re: Myth: PAE increases the virtual address space beyond 4GB

vince — Wed, 18 Aug 2004 15:11:00 GMT

Wouldn't increasing the page size also alleviate this? (Let's pretend for a second that wouldn't break a bunch of applications that don't correctly query the page size from the OS)

I would think bigger page size = less address mapping entries, and thus more physical memory could be mapped (obviously, in different processes).

re: Myth: PAE increases the virtual address space beyond 4GB

Miles Archer — Wed, 18 Aug 2004 15:32:00 GMT

It's time to move to 64-bit processors...

You said it, man. I suspect the floodgates will open once MS releases 64 bit windows for the Athlon-64.

re: Myth: PAE increases the virtual address space beyond 4GB

Steve Dispensa — Wed, 18 Aug 2004 15:45:00 GMT

I have a dual opteron 240 that I use to test drivers with, and I've been running the amd64 preview of winxp for a while now. It works really well, and it is super fast. I agree with you on the Athalon 64 - it's gonna be a killer.

re: Myth: PAE increases the virtual address space beyond 4GB

timchen — Wed, 18 Aug 2004 16:05:00 GMT

There is one thing quite interesting about /PAE. On some machines with 4GB RAM installed, Task Manager shows only 3.5GB physical memory. However, if you switch to the PAE kernel, all 4GB is shown.

Some says that it's because the BIOS reserves 512MB "physical" address space just below the 4GB line and put the real 512MB RAM above the line. Thus only the PAE kernel sees that 512MB RAM.

I'm not sure if the explanation is true or not, but the symptom is confirmed.

re: Myth: PAE increases the virtual address space beyond 4GB

Claus — Wed, 18 Aug 2004 17:47:00 GMT

I have also heard that BIOS story, about how it hides memory from task manager, but I'd sure love to hear that story confirmed or refuted by someone "in the know"....

Claus

re: Myth: PAE increases the virtual address space beyond 4GB

Carmen Crincoli — Wed, 18 Aug 2004 18:43:00 GMT

That's exactly what happens. The BIOS (or sometimes devices) remap the memory above the 4GB range, which means without /PAE, Windows can't address it. Once you turn on /PAE, we can see it all again.

re: Myth: PAE increases the virtual address space beyond 4GB

Ben Hutchings — Wed, 18 Aug 2004 20:34:00 GMT

A standard AGP or PCI bus only has 32-bit address lines so MMIO areas for video, network and disk interfaces among other things normally need to be within the first 4 GB of physical memory. (Some hardware allows remapping between PCI and system bus addresses, but I don't think this is normally done on the PC aside from the use of GARTs for video.) So in order for a standard PC to do anything useful it can't have a whole 4 GB of RAM in the first 4 GB of physical address space, which is why the BIOS prevents that.

re: Myth: PAE increases the virtual address space beyond 4GB

Jeremy Morton — Wed, 18 Aug 2004 22:38:00 GMT

Hmmm, I wonder if that's why our 32GB /PAE machine lists the memory as 31.0 GB.

re: Myth: PAE increases the virtual address space beyond 4GB

Myron A. Semack — Thu, 19 Aug 2004 12:27:00 GMT

Memory-mapped hardware (like almost every PCI device in your system) eats up a chunk of the 4GB physical address space. For example, one of the boards I'm writing drivers for now has a 4MB and an 8MB address window. This cuts into the amount of RAM Windows can see (with 32-bit addressing anyway).

There are some 64-bit PCI devices, which can live above 4GB, but to use them (as a 64-bit device) you need a 64-bit PCI motherboard.

Some devices are I/O mapped (usually legacy ISA devices), which don't eat into the 4GB address space. It's pretty rare to see an exclusively I/O mapped device these days, though.

Enabling PAE increases the physical address space, so Windows can fit all 4GB of RAM AND your PCI devices.

re: Myth: PAE increases the virtual address space beyond 4GB

DrPizza — Thu, 19 Aug 2004 12:29:00 GMT

Of course, 64-bit machines have the same problem with 32-bit PCI devices.

The AMD abominations have an on-CPU hardware IOMMU to make this work (the 32-bit devices think that what they're getting is memory below 4 GiB but it could be anywhere). Intel's x86-64 implementations don't; it'd go on the Northbridge for Intel, but their chipsets don't have one.

AMD's IOMMU is a hacked up AGP GART. It's not as capable as a purpose-built IOMMU would probably be, and so inefficient in comparison (to e.g. SPARC64). But it's still better than no IOMMU at all (which is what you get on Intel).

The proper solution, of course, is to use 64-bit PCI devices (PCI-X, PCIe, both of which notionally require 64-bit support, though both of which have devices that are broken and only really work with 32-bit addresses...).

The story of the 3GB server switch

Savage Nomads — Fri, 20 Aug 2004 20:43:00 GMT

  As Evan already mentioned on his blog, Raymond Chen has a great series on /3GB switch on his blog. What is really cool is that Raymond takes on some myths about the /3GB switch and  the fact that he...

Windebug: 3GB and PAE switches « Salt * Wet * Bytes

Windebug: 3GB and PAE switches « Salt * Wet * Bytes — Thu, 13 Mar 2008 07:43:16 GMT

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