Using 4GB of Ram with XP Pro at DVinfo.net

Jeff Harper · March 30th, 2008, 09:11 AM

For those interested it is possible to utilize up to 4 GB of ram with XP pro in a 32 bit environment. Simply add a switch to your boot.ini file. The switch is /PAE, shut down, then re-start.

The 4GB of ram still WILL NOT show up, but my understanding is that it will still be utilized.

This is fully supported by Microsoft and is documented on their website. This is not a hack.

Because it doesn't show up there are those who believe the additional memory isn't working, but that is not necessarily the case.

If you are interested further, Google "XP pae switch" etc., and read up on it.

Kim Olsson · March 30th, 2008, 03:16 PM

Actually, yes Windows can read the extra 1GB RAM, but only for hardware...

Due to an architectural decision made long ago, if you have 4GB of physical RAM installed, Windows is only able to report a portion of the physical 4GB of RAM (ranges from ~2.75GB to 3.5GB depending on the devices installed, motherboard's chipset & BIOS).

This behavior is due to "memory mapped IO reservations". Those reservations overlay the physical address space and mask out those physical addresses so that they cannot be used for working memory. This is independent of the OS running on the machine.

What this means is a typical system may see between ~256MB and 1GB of address space below 4GB reserved for hardware use that the OS cannot access. Intel chipset specs are pretty good at explaining what address ranges gets reserved by default and in some cases call out that 1.5GB is always reserved and thus inaccessible to Windows.

When looking at memory in systems (be it desktop or notebook) there are three questions to ask that will tell you the maximum amount of memory your O/S will be able to use:

1. What O/S Edition have you installed?

a. 32-bit Windows is limited to a maximum of 4GB and cannot see any pages above 4GB.

b. 64-bit Windows can use between 8GB and 128GB depending on the OS

Jeff Harper · March 30th, 2008, 04:05 PM

I didn't know that the extra ram is available for hardware only, but I'll take it! Especially for printing while multi-tasking. Printing seems to take an inordinate amount of memory.

Matthew Chaboud · March 31st, 2008, 11:12 AM

XP/SP2 uses the PAE kernel by default, but the decision was made with SP2 to stop giving drivers addresses above 4GB for stability reasons.

Translation: That RAM ain't gettin' used, sadly.

Still, it seems that 3.18GB is a common number, and you'll be ready when you push up to an x64 OS (provided that you have drivers for everything).

John Miller · March 31st, 2008, 11:27 AM

I suspect the reason was more to do with marketing. i.e., the client versions of Windows are much, much cheaper that the server versions. The latter can take full advantage of PAE. The clients have the same kernel (someone proved this with a hack that turned XP into Server 2003). The PAE limitations are described here:

http://www.microsoft.com/whdc/system...AE/PAEdrv.mspx

Jeff Harper · March 31st, 2008, 11:56 AM

Yes, Matthew, I'm trying out Vista 64 bit later this week and doubling my memory. I expect lots of drama, but I am getting bored with my machine. I want to fully prepared for Vegas 64 bit, for which I have unrealistically high hopes for. Why is it I always expect the next version or upgrade to be the "magic" one?

I have been looking at the 8 core, but still too expensive, and my current quad really flies. 8 cores will be nice for cutting down time rendering two-pass renders, etc, but at this point I just can't justify upgrading hardware.

March 30th, 2008, 09:11 AM	#1
Jeff Harper Inner Circle Join Date: Jun 2005 Location: Cincinnati, OH Posts: 8,425	Using 4GB of Ram with XP Pro For those interested it is possible to utilize up to 4 GB of ram with XP pro in a 32 bit environment. Simply add a switch to your boot.ini file. The switch is /PAE, shut down, then re-start. The 4GB of ram still WILL NOT show up, but my understanding is that it will still be utilized. This is fully supported by Microsoft and is documented on their website. This is not a hack. Because it doesn't show up there are those who believe the additional memory isn't working, but that is not necessarily the case. If you are interested further, Google "XP pae switch" etc., and read up on it. Last edited by Jeff Harper; March 30th, 2008 at 12:05 PM.

March 30th, 2008, 03:16 PM	#2
Kim Olsson Major Player Join Date: Oct 2007 Location: Sweden Posts: 263	Actually, yes Windows can read the extra 1GB RAM, but only for hardware... Due to an architectural decision made long ago, if you have 4GB of physical RAM installed, Windows is only able to report a portion of the physical 4GB of RAM (ranges from ~2.75GB to 3.5GB depending on the devices installed, motherboard's chipset & BIOS). This behavior is due to "memory mapped IO reservations". Those reservations overlay the physical address space and mask out those physical addresses so that they cannot be used for working memory. This is independent of the OS running on the machine. What this means is a typical system may see between ~256MB and 1GB of address space below 4GB reserved for hardware use that the OS cannot access. Intel chipset specs are pretty good at explaining what address ranges gets reserved by default and in some cases call out that 1.5GB is always reserved and thus inaccessible to Windows. When looking at memory in systems (be it desktop or notebook) there are three questions to ask that will tell you the maximum amount of memory your O/S will be able to use: 1. What O/S Edition have you installed? a. 32-bit Windows is limited to a maximum of 4GB and cannot see any pages above 4GB. b. 64-bit Windows can use between 8GB and 128GB depending on the OS

March 30th, 2008, 04:05 PM	#3
Jeff Harper Inner Circle Join Date: Jun 2005 Location: Cincinnati, OH Posts: 8,425	I didn't know that the extra ram is available for hardware only, but I'll take it! Especially for printing while multi-tasking. Printing seems to take an inordinate amount of memory.

March 31st, 2008, 11:12 AM	#4
Matthew Chaboud Regular Crew Join Date: Sep 2006 Location: Chicago, IL, USA Posts: 93	XP/SP2 uses the PAE kernel by default, but the decision was made with SP2 to stop giving drivers addresses above 4GB for stability reasons. Translation: That RAM ain't gettin' used, sadly. Still, it seems that 3.18GB is a common number, and you'll be ready when you push up to an x64 OS (provided that you have drivers for everything).

March 31st, 2008, 11:27 AM	#5
John Miller Major Player Join Date: Oct 2005 Location: Hillsborough, NC, USA Posts: 968	I suspect the reason was more to do with marketing. i.e., the client versions of Windows are much, much cheaper that the server versions. The latter can take full advantage of PAE. The clients have the same kernel (someone proved this with a hack that turned XP into Server 2003). The PAE limitations are described here: http://www.microsoft.com/whdc/system...AE/PAEdrv.mspx

March 31st, 2008, 11:56 AM	#6
Jeff Harper Inner Circle Join Date: Jun 2005 Location: Cincinnati, OH Posts: 8,425	Yes, Matthew, I'm trying out Vista 64 bit later this week and doubling my memory. I expect lots of drama, but I am getting bored with my machine. I want to fully prepared for Vegas 64 bit, for which I have unrealistically high hopes for. Why is it I always expect the next version or upgrade to be the "magic" one? I have been looking at the 8 core, but still too expensive, and my current quad really flies. 8 cores will be nice for cutting down time rendering two-pass renders, etc, but at this point I just can't justify upgrading hardware.