Windows XP supports 4GB of RAM, period.
Many people say that Windows XP doesn’t fully support 4GB of RAM. That’s not true, because Windows XP supports exactly 4GB of usuable RAM, by using PAE.
If you can’t use 4GB of RAM, and have PAE enabled, you have bought hardware that doesn’t support 4GB of RAM. There’s a KB entry, which details some of the problems. If you want to use the full 4GB, buy better hardware.
Craig:
I do not agree with your statement “If you can’t use 4GB of RAM, and have PAE enabled, you have bought hardware that doesn’t support 4GB of RAM”…
My BIOS reports 4GB RAM as does Intel Desktop Control utility (run from Windows). I have enabled PAE and the /3GB switch and combinations of the two but the amount of RAM Windows reports does not change by 1 byte.
The Microsoft KB article explains the symptoms nicely but is about as much help as an ashtray on a motorcycle.
24. October, 2007, 09:55neil@anon.penet.fi:
The grandparent is omitting the fact that many hardware devices occupy a portion of the physical address space. You’re very unlikely to have the full 4GB available to you with XP SP2 even with PAE enabled (which is the default by the way.)
1. December, 2007, 04:46wizualizacje:
Thanks for the info.
4. February, 2008, 15:37djswarm:
Approximately 0.5 gb of ram is resevered by the system for the PCI-e bus, shared video ram and what not, plus drivers often eat up some of it. MS went from reporting total RAM to reporting available RAM so you’ll never see more than about 3.5 gb on a 4 gb system no matter what switches you use.
Some chipsets will let you tweek this number, tweek at your own risk, but on the whole XP is just being honest for once about what is there.
/PAE introduces some issues of its own since most client drivers are written without any expectation of /PAE. See the MS KB on XP and /PAE for details.
Personally I would say if you need gobs of memory you really should be running 2003 x64 or 2008 (xp x64 sucks) as they handel 4+ gb way more efficiently. Or you could use a real OS like RHEL 5.1 x64 :P
XP is rumored to not really use the second GB very effectively. I say rumored cause I haven’t had the need to verify that, but the people who told me were uber geeks I trust to know such things.
8. February, 2008, 08:26Computer Guy:
If XP is reporting what ram I can actually use, why does a single stick of 512, say 512?
Two 1Gb sticks say 2Gb.
I think you have a flaw in your logic, and no, Red Hat 5 is not the answer. ( far from it )
If you must use windows, and really need lots of memory, Windows XP x64 is the way to go.
But, if you are going to have lots of memory, do yourself a favor, get a RAID card.
Here is a box I built a few months ago, and it is SCREAMING fast
20. April, 2008, 09:18Intel MB
Xeon dual-core 2.4
8Gb PC6400
XP x64
3 SATA II drives RAID-5 ( pci adapter card )
PCI-x16 512Mb video
so yeah...:
^ The problem is that XP32 has a 4GB address space - it can only address 2^32 addresses. These addresses are taken up not just by your main system RAM, but also graphics card RAM and other things.
So if you have 2 GB of RAM you don’t have any problems because XP32 can address the 2GB of your main RAM as well as perhaps 512MB of video memory and some amount of other addresses used for other stuff. The total of all these addresses is less than 2^32.
With 4GB of RAM in your computer, the total number of addresses will be greater than 2^32, and some portion of your main RAM will be inaccessible.
24. April, 2008, 21:51Ernest:
I have 4 gigs of ram, and an XP/Vista32 dual boot system. I have PAE enabled, but XP still only sees 2 gb of ram, and vista sees all 4 gb. So here we are. :(
7. June, 2008, 06:13Brent Williams:
Just installed 4 gigs of ram into my XP system. (Had 2 gigs - saw all of it, upgraded to 4 gigs) Bios sees all 4 gigs but system only sees 2.75 gigs. I have two 512 meg 8800 GT’s running in SLI. My friends computer is practically identical except he only has 1 8800 GT and his available ram is 3.3 gigs.
Are my video cards sucking up a gig of my available ram? Seems odd. I’m running an nVidia 680i SLi motherboard, Intel QX6850 Extreme Quad Core, overclocked to 3.66, dual 8800 GT’s in SLi mode, 2 SATA 750 gig drives in RAID 0 mode (1.36T stripe) and 4 gigs (4 x 1 gig) Corsair Dominator DDR2 ram running at 2.2 volts (1066 MGz overclocked). Was it worth buying the extra 2 gigs or what? If I’m only seeing an extra 750 megs of ram out of it?
19. June, 2008, 15:43nick:
I also have 4gb ram installed on my mobo, xp pro only sees 2.5gb with my dual 7900gto’s 512 cards.
20. June, 2008, 20:22i have a msi p6n 680i mobo, with 4x 1gb corsair value select 667. bios and every other program sees the 4 sticks, apart from xp and vista ultimate 32(dual boot).
KJ:
Video RAM is included in Windows’s address space. So, of you have 1024 MB of video RAM, Windows will have only 3GB of address space left for system RAM. An additional 500MB of address space is usually needed for other hardware, leaving you with 2.5GB of available RAM.
12. July, 2008, 05:51Steven Kirkland:
I believe the greatest problem in the conveyance of understanding is in the explanation of Windows XP and memory. Aside from ≈ 500MB of memory map addressing for BIOS/drivers/other basic hardware, you also lose part of that memory addressing to video ram. Simply put, you’re not losing ram to your system requiring it, XP worked for years on 128MB using the same 4GB addressing for hardware, you’re losing the upper portions of `memory addressing` space starting at 4GB and working it’s way downwards dependent upon how much is required for windows to use your goodies.
22. July, 2008, 19:44Charles:
This “issue” is not a Windows XP issue, but a 32-bit system issue. As said above, 32-bit means a maximum of 2^32 bytes of addresses. And these addresses does not mean it should be only your RAM addresses, but your hole computer hardware addresses.
So, again, if you have 4GB of RAM, it won’t use all of it, because the system requires some other addresses (like those 512MB from your video memory), and then winXP 32-bit just won’t be able to access your hole RAM memory addresses.
In 64-bit systems, 2^64 = 128GB. So you can use your, let’s say, 8GB of RAM plus a dual 512MB nvidia cards and there will still be plenty of addresses for the operating system use.
12. August, 2008, 17:19andrew:
where can i find this pae on my system please.
23. August, 2008, 11:57thanks in advance
David Reis:
Enable 4GB tuning and PAE.
Just read:
http://www.cadforum.cz/cadforum_en/qaID.asp?tip=5551
If you want to make accessible 3GB RAM for your CAD applications in Windows Vista, use the BCDedit.exe tool (Boot Configuration Data Editor). The 3GB RAM mode can be enabled by the command:
bcdedit /set IncreaseUserVa 3072
and disabled by the command:
bcdedit /deletevalue IncreaseUserVa
Run this command from a command window with Administrator priviledge - i.e. (in the Start menu) type CMD and press Ctrl+Shift+Enter, or select “Command Prompt” (Accessories), right-click on it and choose “Run as Administrator”.
…
You could download and install VistaBootPRO 3.3 and enable Physical Extension Addressing on windows vista too.
But on taskmgr you will only see 3gb available.
28. August, 2008, 04:48David:
Although thi is an old issue, i still see fairly recent comments-questions that clerly show that sometimes the answer to issue is not properly understood.
This is the same explenation through an over simplified analogy:
To people to be able to recieve mail thy must have an address that will enable the post office to know to where to direct it to. Lets say for this analogy that for technical reasons the post office can only allocate addresses in the form of two digit numbers - that means 0-99 (covers 100 houses). If the region holds 120 houses, 30 of which have high priority to mail what will happen then? Those 30 will recieve an address and will have mail delivered to them, from the other 90 houses only 70 of them can get addresses, all the rest wont be able to get mail(they have no address).
In terms of the memory mapping - 2^32 = 4GB is the number of addresses available. The system resources (System ROM, APIC(s), Integrated PCI devices - such as network connectors and SCSI controllers, Graphics card and more) have priority (otherwise they wont be accessible) and the rest - Less then 4GB- is left for the RAM.
5. November, 2008, 18:48Now its not that the remainder is used for anything\eaten up and so on - there simply not mapped, which means they have no address that can be used to refer to them for read\write operations.