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I recently upgraded my computer's RAM to 4 GB. My 32-bit Windows installation shows only 3574 MB of the memory. How can I make Windows use the full amount of RAM?

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This was answered only this morning in… – Tom Aug 21 '09 at 10:42
This needs to be upped by like 2000 votes cause it seems like everyday that I run into yet another person that asks "where is my memory?" Microsoft should just take that out of the dialog box . . . – surfasb Jul 1 '11 at 4:16
Dude, you need a 64 bit OS. Seriously, start saving your money now. Then this question would become a moot point when you have a system that will actually support all the memory you can stuff in the motherboard. – Fiasco Labs Aug 18 '12 at 5:37
Feel carefully around the system unit for any moisture or dampness. It could be a 'memory leak'. – EleventhDoctor Sep 24 '13 at 10:07
@ala : Because Of the kernel memory split... (1Gb allocate by the kernel). It seems you're already using the /3gb switch... – user2284570 Apr 4 '14 at 20:06
up vote 61 down vote accepted

You can't:

See Dude, Where's My 4 Gigabytes of RAM?

if you want to fit memory and devices into a 32-bit address range: not all of the available 4GB of address space can be given over to memory.
So what actually happens if you go out and buy 4GB of memory for your PC?
There's a hole in your memory map for the IO. (Now it's only 25% of the total address space, but it's still a big hole.) So the bottom 3GB of your memory will be available, but there's an issue with that last 1GB.

The only practical solution is to install a 64-bit operating system. In Windows Vista and later, 32-bit and 64-bit license keys are interchangeable. If you can get Windows installation media for the 64-bit version of your operating system, you can reinstall using your original license key.

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I don't understand - isn't 2^32 4 gigabytes? – Matt Aug 21 '09 at 15:33
Ahh, nevermind, actually read the linked article – Matt Aug 21 '09 at 15:34
good to know this – LiuYan 刘研 Jun 13 '11 at 9:30
@VonC : What about this? – user2284570 Apr 4 '14 at 19:33
@user2284570 true, PAE can help, but who is still on WXp? ;) – VonC Apr 4 '14 at 19:36

First, Windows XP (32bit) only supports 4 GBs. That doesn't just apply to Windows XP. Instead, it applies to 32bit desktop Windows - 32bit Linux systems with PAE and many 32bit Windows Server editions support more than 4GB per the linked article. You will never see over 4 GBs if you are using a 32-bit Windows XP. However, I did find a more detail reason why your system shows less available ram than what is actually installed.

From The 3GB-not-4GB RAM problem (Microsoft MSDN blog):

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.

Significant chunks of address space below 4GB (the highest address accessible via 32-bit) get reserved for use by system hardware:

  • BIOS – including ACPI and legacy video support
  • PCI bus including bridges etc.
  • PCI Express support will reserve at least 256MB, up to 768MB depending on graphics card installed memory

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.

There is more information if you check out the source. However, you may be able to take advantage of the full 8 GBs installed if you followed these instructions.

Speaking from personal experience, this is no new problem. I ran in to this same problem a few years ago when I built a 4 GBs 32-bit Windows Vista system when Vista was first released. There were countless forum posts online related to this same topic.

Read also Microsoft KB 929605 - The system memory that is reported in the System Information dialog box in Windows Vista is less than you expect if 4 GB of RAM is installed.

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Microsoft documents here that Windows XP supports up to 128GB of RAM, if the underlying architecture is 64-bit. Can you please update your answer? – Dan Dascalescu Dec 30 '12 at 3:12
@SgtOJ : What about this? – user2284570 Apr 4 '14 at 19:45
@SgtOJ: The 4 GB RAM limit does NOT apply to "all 32-bit operating systems". A 32-bit OS that fully supports PAE (like Windows Server 2003) can happily use more than 4 GB RAM. "32 bits" restricts the virtual address space, not physical. – Jamie Hanrahan Sep 10 '14 at 19:59
@JamieHanrahan that still means a single program can't use more than 4GB. And I think he meant the IO reservation overhead is OS independent. – jiggunjer Dec 19 '15 at 17:57
@jiggunjer Nowhere in the question or the answer I was commenting on was it said or implied that what was being asked about was what a "single program" could use. Nor is the I/O reservation strictly OS independent. – Jamie Hanrahan Dec 19 '15 at 18:29

If you are using a 32Bit Windows XP, then it's not possible. Since there is not enough Address Space available for making use of the extra RAM

The best solution would be to upgrade to a 64-Bit Version of Windows as it supports 4+ GB of RAM.

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2^32 is enough "address space" to fit 4 GiB. You mistake address space and physical memory. Every process on modern Windows versions sees the full 4 GiB address space even though normally only 2 GiB are usable due to the UM/KM partitioning of the virtual address space. – 0xC0000022L Mar 29 '12 at 14:36
@0xC0000022L: That's not the issue being discussed here. – Ben Voigt Aug 31 '12 at 19:37
According to Microsoft, most releases of Windows 7 (including 32-bit), support up to 192GB of RAM. The only requirement is that the underlying architecture is 64-bit. So the best solution would be to upgrade from XP to Windows 7. Can you please update your answer? – Dan Dascalescu Dec 30 '12 at 3:11
@Shaharyar : What about this? – user2284570 Apr 4 '14 at 19:50

As the others have said, it's not possible.


Each piece of memory has an address. The operating system specifies the length of the address. For older operating systems the address length is 32 bits, which only allows 2 ^ 32 (4 294 967 296) addresses. Now, looking at that number, it might seem like your operating system should support 4 GB, but all other hardware (most significantly, the video ram in your video card) that has internal memory also gets mapped into that address space.

It's like you're trying to distribute 1,200,000 six digit telephone numbers.

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You are mixing up RAM and address space. – 0xC0000022L Mar 29 '12 at 14:34
@0xC0000022L: No he's not. He's talking about physical address space, while RAM must be mapped into to be usable. – Ben Voigt Aug 31 '12 at 19:36
@BenVoigt : What about this? – user2284570 Apr 4 '14 at 19:48
PAE is great, if all kernel code is written to use it. Unfortunately a lot of kernel code is drivers written by third parties who can barely get their stuff stable with a flat memory model. Enabling PAE requires throwing away all existing drivers with the flat model assumption. That's why Windows server enables PAE and client does not. See steveb's answer. – Ben Voigt Apr 4 '14 at 19:53
@BenVoigt : Since 1997, Drivers which assume memory <4GB are rare because ALL WDM drivers can be installed on Windows 2000/2003... – user2284570 Apr 4 '14 at 20:18

Old motherboard only supported 4Gb of physical RAM, modern ones support a lot more. The Intel P4 CPU could always address more than 4Gb (by using something called Physical Address Extension, or PAE)

Microstoft made a 'marketing decision' not to support more then 4Gb physical address space in the 'client' versions of all the 32bit versions of it's Operating Systems - that includes all of then from w2k to 32bit Windows 8 (if you visit the MS KB pages you will note that the 32bit Server versions can support a lot more - up to 128Gb I believe with the 32bit 'Enterprise Edition') - which is why 'proper' 32bit OS's (Linux etc) on exactly the same hardware could always access ALL the physical RAM.

(side note - 32bit Win7 users got so fed up with this 'stupid' limit that they 'hacked' the Kernal to allow ascces to the full physical RAM .. needless to say, this breaks your Licence and is actually illegal in US (DCMA)

Windows XP 32bit 'reserves' part of the lo 4Gb address space for driver use. There is no reason why you can't 'address' physical RAM beyond 4Gb (using PAE) however many OLDER DRIVERS around at the time of Windows XP Pro failed to do so and you would get a 'BSOD' error (modern drivers use PAE correctly and IN THEORY there is no reason why they need more than about 32Mb of 'lo' space pages (as a 'window') in total even to support a 2Gb Graphics card (the 2gb will be in 'hi' space i.e. above 4Gb mark)

In XP SP2, MS almost tripled the amount of address space 'reserved' for drivers in an effort to eliminate the BSOD's and it's been that ever since (MS don't want to 'debug' drivers == they want to sell you the 64bit OS instead). The ACTUAL hardware has almost zero effect on this 'reserved' space.

SO, if you have 4Gb or more of physical RAM, the most XP Pro sp2+ will see is about 3.25Gb .. the rest CAN be accessed using various different RAM Disk Drivers (not the MS one of course) = go look for something that uses the 'Invisible Address space' or 'high 4gb space' on XP Pro) .. using the 'missing' .75Gb as part of your 'virtual memory' can speed up your system ...

Note that the /3G switch can allow 32 bit applications to access up to 3Gb on a 32 bit OS (if they have the LargeAddressAware flag set in their .exe header - in which case they cabn access up to 4Gb on a 64bit OS) but on XP you need to specify /USERVA as well to stop the OS being 'squeezed out' of RAM :-)

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Windows XP SP1 support >4GB Of RAM if you add the /PAE switch to boot.ini – user2284570 Apr 4 '14 at 19:53

If you want to make use of the complete 4GB, the only option is to upgrade to a 64bit operating system:

  • Windox XP 64
  • Windows Vista 64
  • Linux
  • Max OS/X (ok, you may need a new PC for that as well)

The normal Windows XP is only 32bit, and this limits memory to 3.5gb max. Check the link VonC posted for details about this topic.

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Being 32 bit doesn't limit memory to 3.5Gb max. It's other devices eating the available address space, so 3.75Gb are possible too. -1 – MaxVT Aug 21 '09 at 11:45
That's why I said "check the link for details" :-) – FrankS Aug 21 '09 at 11:50
Correct. Microsoft lists that XP 64-bit supports up to 128GB of RAM. – Dan Dascalescu Dec 30 '12 at 3:13

If you use integrated graphics then you can turn that down, and that will mean the system will be able to address more of your system RAM.

But, best solution is definetly to get a x64 OS.

No reason not too anyways. Most programs work in x64 with XP/Vista/7 having a x86 emulation layer. Its just drivers that might be the issue.

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x64 drivers on Windows XP Pro x64 are garbage, but the Vista and 7 x64 drivers have been very mature for a while now. – MDMarra Aug 21 '09 at 11:38

it shows 3574MB Ram because thats how much RAM is FREE and available for the computer to use at any time. This figure is the RAM that your computer has total access to because the programs and settings pre-installed onto the PC use a set amount of RAM which, in your case, happens to be 426mb. My 6GB 64bit PC shows 5.8GB available RAM isntead of the full 6GBs. Your best bet would be to look into upgrading your PC

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If 3.5 GB is the free RAM, then why is Windows still dividing it into free RAM and used RAM? It's an operating system, it should know of everything, especially if something is using up 0.5 GB of memory. Installed programs don't take up RAM until they are started. – gronostaj Jul 2 '13 at 13:37

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