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?

  • 2
    This was answered only this morning in superuser.com/questions/27006/…
    – Tom
    Aug 21, 2009 at 10:42
  • 3
    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, 2011 at 4:16
  • 2
    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. Aug 18, 2012 at 5:37
  • 2
    Feel carefully around the system unit for any moisture or dampness. It could be a 'memory leak'. Sep 24, 2013 at 10:07
  • @ala : Because Of the kernel memory split... (1Gb allocate by the kernel). It seems you're already using the /3gb switch... Apr 4, 2014 at 20:06

9 Answers 9


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.

  • I don't understand - isn't 2^32 4 gigabytes?
    – Matt
    Aug 21, 2009 at 15:33
  • 7
    Ahh, nevermind, actually read the linked article
    – Matt
    Aug 21, 2009 at 15:34
  • good to know this Jun 13, 2011 at 9:30
  • @VonC : What about this? Apr 4, 2014 at 19:33
  • @user2284570 true, PAE can help, but who is still on WXp? ;)
    – VonC
    Apr 4, 2014 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.

  • 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? Dec 30, 2012 at 3:12
  • @SgtOJ : What about this? Apr 4, 2014 at 19:45
  • 1
    @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. Sep 10, 2014 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, 2015 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. Dec 19, 2015 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.

  • 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. Mar 29, 2012 at 14:36
  • @0xC0000022L: That's not the issue being discussed here.
    – Ben Voigt
    Aug 31, 2012 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? Dec 30, 2012 at 3:11
  • @Shaharyar : What about this? Apr 4, 2014 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.

  • You are mixing up RAM and address space. Mar 29, 2012 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, 2012 at 19:36
  • @BenVoigt : What about this? Apr 4, 2014 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, 2014 at 19:53
  • 1
    @BenVoigt : Since 1997, Drivers which assume memory <4GB are rare because ALL WDM drivers can be installed on Windows 2000/2003... Apr 4, 2014 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 :-)

  • 1
    Windows XP SP1 support >4GB Of RAM if you add the /PAE switch to boot.ini Apr 4, 2014 at 19:53

It is a rather complicated question as it takes several components to work out. Those components are:

  • Central Processing Unit
  • Memory Controller Unit
  • BIOS / (U)EFI
  • Operating System
  • Application

The last item does not relate to the issue directly, but I've added it for a completeness sake.

In a nutshell, all of the above components have to support more than 4 GB of memory in order to succeed.

Central Processing Unit

Lets start with a CPU. But first it is important to note, that a CPU does not have any magical ability to use any memory at all. Instead it could just access any address within its addressable space. Now here comes the bitness-thing. 32-bit CPU is capable of accessing addresses in range from 0 to 4 GiB. This range however has to accommodate not only the available RAM, ROM, SMRAM, Graphics Address Remapping Table but also all the required Memory-mapped I/O and many more, which might require substantial part of this address range. As a consequence it might be perfectly possible, that some memory addresses were shadowed by those uses and therefore are not available for operations on RAM. In order to regain hidden memory it is necessary to extend the CPU accessible address space. In case of 32-bit CPU Physical Address Extension could help. Which allows addressing up to 64 GiB. 64-bit CPU operating in native 64-bit mode is by default capable of accessing up to 16 EiB, which is rather huge and does not require such tricks. But on the other hand it is impractical for a 64-bit CPU to use 64-bit addressing, so in reality they are still limited by their address bus to a lesser range than 16 EiB.

Memory Controller Unit

Initially it was located in Northbridge, but was moved onto the processor die by AMD beginning with their AMD64 processors and by Intel with their Nehalem processors. Even if a CPU with PAE is capable of accessing 64 GiB by itself, it still needs a MCU to access RAM. Simply put: a CPU accessing particular address gets its reply either from PCI device, BIOS ROM flash or MCU, if the address requested resides in RAM, and so on. It is not uncommon however for a MCU (be it in Northbridge or in CPU) to support much less addressable space than 64 GiB for marketing reasons for example.


BIOS / (U)EFI are also important as it is their duty to configure the hardware before anything can actually operate. Among other things they are responsible for mapping the entire processor addressable space with memory ranges of hardware devices, buses, RAM and so on. Interestingly it might happen that the whole RAM split in pieces and mapped at several available non-contiguous spots. The problem with BIOS is that it's developers might not expect more than 3 GiB of RAM usage within their system and as such BIOS will not properly map all of the memory to be used. Most likely this will not happen to (U)EFI.

Operating system

Operating system should also support addressing more than 4 GiB memory. To be more specific, it should configure a CPU to use PAE. Some 32-bit OSes are perfectly capable of using PAE and accessing more than 4 GiB, others have software-limit on maximum memory, even though they enable PAE for enhanced security (Windows XP being an example). Once again 64-bit OSes naturally do not have such difficulties and normally support more than 4 GiB of memory.


The last point is from a different world, since applications commonly operate in a Virtual Address Space, I have added it for a completeness sake. Nonetheless it takes some additional steps while compiling for application to be able to use more memory. By default 32-bit application only capable of using 2 GiB of memory. The same 32-bit application could address up to 4 GiB of memory, provided that OS does support that and has enabled CPU PAE. More specifically, it could use up to 3 GiB of memory, the other 1 GiB would be addressable as common libraries and OS kernel, but not writeable (ultimately this depends on OS architecture rather than application itself). Again, 64-bit applications natively support addressing more than 4 GiB of memory and do not have such troubles at the first place.

To put all of this into perspective, lets consider several examples.

Consider Intel® 82945G Memory Controller. Intel website states, that it does not support PAE and is limited to 4 GB of RAM. Lets consult paragraph 9 System Address Map of the datasheet. It has lots of information. Most interesting is Figure 9-3. Main Memory Address Range: Figure 9-3. Main Memory Address Range and the paragraph 9.2 Main Memory Address Range (1 MB to TOLUD), which reads as follows:

This address range extends from 1 MB to the top of physical memory that is permitted to be accessible by the (G)MCH (as programmed in the TOLUD register). All accesses to addresses within this range are forwarded by the (G)MCH to main memory unless they fall into the optional TSEG, optional ISA Hole, or optional IGD stolen VGA memory. The (G)MCH provides a maximum main memory address decode space of 4 GB (2 GB for the 82945GC/82945GZ /82945PL). The (G)MCH does not remap APIC or PCI Express memory space. This means that as the amount of physical memory populated in the system reaches 4 GB (2 GB for the 82945GC/82945GZ/82945PL), there will be physical memory that exists yet is non- addressable and therefore unusable by the system.

As it says, since it does not support PAE, it is ultimately unable to remap PCI and every other MMIO above 4 GiB, therefore some of the memory will be shadowed and unused. Which means that even if it is used with a CPU supporting 36-bit addressing, memory above 4 GiB will still be unavailable and some of the memory close to 4 GiB limit will also be unused.

Conversely, lets consider Intel® 82955X Memory Controller, which is related to the previous one, but it is specified to support 8 GB contrary to the previous one! Datasheet paragraph 2.1 Host Interface Signals suggests, that it does use 36-bit address bus to the CPU, which is a good sign. Paragraph 7 System Address Map has further insights. Compare Figure 7-1. System Address Ranges to the one we saw before: Figure 7-1. System Address Ranges This clearly suggests that it is capable of utilizing more than 4 GiB of RAM, provided that it is used with the CPU also capable of 36-bit addressing. It is also worth noting that paragraph 7 System Address Map states that MCH supports 64 GB of addressable memory space. However it is further clarified, that maximum usable RAM is limited to 8 GB. This once again declares the difference between addressable and usable as I was trying to convey in the CPU section.

To conclude this, I'd like to note that two related chipsets have in fact huge difference in memory handling.

Lets inspect Intel® Pentium® Processor Extreme Edition 840. It does support 64-bit instruction set, but does not support PAE. Which means that if run in 32-bit mode, it will be unable to use more than 4 GiB of RAM (please, correct me if I'm wrong with this assumption). Lets check the datasheet and table Table 4-3. Signal Description. Even though that this CPU is 64-bit capable, it is still limited by it's address bus being 36-bit wide, thus limiting it to accessible address range starting from 0 to 64 GiB.

Now lets examine a more recent CPU. Take Intel® Core™ i7-7700 Processor for example. It is advertised to support up to 64 GB of RAM. Now lets check the datasheet of it. Paragraph 2.3 System Address Map convinces us that this CPU's addressable range is from 0 up to 512 GiB due to 39-bit address bus. Note that it is still far from the theoretical 16 EiB range for a 64-bit CPU. Surprisingly though it further clarifies that this CPU does support up to 32 GiB of usable RAM maximum (contrary to 64 GB on the website). There is also an image of interest here Figure 2-2. System Address Range Example: Figure 2-2. System Address Range Example It does a good job illustrating the issue with memory shadowing by PCI devices etc. Also it suggests a way of overcoming this issue by mapping a range from real RAM to upper address range within CPU-addressable range. Thus reclaiming that hidden memory for use. Note that this remapping is done by BIOS/(U)EFI upon hardware initialization and it is exactly for this reason I did include BIOS/(U)EFI in bullet-list above.

I have taken Intel products as an examples, but the same is true for AMD.

Now lets discuss how Operating Systems are involved. The Linux kernel includes full PAE mode support starting with version 2.3.23 and it is perfectly possible to have more than 4 GiB of memory usable with Linux kernel on a 32-bit system. However be aware of an undetermined issue with 32-bit Linux kernel on a system with more than 4 GiB of RAM on some hardware (in my experience it has something to do with TLB shootdowns). Anyway Wikipedia has a good information on PAE support. Interesting is how Windows XP is limited to 4 GB, whereas Windows Server 2003 Enterprise/Datacenter SP2 is capable of full 64 GB. Same is for other Desktop-Server Windows pairs.

As you can see there are many combinations among components listed and in some cases replacing a CPU or OS might help. It is much harder to deal with BIOS/(U)EFI or MCU since the former is proprietary and built-in into the system and not easily modifiable, the latter is built into the CPU or soldered on board.


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.

  • 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, 2009 at 11:45
  • That's why I said "check the link for details" :-)
    – FrankS
    Aug 21, 2009 at 11:50
  • Correct. Microsoft lists that XP 64-bit supports up to 128GB of RAM. Dec 30, 2012 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.

  • 1
    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, 2009 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

  • 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, 2013 at 13:37

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