It's probably just your motherboard. Even though alot of motherboards state that they support up to 4gb, they will only read up to 3-3.5 gb of it. Look through your motherboard manual, it will most likely tell you this. I have a MSI K8N diamond Plus top of the line MB with 4x1gb OCZ platinum, but it only show up as 3 GB. You still need the 4th GB if you want to run dual channel. Has nothing to do with your operating system whatsoever, as the previous poster tried to explain. There is nothing you can do about it.
Firstly, this will affect systems with 4 x 1024 MB DIMMs, dual-channel or not, it will also affect Socket 754 platforms with 2 x 2048 MB DIMMs, and Socket 754 is single-channel by nature. It has affected workstation operating systems (lacking PAE-36) for years.
I can guarantee with Windows XP x64 Edition he'll see all 4 GB in System Properties and thus be able to address all of his physical memory. Versus now he only sees 3.25 GB in System Properties. (Assuming his platform as a whole supports x64 via EM64T or AMD64 extentions)
Various devices have used 'normally unused by memory' parts of the 32 bit address range for accelerated I/O for years, decades even. It is because now people have large amounts physical memory the range gets mapped to the devices for accelerated I/O, and thus can't be mapped to memory.
(
eg: Some BIOS have an option to recover part of the range, or decicate part of the range for SLI use. SLI Performance is substantially higher when a wide range is dedicated to it for accelerated I/O purposes.)
The same will also apply to memory stats in Task Manager, Performance tab, and what WinVer.exe reports.
Motherboard manuals 'water down' explanations.
Since you claim to know what causes it, can you please also perform the above Device Manager check and screenshot the results ?, or are you affraid you might learn something ?
Heck, I run a Tyan K8WE, and only get 2.75 GB of my 4 GB under Win32, leaving 2 'lopsided' NUMA nodes aswell as being unable to physically address all my memory under Win32 (because of heaps of devices using said parts of the 32 bit address range).
Assuming you know how chipset platforms work, and know basic hex, refer to this image:
As you can clearly see from B000,0000 to FFFF,FFFF is in use by devices on the PCI bus, and system board more towards the end of the 32 bit address space.
Being able to map physical memory in the 'available' range from 0000,0000 to B000,000 = gives a maximum mapping range of 2,952,790,016 bytes to physical memory by the (32 bit) Operating System. (Perhaps minus that 256 KB range from A,0000 to D,FFFF at certain times)
2,952,790,016 bytes = 2.75 GB - The exact amount my System Properties window displays.
With less hardware (than my example above) using the address space typical desktop machines will commonly have between 3 GB to 3.5 GB 'available range' to be mapped to physical memory.
This is one of many reasons why Microsoft have been offering free upgrades from Windows XP Pro to Windows XP x64 Edition (in exchange for your old licence though - Downside is having WinXP x64 only may limit which applications you can run in some circumstances. eg: No more backwards support for 16 bit EXEs, some installers are 16 bit and not 'substitued automatically' by the OS for 32 bit ones yet, x64 driver support for hardware, etc, etc...).
However, on a whole WinXP x64 Edition is no-where near as bad as people say it is, and the address range available to it is substantial, we are unlikely to have this 'problem' again (similar to when we went from 24-bit to 32-bit memory addressing. Yes many 16-bit processors, even some early 32-bit ones, only had a 24-bit address range at one time for RAM, back in the days when 4 MB was 'a lot of memory'.)
See:
http://en.wikipedia.org/wiki/Physical_Address_Extension ; for a general overview of PAE (64 GB = 36 bit address range). Even though the Windows Kernel supports it, it has only (recently) combined with the NX/XD-bits to enable Enhanced Virus Protection to stop code inserted using Buffer Over-runs from executing.
The PAE 36-bit addressing feature is 'disabled' in the kernel on the 'workstation' versions of Windows. It is only used for NX/XD bit trickery (if available) once Service Pack 2 is added to Windows XP Pro/Home. (Some people may notice it says 'Physical Address Extension' in their system properties, when using 4 GB or less, and this is why.
ie: The 'workstation' versions of the Windows OS can't use PAE to map/address over 4 GB, or up to 64 GB, of RAM. They can however use it for other purposes as Microsoft deam fit (such as combing it with the NX-bit to gain security). Remember to bear in mind typically 768 MB of the address space is used for accelerated I/O, and this is already becoming a smaller and smaller percentage of total memory.
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