Sounds like a physical limitation of the motherboard, i once had a workstation motherboard with a similar issue. It supported 4 gigs but only showed 2.9 gigs usable with x64 bit. I later found out that it was the motherboard that had a physical limitation not the operating system.
Only way to fix this is to get a newer motherboard or if your bios supports it there should be an option in there.
This is the thread i got my information from my workstation i was building.
http://h30499.www3.hp.com/t5/Business-PCs-Deskpro-EVO/Memory-DC7600/td-p/619877
If your too lazy to read through it heres the main part:
Kevin,
I doubt you'll ever see this but just in case...
I have the same problem. It is NOT related to the OS, since I have tried Linux, Win XP and Win 2003 Enterprise (which has an 8gb limit).
I called HP and after 45 minutes I found out this is a known archiectural limit. HP emailed me the following.
I have an additional graphics card, and a RAID controller in the machine. HP suggest I take them out, and install Win 2003 64 bit to see if the problem goes away. Marvellous...
Hope this helps (explain it at least)
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Title: Personal Workstations - Total Memory Size Reported Incorrectly in Computers Equipped With 4GB Memory
Information
Total Memory Size Reported Incorrectly in Computers Equipped With 4GB Memory
Details
Symptomatic Description:
Total Memory Size Reported Incorrectly in Computers Equipped With 4GB Memory
Root Cause and Technical Description
DESCRIPTION:
The Operating System reported the memory size incorrectly in Workstaion equipped with 4GB of Memory Modules. However, the size of the memory modules are reported correctly in the F10 Setup and POST. Due to architecture reasons not all of the physical memory will be available in systems equipped with 4096 MB of RAM. This issue is not specific to HP workstations. Any current system capable of supporting 4 Gigabytes of RAM will show this issue. For example, the following video controllers report the different memory size in W8000 with 4GM Memory :
- Nvidia Quadro4 900 XGL : 3696 MB
- ATI FireGL 8800 : 3440 MB
- ATI Quadro2 Pro : 3696 MB
- Matrox G450 Dual Head : 3888 MB
SCOPE:
This issue can occur on any computer with 4GB memory.
ROOT CAUSE:
To get better performance many devices use Memory Mapped I/O (MMIO) to transfer data between system memory and the device. Typical devices are graphics adapters, network interface cards, SCSI cards, and IEEE 1394 devices.
To do MMIO a device will reserve a block of memory addresses. These addresses reside near the top of the addressable memory space. The MMIO range required for storage and I/O devices is usually small; somewhere around 1 MB each is typical. It can be much larger for devices like caching RAID cards.
The MMIO address requirements of graphics adapters are usually much higher. This larger MMIO address requirement is needed to achieve the high performance required of workstation class graphics. (This is also true of high-end consumer/game graphics devices.) It is standard practice for a graphics device to reserve a MMIO area equivalent to the size of the frame buffer so that 1:1 memory accesses can be performed.
Some current graphics adapters require twice the frame buffer. It is also common to see a few more megabytes of address space reserved in addition to the frame buffer for other uses. The AGP aperture memory is also an MMIO region. The AGP aperture is configurable in the BIOS setup and is typically set to 64 MB.
Current systems have 32 address lines. This means that they can address
4,294,967,296 bits or 4 GB. When 4GB of physical memory is installed, these reserved address ranges actually reside in physical memory, making that memory unavailable for the operating system.
There is another constraint that further reduces the available memory.
The PCI 2.2 specification dictates that root PCI bus must be allocated one block of MMIO addresses. This block of addresses is subdivided into the regions needed for each device on that PCI bus. And each of those device MMIO regions must be aligned on addresses that are multiples of the size of the region. For example, a 256MB region must be aligned on addresses that are multiples of 256MB (10000000h).
The system BIOS itself needs an MMIO range that is always fixed at the top very top of memory. The BIOS is typically 512 KB so it reserves FFF80000h to FFFFFFFFh. This range of addresses reserved by the BIOS cannot be moved.
In practice, altogether this means that the MMIO ranges at the top of the memory addresses become fragmented and more memory than expected is consumed by MMIO. Resolution
RESOLUTION:
This issue is not specific to HP workstations and the system is working as designed.
See HP White Paper for clarification:
http://www.hp.com/workstations/white_papers/docs/RAM_alloc_WindowsXP.pdf
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