Can I test 4GB DIMMs on XP? - 3.2GB is available, 800MB missing.

[David (PassMark)]

I had a quick look on the web, and there is a HUGE amount of misinformation about this limit on the web. Some people claim it is related to PAE, or the paging file, or DEP, etc... This is all wrong.

But you are essentially correct. Windows XP has a 4GB limit on the amount of physical memory it can address. But the BIOS and many of the hardware devices in a typical PC also use some of this physical address space. The video card being one of the big users. This is done for MMIO (Memory Mapped Input Output) and allows the hardware device to transfer data to and from the other devices in your system (mainly the CPU). This memory is allocated from the 4GB level down. The more devices you have installed the less address space you'll have left over for system RAM.

You can see this in the image below.



There is nothing you can do about this in Windows XP. If you have 4GB of RAM, between 600MB and 800MB of your RAM is wasted and not used nor can it be accessed. So it will appear like you only have between 3.2GB or 3.4GB of RAM available.

The real solution is to switch to Windows 64bit. The hardware devices will still be mapped into the same locations, but there is physical address space available for your entire 4GB.

There is a 64bit version of BurnInTest.

[jrxnelson]

I have a platform ... and I put four 1GB dimms in it.

WindowsXP-Pro reports 3192MB. I think this may be to PCI config space and chipset graphic memory.

Can I still run BitPRO to test 4GB?

Thx!