Why is it that 32bit Windows XP can only address 3GB [duplicate]

Possible Duplicates:
Windows XP and RAM 3.5GB+
How to Check RAM Size without Looking at Motherboard

If Windows XP is a 32-bit operating system, and 2^32 equals 4GB, why can't it support 4GB of memory?

There are other types of memory that also need to be accessed, such as the video memory.

512mb is often used for your vram while the other 3.5 is used for your system ram.

This is an in-depth discussion of the memory limitations of 32 bit technology and why you almost never see 4 GB of RAM in 32 bit systems. Written by a recognized expert in Windows systems, Mark Russinovich.

http://blogs.technet.com/markrussinovich/archive/2008/07/21/3092070.aspx