32-bit will read 4GB, but it cannot use it because it can only address 4GB. Here is a good explanation of it
http://www.maximumpc.com/forums/viewtopic.php?t=71236
Picture should be flipped upside down though, it starts with the highest address, which is the "bottom" of memory.
OS/Kernel takes the top.
something else very interesting is the 64 bit os's can address alot more memory, but the OS ensures that it is only allocated in blocks that can be addressed in 32bit registers, in case truncation occurs.
I just finished a research project on this for my Operating systems class.
here is an excerpt from my presentation notes
Virtual Memory
XP Pro uses 4kb pages
32Bit systems limited to 4GB virtual address space
2GB User Applications
2GB System Applications
All memory onboard is included, even video card.
64bit systems can address up to 8 terabytes of space
Although 8TB of space is addressable, the system insures that every memory allocation is under 2GB
Virtual Memory
64 Bit Virtual Memory Issues
Pointers are 64 bit, but the system will only allow up to 2GB because if a 64 bit pointer is truncated at 32bits the data is not lost
64 Bit XP main memory published limit is 128GB, although some packaging states 16GB is the limit and some users state problems with more than 16gb main memory