3 Gb 32bit OS

[robamb2002]

I've been researching the memory address capacity of a 32 bit operating system and have not found out why it only recognizes 3 Gb when the theoretical max is 4 Gb. Theoretically a 32 bit system should recognize 4 gigabytes and a 64 bit system should recognize 16 exabytes (16,000,000 Gb). My question is "where is the extra Gb?"

[kpauls]

One possibility is reserved address space for memory-mapped drivers file IO operations and such.

Looking at this MSDN article it looks like it's an arbitrary software limitation, but perhaps it's imposed in order to achieve 1:1 mapping of virtual to physical address space (faster gaming possibly?).

Just some semi-educated guesses.