Holy cow, I wrote a book!
This is another non sequitur.
PAE increases the amount of physical memory that can be
addressed by the processor, but that is unrelated to virtual
(Remember that PAE stands for
Physical Address Extensions.)
PAE increases the physical address space
(the address space that the CPU can use
to access the memory chips on your computer) from 32 bits
to 36 on a Pentium 2,
for a theoretical maximum physical memory capacity of 64GB.
However, the size of a pointer variable hasn't changed.
It's still 32 bits (for a 32-bit processor), which means
that the virtual address space is still 4GB.
With PAE enabled, the page table and page directory entries
double in size (to accomodate the additional bits in the page frame),
which significantly increases the amount of memory required for
page tables and page directories (since each page table describes
only half as much memory as it used to).
Notice that this has as a consequence that
PAE and /3GB conflict with each other
to some degree.
If you turn on both PAE and /3GB, then the kernel will limit itself
to 16GB of physical memory.
there isn't enough address space in the kernel
to fit all the necessary memory bookkeeping into the 1GB of memory
you told the kernel to squeeze itself into.
(On an AMD processor, the physical address space expands to 40 bits,
for a theoretical maximum of 1TB. However, the memory manager uses only
37 of those bits, for an actual maximum of 128GB. Why?
For the same reason that the kernel limits itself to 16GB in /3GB mode:
Not enough address space.
It's time to move to 64-bit processors...)