Martin Husemann <martin@duskware.de> writes:

> There is no support for using this extension in NetBSD yet (and I have
> zero clue how intrusive that would be)

There are two bits that are relevant in the CPUID mask there: PAE and
PSE-36. PAE is the "physical address extention" and is the more common
way of extending an x86 system to handle 36-bit physical addresses in
the pmap. Essentially, it makes the page table entries twice as large,
and breaks the page tables down as 2-9-9-12 bits instead of the
current 10-10-12 bits.

The pmap used by the amd64 port is very, very similar to what an
i386 with PAE would use. If you wanted to add PAE support, the amd64
pmap would be the place to look.

PSE-36 is a different, newer mechanism that also permits accessing
2^36 bytes of physical memory, but in a much "chunkier" way - by
increasing the page size to 4M, and keeping it there. I'm not sure
what the application of that would be, really.

        - Nathan