I came across this while looking around Dennis Ritchie's web page.
They're a set of notes which date back to 1977, and are about the new
32bit computer in the works at DEC.  (i.e. the VAX)  They're mostly of
historical note.

  http://cm.bell-labs.com/cm/cs/who/dmr/vax1.html

One paragraph which caught my attention was the following:

   The memory mapping is not so good, mainly because it does not seem easy
   to use the very large virtual address space. If information is placed
   at random the page tables become huge (2^21 words!). However, the user
   page tables can themselves be paged, and this may provide an out.
   [...]

I remember my Computer Organization and Assembly Language professor
mentioning this, but I either figured I had mis-heard him or I thought
that he was on drugs.  I know that one of the issues we have with
NetBSD/vax is that we've got pretty a fairly limited maximum process size
because of the problems with large page tables.  Within the design of
NetBSD, would it be possible to make use of this feature of being able to
page out our page tables? :-)

-brian.
--- Brian Chase | bdc@world.std.com | http://world.std.com/~bdc/ -----