36bit still in use ?

[Eric Smith]

On Mon, Dec 1, 2014 at 5:03 AM, Lars Brinkhoff <lars at nocrew.org> wrote:
> Rob Doyle <radioengr at gmail.com> writes:
>> Given that a PDP-10 only supports a virtual address space of 256 KW,
>
> That's not quite right.  The "Model B" KL supported a 23-bit virtual
> address, and later clones extended that to 30 bits.

I'm not sure why DEC limited the section table on the Model B KL10 to
32 sections, resulting in the 8MW per-process virtual memory limit,
but they reserved 12 bits in the section number to allow for future
support of 4096 sections, which would result in a 1GW limit. There was
room in the EPT/UPT to expand the section table somewhat, perhaps to
as many as 128 entries, but obviously not to 4096 entries. There's
evidence that DEC intended to go to a two-level section table in
Jupiter, with the EPT/UPT having the first level of section table,
pointing to pages for the second level section tables. It's my
understanding that XKL followed that basic plan, though perhaps the
details might not match Jupiter (which doesn't matter since Jupiter
didn't ship).