wikipedia article

Marcus Watts mdw at
Tue Jun 13 06:45:34 UTC 2006

Various wrote:
> From: Otto Moerbeek <otto at>
> To: Ted Mittelstaedt <tedm at>
> > What was the bit size of the CPU's originally used to write UNIX in Bell
> > Labs?
> What's more, iirc the MMU of the pdp11 isn't what we call a MMU today,
> it could not even do paging.

The pdp-11 mmu could handle program relocation, segmentation (after
a fashion) and memory protection.  I'm not sure what more you
could expect from an mmu.  What you mean by "paging" is
probably "demand paging", which means the ability to run a program
without requiring that it be entirely resident.  The key
feature you need for that is a guarantee that any instruction fault
caused by missing memory can be either restarted or continued.
In most architectures that's a question of cpu design not mmu.

In the case of the pdp-11 that's mostly a moot point.  The pdp-11 only
provides for mapping the 64k of memory space into into 8 segments
(addressable on 64-byte "clicks") and there's just not much win to
demand paging 8 "pages".  (actually 6 x 8 pages; there was kernel,
user, and supervisor mode, & each had separate instruction and data
spaces, but supervisor mode was rarely used in Unix environments, and
only a few large user mode programs ran using split I/D space.)  For
what it's worth, though, I *think* it was possible to restart most
instructions on the /45 and /70, which were the "big" machines and the
primary target of most later pdp-11 work.  In fact, some use was made
of this feature -- automatic stack growth.  If you look through ancient
Unix source, you'll find interesting bits of kernel code that manage

There's actually a cheesy way to do demand paging with microprocessors
that don't support demand paging (such as the original 68000--another
"16 bit" machine).  The way to do this is to run two processors in parallel
but skewed by one instruction.  If the first one does a bad memory fetch,
then the second one will not have fetched the instruction causing the
fault so contains restartable machine state.  Masscomp sold a machine
like this once.

				-Marcus Watts

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