[TUHS] Unix & Memory Management Units (MMU)

Erik E. Fair fair-tuhs at netbsd.org
Thu Dec 8 02:46:38 AEST 2016

Which version of Unix first ran on a computer with virtual addressing (address translation) so that a process with non-position independent code (PIC) can be loaded anywhere in RAM that the kernel decided to put it, and memory protection such that no process could accidentally or deliberately access RAM not allocated to it by the kernel (or a SIGSEGV would be delivered to it)?

Put another way, when did Unix processes stop playing Core War with each other? (OK, so long as no more than one is resident at a time, they can't play Core War with each other, but there still needs to be a mechanism to protect the kernel from inadvertent (or advertent) pointer use).

Which is to say, when did Unix run on (and properly use) computers with memory management units (MMU)?

My guess from a quick look at the history of the DEC PDP-11 is that the target computer was likely a PDP-11/35 or PDP-11/40 with a KT11-D "memory management" module.

One imagines that many pointer mistakes (bugs) in assembly or C were discovered and squashed in that version, modulo the historical unhappiness resulting from address zero containing a zero if dereferenced ("NULL pointers") in process address space.

What year did that come about?

By the time I got to Unix (2.8BSD on the Cory Hall DEC PDP-11/70), those features (virtual addresses, memory protection from the kernel) had apparently been part of Unix for a long time - certainly earlier than Version 6.

This is distinct from demand-paged virtual memory which so far as I know was developed on the DEC VAX-11.


	Erik <fair at netbsd.org>

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