[TUHS] /dev/drum

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Apr 26 08:55:51 AEST 2018

    > From: Johnny Billquist

    > PDP-11 have 8K pages.

Segments. :-) (This is an old argument between Johnny and me, I'm not trying
to re-open it, just yanking his chain... :-)

    > On a PDP-11, all your virtual memory was always there when the process
    > was on the CPU

In theory, at least (I don't know of an OS that made use of this), didn't the
memory management hardware allow the possibility to do demand-paging? I note
that Access Control Field value 0 is "non-resident".

Unix kinda-sorta used this stuff, to automatically extend the stack when the
user ran off the end of it (causing a trap).

    > you normally did not have demand paging, since that was not really
    > gaining you much on a PDP-11

Especially on the later machines, with more than 256KB of hardware main
memory. Maybe it might have been useful on the earlier ones (e.g. the -11/45).


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