[pups] Unix and PDP11/20 (was PDP9?)

Dennis Ritchie dmr at plan9.bell-labs.com
Wed Sep 4 14:21:40 AEST 2002

As Billquist, Wilson and others pointed out, the first
versions of Unix did run on machines with no memory
mapping or protection, and indeed context-switching
was accomplished by swapping.  By 1973 we had the luxurious
11/45, to considerable relief.

I'm not positive about the logo on our first PDP-11.
On the earliest handbook I have, the front panel photo
just shows "PDP11", though inside the handbook
it does talk about the two models (11/10 and 11/20).
Both had the same KA11 processor, but the basic
11/10 sported 1024Kw ROM memory plus a generous
128 words of RAM, while the 11-20 had
4096Kw core RAM, and the ASR33 Teletype was included.
You could add more RAM to the 11/20.

Incidentally, the machine's handbook was a wonder.
In 104 pages (each 5.25x8 inches), it described the whole
system: not only the instruction set but the theory
of the Unibus (including some logic diagrams) together with
programming specifications for the TTY, the clock,
and the paper tape reader.


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