[TUHS] Harvard and Von Neumann Architectures and Unix

Noel Chiappa jnc at mercury.lcs.mit.edu
Sat Nov 25 07:43:42 AEST 2017

    > From: Will Senn <will.senn at gmail.com>

    > I am curious about how the Harvard Architecture relates to Unix,
    > historically. If the Harvard Architecture is predicated on the
    > separation of code from data in order to prevent self-modifying code (my
    > interpretation)

That's not the 'dictionary' definition, which is 'separate paths for
instructions and data'. But let's go with the 'no self-modifying code' one for
the moment.

The thing is that self-modifying code is pretty much an artifact of the dawn
of computers, before the economics of gates moved from that of tubes, to
transistors, and also before people understood how important good support for
subroutines was. (This latter is a reference to how Whirlwind did subroutines,
with self-modifying code.) Once people had index registers, and lots of
registers in general, self-modifying code (except for a few small, special
hacks like bootstraps which had to fit in tiny spaces) became as dead as the

It's just a Bad Idea.

    > then it would seem to me to be somewhat at odds with a Unix philosophy
    > of extreme abstraction (code, data, it's all 0's and 1's, after all).

The people who built Unix were fundamentally very practical. Self-modifing
code is not 'practical'. (And note that Unix from V4:


onward has support for pure text - for practical reasons).

    > the PDP-11 itself, with the Unibus and apparently agnostic ISA seem to
    > summarily reject the Harvard Architecure...

You could say that of a zillion computers. The only recent computer I can
think of offhand with separate instruction and data paths was the AMD 42K
(nice chip, I used it in a product we built at Proteon). They had separate
ports for instructions and data purely for performance reasons. (Our card had
a pathway which allowed the CPU to write the instruction memory, needed during
booting, obviously; the details as to how we did it escape me now.)

    > From: Jon Steinhart

    > For all intents and purposes instructions were separate from data from
    > the PDP 11/70 on.


And the other -11 memory management (as on the /40, /23, etc) does allow for
execute-only 'segments' (they call them 'pages' in the later versions of the
manual, but they're not) - again, separating code from data. Unix used this
for shared pure texts.

And note that those machines with separate I+D space don't meet the dictionary
definition either, because they only have one bus from the CPU to memory,
shared between data and instruction fetches.


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