The Strange Birth and Long Life of Unix - IEEE Spectrum

[D. Hugh Redelmeier]

| From: Antonio Sun <antoniosun-N9AOi2cAC9ZBDgjK7y7TUQ at public.gmane.org>

| http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1

I started to read the code and was puzzled by something I found in the
first few lines:  apparently redundant moves to lsh in
  <http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u5.s>

It turns out that it won't run on a PDP-11/20 without the KE11A
Extended Arithmentic Element, something I've never used.  lsh is a
register in that device that causes another register pair (called mq
and ac) to shift.  Without the EAE, the PDP-11/20 had only single bit
shift instructions.

The symbols are mentioned in the assembler manual
<http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/man/man1/as.1>
but not their actual meaning.

This is an interesting example of devices being addressed as if they
were memory, a noteable feature of the PDP-11.

Anyway, the moves were not redundant: each one caused a shift.

The code has lots of comments.  I think later kernel assembly code had a 
lot fewer.


It would have been more interesting (and alien) to me to see the PDP-7 
version of Unix that preceded this one.  I've never written assembly 
language for the PDP-7, but I kind of know it.  In some ways it is a 
stretched PDP-8, something I have used.  Odd fact: the PDP-7 supported 
both one's complement and two's complement arithmetic.
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