[TUHS] [tuhs] PDP-11 and early Unix

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Mar 15 06:22:53 AEST 2022

    > From: Clem Cole

    > I'm thrilled to see the 11 kept alive.

Ditto. The most elegant archtecture ever, IMO.

    > I was under the impression ??Ken?? had created them for B
    > independently (which, of course, was first on the PDP-7).

"The Development of the C Language", by Dennis M. Ritchie:



  The PDP-7, however, did have a few `auto-increment' memory cells, with the
  property that an indirect memory reference through them incremented the cell.
  This feature probably suggested such operators to Thompson; the
  generalization to make them both prefix and postfix was his own. Indeed,
  the auto-increment cells were not used directly in implementation of the
  operators, and a stronger motivation for the innovation was probably his
  observation that the translation of ++x was smaller than that of x=x+1.

Note the "probably"; unless Ken remembers, and says something, that's
probably the best we are going to get.

    > I did not think the PDP-7 ISA includes addressing modes in the same
    > manner as the 11. .. I thought PDP-7 is a very simple instruction (and
    > small) with an AC, Link/Indirection and a PC - it reminded me of the
    > PDP-8 more than anything else

The PDP-4, -7 and -9 are all the same architecture (very similar to the
PDP-1, but simplified a bit), differing only in implementation. (Most PDP-7
code will run on a -9, un-modified.) Basic instructions look like:

  Instructions had a 4-bit opcode ('000'-'054'), 1 bit of indirect, and 13
  bits of address. It was a load-store architecture, with a single accumulator.

So, yes, similar to an -8. There are other opcodes for non-memory operations
('074' opcode), and I/O ('070'), using bits in the 'address' field. ('060'
opcodes were for the optional EAE.) All of the -4/-7/-9 had the
'auto-increment on locations 010-017' when indirecting through them' feature.
Bitsavers has fairly complete docs on them all:



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