[ih] A bit more re John McCarthy

[Lars Brinkhoff]

Jack Haverty wrote:
> The PDP-10 instruction set included ROTC - ROTateCombined - which
> treated two registers as a single memory location and ROTated bits
> between the two.  IIRC bits ROTated off the right end of one register
> were moved into the right side of the adjacent register. [...]
> Greenblatt at one point decided it would also be useful for Chess to
> have a different instruction available - where the bits rotated out
> the right end went into the LEFT side of the adjacent register.

This calls for an infodump!

Positive shifts rotate to the left, negative to the right.  (On the
PDP-10, "left" and "right" are firmly associated with "more significant"
and "less significant".)  If A and B are adjacent accumulators
(registers), rotating one step looks like this.  Will only look good
with a monospace font.

                           ROTC A,1
      ----------------------------------------------------
     |  --------------------      --------------------    |
      -| AAAAAAAAAAAAAAAAAB | <- | BBBBBBBBBBBBBBBBBA | <-
        --------------------      --------------------

The new instruction was called CIRC, and one step looks like this.

                           CIRC A,1
   ----------------------------
  |  --------------------      |    --------------------
   -| AAAAAAAAAAAAAAAAAB | <-   -> | ABBBBBBBBBBBBBBBBB | -
     --------------------    |      --------------------   |
                              -----------------------------

Another useful feature hacked into the MIT machines was a software
controlled instruction single step.  (Single stepping could be done from
the front panel, but that's not useful for debuggers under timesharing.)

The above additions were also made to the KL10 microcode running on
MIT-MC, and the micocode on the four replacement KS10 machines later.

Another much more obscure addition to the instruction set was a
PC-relative addressing mode.  It was only available on the AI lab
PDP-10, but I have not seen any code that uses it.