that plays tic-tac-toe
by A. K. Dewdney
indirectly kicks an "output
duck," a bird-shaped construction. The output duck swings
down from its perch so that its beak points at a number- which
identifies the computer's next move in a game of tic~tac-toe.
does the read head scan as it feels its way down the monolith?
Nothing less than 48 rows of Tinkertoy "memory spindles"
encoding all the critical combinations of X's and O's that might
arise during a game [see illustration on opposite page].
Each spindle is a sequence of smooth spools connected axially
by sticks and arranged in nine groups of three each, one group
for each square of the tic-tac-toe board. The presence or absence
of spools from a group indicates that a corresponding square
of the tic-tac-toe board is vacant or is occupied by an X or
computer is not fully autornatic: a human operator must crank
the read head up and down and must manage its input. After the
computer's opponent makes a move, the operator walks to the front
of the machine to adjust the core piece inside the read head,
registering the contestant's move. The operator then pulls on
a string to cock the core piece for its impending whirl of recognition.
When it discovers a memory that matches the current state of
the game, the core piece spins, and the computer indicates its
The best way
to understand how the machine works in detail is to recount the
story of its creation at the hands of the M.I.T. students: Erlyne
Gee, Edward Hardebeck, Daniel Hillis, Margaret Minsky and brothers
Barry and Brian Silverman. Most of the group has long since graduated
and entered various computer professions. Perhaps the best-known
team member is Hillis. He was the moving force behind Thinking
Machines, Inc., which produces the well-known parallel supercomputer
called the Connection Machine. (Perhaps Tinkertoys have something
to teach us.)
In 1975, when
Hillis and Brian Silverman were in their sophomore year, they
participated in a class project to build something digital from
Tinkertoys. The students sat down to play. One made an invertera
logic device that converts a binary 1 signal to a 0 signal and
conversely. Another made an OR gate; if either of the device's
two input signals happened to be a 1, then its output would also
be a 1. It quickly became clear to the students that Tinkertoys
were "computation universal," the theoretical term
for a set of components from which a fully program
"I first had that experience
[universality of computation] before I went to school. There
weren't any [computersl yet, but we had toy construction sets.
One was called TinkerToy.... What's strange is that those spools
and sticks are enough to make anything."
in preface to LogoWorks
many of us remember Tinkertoys, those down-home kits of colored
wooden sticks and spools with holes in them? Amid our childhood
constructions of towers or cranes, how many of us pondered the
outer limits of the Tinkertoy world? Did we conceive of contraptions
that reached the ceiling? Perhaps, but we lacked the kits or
the time to make it
happen. Such a Tinkertoy fantasy
took place several years ago when a student group from the Massachusetts
Institute of Technology constructed a computer entirely (well,
almost entirely) out of Tinkertoys!
From a distance
the Tinkertoy computer resembles a childhood fantasy gone wild
or, as one of the group members remarked, a spool-and-stick version
of the "space slab" from the movie 2001: A Space
Odyssey. Unlike the alien monolith, the computer plays a
mean game of tic-tac-toe. A Tinkertoy framework called the read
head clicks and clacks its way down the front of the monolith
At some point the clicking mysteriously stops; a "core piece"
within the framework spins and then with a satisfying "kathunk"