Some years ago, I (GLS) was snooping around in the cabinets
that housed the MIT AI Lab's PDP-10, and noticed a little switch
glued to the frame of one cabinet. It was obviously a homebrew
job, added by one of the lab's hardware hackers (no one knows
who).
You don't touch an unknown switch on a computer without
knowing what it does, because you might crash the computer. The
switch was labeled in a most unhelpful way. It had two positions,
and scrawled in pencil on the metal switch body were the words
`magic' and `more magic'. The switch was in the `more magic'
position.
I called another hacker over to look at it. He had never seen
the switch before either. Closer examination revealed that the
switch had only one wire running to it! The other end of the wire
did disappear into the maze of wires inside the computer, but
it's a basic fact of electricity that a switch can't do anything
unless there are two wires connected to it. This switch had a
wire connected on one side and no wire on its other side.
It was clear that this switch was someone's idea of a silly
joke. Convinced by our reasoning that the switch was inoperative,
we flipped it. The computer instantly crashed.
Imagine our utter astonishment. We wrote it off as
coincidence, but nevertheless restored the switch to the `more
magic' position before reviving the computer.
A year later, I told this story to yet another hacker, David
Moon as I recall. He clearly doubted my sanity, or suspected me
of a supernatural belief in the power of this switch, or perhaps
thought I was fooling him with a bogus saga. To prove it to him,
I showed him the very switch, still glued to the cabinet frame
with only one wire connected to it, still in the `more magic'
position. We scrutinized the switch and its lone connection, and
found that the other end of the wire, though connected to the
computer wiring, was connected to a ground pin. That clearly made
the switch doubly useless: not only was it electrically
nonoperative, but it was connected to a place that couldn't
affect anything anyway. So we flipped the switch.
The computer promptly crashed.
This time we ran for Richard Greenblatt, a long-time MIT
hacker, who was close at hand. He had never noticed the switch
before, either. He inspected it, concluded it was useless, got
some diagonal cutters and diked it
out. We then revived the computer and it has run fine ever
since.
We still don't know how the switch crashed the machine. There
is a theory that some circuit near the ground pin was marginal,
and flipping the switch changed the electrical capacitance enough
to upset the circuit as millionth-of-a-second pulses went through
it. But we'll never know for sure; all we can really say is that
the switch was magic.
I still have that switch in my basement. Maybe I'm silly, but
I usually keep it set on `more magic'.
1994: Another explanation of this story has since been
offered. Note that the switch body was metal. Suppose that the
non-connected side of the switch was connected to the switch body
(usually the body is connected to a separate earth lug, but there
are exceptions). The body is connected to the computer case,
which is, presumably, grounded. Now the circuit ground within the
machine isn't necessarily at the same potential as the case
ground, so flipping the switch connected the circuit ground to
the case ground, causing a voltage drop/jump which reset the
machine. This was probably discovered by someone who found out
the hard way that there was a potential difference between the
two, and who then wired in the switch as a joke.
Node:Some AI Koans, Next:OS and JEDGAR, Previous:A Story About
Magic, Up:Appendix A