A Story About 'Magic'
-=-=-=-=-=-=-=-=-=-=-=
There are two ways to write bug-free code; only the third way works.
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 {dike}d 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.
výheň