[2457] in Depressing_Thoughts
Re: no more magic
marc@ATHENA.MIT.EDU (marc@ATHENA.MIT.EDU)
Tue Oct 22 16:49:51 1991
Here's a longer version, from ~marc/stuff/jargon/jargon2.8.3:
Two Stories About `Magic' (by Guy Steele)
=========================================
When Barbara Steele was in her fifth month of pregnancy in 1981, her
doctor sent her to a specialist to have a sonogram made to determine
whether there were twins. She dragged her husband Guy along to the
appointment. It was quite fascinating; as the doctor moved an
instrument along the skin, a small TV screen showed cross-sectional
pictures of the abdomen.
Now Barbara and I had both studied computer science at MIT, and we
both saw that some complex computerized image-processing was involved.
Out of curiosity, we asked the doctor how it was done, hoping to learn
some details about the mathematics involved. The doctor, not knowing
our educational background, simply said, "The probe sends out sound
waves, which bounce off the internal organs. A microphone picks up
the echoes, like radar, and send the signals to a computer --- and the
computer makes a picture." Thanks a lot! Now a hacker would have
said, "... and the computer *magically* (or {automagically})
makes a picture", implicitly acknowledging that he has glossed over
an extremely complicated process.
Some years ago I 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
only had 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 ran 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'.
