53 lines
3.5 KiB
Plaintext
53 lines
3.5 KiB
Plaintext
A Story About ‘Magic'
|
||
|
||
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.
|
||
|