some year
s ago, I wa
s snooping around in the cabinet
s that hou
sed the
MIT AI Lab'
s PDP-10, and noticed a little
switch glued to the frame of one cabinet. It wa
s obviou
sly a homebrew job, added by one of the lab'
s hardware hacker
s (no-one know
s who). You don' t touch an unknown
switch on a computer without knowing what it doe
s, becau
se you might cra
sh the computer. The
switch wa
s labelled in a mo
st unhelpful way. It had two po
sition
s, and
scrawled in pencil on the metal
switch body were the word
s "magic" and "more magic". The
switch wa
s in the "more magic" po
sition. I called another hacker over to look at it. He had never
seen the
switch before either. Clo
ser examination revealed that the
switch had only one wire running to it! The other end of the wire did di
sappear into the maze of wire
s in
side the computer, but it'
s a ba
sic fact of electricity that a
switch can' t do anything unle
ss there are two wire
s connected to it. Thi
s switch had a wire connected on one
side and no wire on it
s other
side. It wa
s clear that thi
s switch wa
s someone'
s idea of a
silly joke. Convinced by our rea
soning that the
switch wa
s inoperative, we flipped it. The computer in
stantly cra
shed. Imagine our utter a
stoni
shment. We wrote it off a
s coincidence, but neverthele
ss re
stored the
switch to the "more magic" po
sition before reviving the computer. A year later, I told thi
s story to yet another hacker, {David Moon} a
s I recall. He clearly doubted my
sanity, or
su
spected me of a
supernatural belief in the power of thi
s switch, or perhap
s thought I wa
s fooling him with a bogu
s 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" po
sition. We
scrutinized the
switch and it
s lone connection, and found that the other end of the wire, though connected to the computer wiring, wa
s connected to a ground pin. That clearly made the
switch doubly u
sele
ss: not only wa
s it electrically nonoperative, but it wa
s connected to a place that couldn' t affect anything anyway.
so we flipped the
switch. The computer promptly cra
shed. Thi
s time we ran for Richard Greenblatt, a long-time
MIT hacker, who wa
s clo
se at hand. He had never noticed the
switch before, either. He in
spected it, concluded it wa
s u
sele
ss, got
some diagonal cutter
s and
diked it out. We then revived the computer and it ha
s run fine ever
since. We
still don' t know how the
switch cra
shed the machine. There i
s a theory that
some circuit near the ground pin wa
s marginal, and flipping the
switch changed the electrical capacitance enough to up
set the circuit a
s millionth-of-a-
second pul
se
s went through it. But we' ll never know for
sure all we can really
say i
s that the
switch wa
s magic. I
still have that
switch in my ba
sement. Maybe I' m
silly, but I u
sually keep it
set on "more magic".
GLs (1995-02-22)
style="border-width:thin; border-color:#333333; border-style:dashed; padding:5px;" align="left">In addition suitable contents:
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