48 lines
5.4 KiB
HTML
48 lines
5.4 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
||
<html xmlns="http://www.w3.org/1999/xhtml"><head><title>A Story About ‘Magic'</title><link rel="stylesheet" href="jargon.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.61.0"/><link rel="home" href="index.html" title="The Jargon File"/><link rel="up" href="appendixa.html" title="Appendix A. Hacker Folklore"/><link rel="previous" href="tv-typewriters.html" title="TV Typewriters: A Tale of Hackish Ingenuity"/><link rel="next" href="koans.html" title="Some AI Koans"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">A Story About ‘Magic'</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="tv-typewriters.html">Prev</a> </td><th width="60%" align="center">Appendix A. Hacker Folklore</th><td width="20%" align="right"> <a accesskey="n" href="koans.html">Next</a></td></tr></table><hr/></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="magic-story"/>A Story About ‘Magic'</h2></div></div><div/></div><p>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).</p><p>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.</p><p>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.</p><p>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.</p><p>Imagine our utter astonishment. We wrote it off as coincidence, but
|
||
nevertheless restored the switch to the ‘more magic’ position
|
||
before reviving the computer.</p><p>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.</p><p>The computer promptly crashed.</p><p>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
|
||
<a href="D/dike.html"><i class="glossterm">dike</i></a>d it out. We then revived the computer and it has
|
||
run fine ever since.</p><p>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
|
||
<a href="M/magic.html"><i class="glossterm">magic</i></a>.</p><p>I still have that switch in my basement. Maybe I'm silly, but I usually
|
||
keep it set on ‘more magic’.</p><p>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.</p></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tv-typewriters.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="appendixa.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="koans.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">TV Typewriters: A Tale of Hackish Ingenuity </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Some AI Koans</td></tr></table></div></body></html>
|