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Jargon used in computing

Appendix A: Hacker Folklore

This appendix contains several legends and fables that illuminate the meaning of various
entries in the lexicon.

The Meaning of `Hack'

"The word {hack} doesn't really have 69 different meanings", according to MIT hacker Phil
Agre.  "In fact, {hack} has only one meaning, an extremely subtle and profound one which
defies articulation.  Which connotation is implied by a given use of the word depends in
similarly profound ways on the context.  Similar remarks apply to a couple of other hacker
words, most notably {random}."

Hacking might be characterized as `an appropriate application of ingenuity'.  Whether the
result is a quick-and-dirty patchwork job or a carefully crafted work of art, you have to
admire the cleverness that went into it.

An important secondary meaning of {hack} is `a creative practical joke'. This kind of hack
is easier to explain to non-hackers than the programming kind.  Of course, some hacks have
both natures; see the lexicon entries for {pseudo} and {kgbvax}.  But here are some examples
of pure practical jokes that illustrate the hacking spirit:

     In 1961, students from Caltech (California Institute of Technology, in Pasadena)
     hacked the Rose Bowl football game.  One student posed as a reporter and
     `interviewed' the director of the University of Washington card stunts (such
     stunts involve people in the stands who hold up colored cards to make pictures).
     The reporter learned exactly how the stunts were operated, and also that the
     director would be out to dinner later.

     While the director was eating, the students (who called themselves the `Fiendish
     Fourteen') picked a lock and stole a blank direction sheet for the card stunts.
     They then had a printer run off 2300 copies of the blank.  The next day they
     picked the lock again and stole the master plans for the stunts --- large sheets
     of graph paper coloured in with the stunt pictures.  Using these as a guide, they
     made new instructions for three of the stunts on the duplicated blanks.  Finally,
     they broke in once more, replacing the stolen master plans and substituting the
     stack of diddled instruction sheets for the original set.

     The result was that three of the pictures were totally different.  Instead of
     `WASHINGTON', the word ``CALTECH' was flashed.  Another stunt showed the word
     `HUSKIES', the Washington nickname, but spelled it backwards.  And what was
     supposed to have been a picture of a husky instead showed a beaver.  (Both
     Caltech and MIT use the beaver --- nature's engineer --- as a mascot.)

     After the game, the Washington faculty athletic representative said: "Some
     thought it ingenious; others were indignant."  The Washington student body
     president remarked: "No hard feelings, but at the time it was unbelievable.
     We were amazed."

This is now considered a classic hack, particularly because revising the direction sheets
constituted a form of programming.

Here is another classic hack:

     On November 20, 1982, MIT hacked the Harvard-Yale football game. Just after
     Harvard's second touchdown against Yale, in the first quarter, a small black
     ball popped up out of the ground at the 40-yard line, and grew bigger, and
     bigger, and bigger.  The letters `MIT' appeared all over the ball.  As the
     players and officials stood around gawking, the ball grew to six feet in
     diameter and then burst with a bang and a cloud of white smoke.

     The `Boston Globe' later reported: "If you want to know the truth, MIT won
     The Game."

     The prank had taken weeks of careful planning by members of MIT's Delta Kappa
     Epsilon fraternity.  The device consisted of a weather balloon, a hydraulic
     ram powered by Freon gas to lift it out of the ground, and a vacuum-cleaner
     motor to inflate it.  They made eight separate expeditions to Harvard Stadium
     between 1 and 5 A.M., locating an unused 110-volt circuit in the stadium and
     running buried wires from the stadium circuit to the 40-yard line, where they
     buried the balloon device.  When the time came to activate the device, two
     fraternity members had merely to flip a circuit breaker and push a plug into
     an outlet.

     This stunt had all the earmarks of a perfect hack: surprise, publicity, the
     ingenious use of technology, safety, and harmlessness.  The use of manual
     control allowed the prank to be timed so as not to disrupt the game (it was
     set off between plays, so the outcome of the game would not be unduly affected).
     The perpetrators had even thoughtfully attached a note to the balloon explaining
     that the device was not dangerous and contained no explosives.

     Harvard president Derek Bok commented: "They have an awful lot of clever people
     down there at MIT, and they did it again."  President Paul E. Gray of MIT said:
     "There is absolutely no truth to the rumour that I had anything to do with it,
     but I wish there were."

The hacks above are verifiable history; they can be proved to have happened.  Many other
classic-hack stories from MIT and elsewhere, though retold as history, have the
characteristics of what Jan Brunvand has called `urban folklore' (see {FOAF}).  Perhaps the
best known of these is the legend of the infamous trolley-car hack, an alleged incident in
which engineering students are said to have welded a trolley car to its tracks with thermite.
Numerous versions of this have been recorded from the 1940s to the present, most set at MIT
but at least one very detailed version set at CMU.

Brian Leibowitz has researched MIT hacks both real and mythical extensively; the interested
reader is referred to his delightful pictorial compendium `The Journal of the Institute for
Hacks, Tomfoolery, and Pranks' (MIT Museum, 1990; ISBN 0-917027-03-5).

Finally, here is a story about one of the classic computer hacks.

     Back in the mid-1970s, several of the system support staff at Motorola discovered
     a relatively simple way to crack system security on the Xerox CP-V timesharing
     system.  Through a simple programming strategy, it was possible for a user program
     to trick the system into running a portion of the program in `master mode'
     (supervisor state), in which memory protection does not apply.  The program could
     then poke a large value into its `privilege level' byte (normally write-protected)
     and could then proceed to bypass all levels of security within the file-management
     system, patch the system monitor, and do numerous other interesting things.  In
     short, the barn door was wide open.

     Motorola quite properly reported this problem to Xerox via an official `level 1 SIDR'
     (a bug report with an intended urgency of `needs to be fixed yesterday').  Because
     the text of each SIDR was entered into a database that could be viewed by quite a
     number of people, Motorola followed the approved procedure: they simply reported the
     problem as `Security SIDR', and attached all of the necessary documentation,
     ways-to-reproduce, etc.

     The CP-V people at Xerox sat on their thumbs; they either didn't realize the severity
     of the problem, or didn't assign the necessary operating-system-staff resources to
     develop and distribute an official patch.

     Months passed.  The Motorola guys pestered their Xerox field-support rep, to no avail.
     Finally they decided to take direct action, to demonstrate to Xerox management just
     how easily the system could be cracked and just how thoroughly the security safeguards
     could be subverted.

     They dug around in the operating-system listings and devised a thoroughly devilish set
     of patches.  These patches were then incorporated into a pair of programs called `Robin
     Hood' and `Friar Tuck'.  Robin Hood and Friar Tuck were designed to run as `ghost jobs'
     (daemons, in UNIX terminology); they would use the existing loophole to subvert system
     security, install the necessary patches, and then keep an eye on one another's statuses
     in order to keep the system operator (in effect, the superuser) from aborting them.

     One fine day, the system operator on the main CP-V software development system in El
     Segundo was surprised by a number of unusual phenomena.  These included the following:

        * Tape drives would rewind and dismount their tapes in the middle of a job.
        * Disk drives would seek back and forth so rapidly that they would attempt to walk
          across the floor (see {walking drives}).
        * The card-punch output device would occasionally start up of itself and punch a
          {lace card}.  These would usually jam in the punch.
        * The console would print snide and insulting messages from Robin Hood to Friar Tuck,
          or vice versa.
        * The Xerox card reader had two output stackers; it could be instructed to stack into
          A, stack into B, or stack into A (unless a card was unreadable, in which case the
          bad card was placed into stacker B).  One of the patches installed by the ghosts
          added some code to the card-reader driver... after reading a card, it would flip
          over to the opposite stacker. As a result, card decks would divide themselves in
          half when they were read, leaving the operator to recollate them manually.

     Naturally, the operator called in the operating-system developers. They found the bandit
     ghost jobs running, and X'ed them... and were once again surprised.  When Robin Hood was
     X'ed, the following sequence of events took place:

          !X id1

          id1: Friar Tuck... I am under attack!  Pray save me!
          id1: Off (aborted)

          id2: Fear not, friend Robin!  I shall rout the Sheriff of Nottingham's men!

          id1: Thank you, my good fellow!

     Each ghost-job would detect the fact that the other had been killed, and would start
     a new copy of the recently slain program within a few milliseconds.  The only way to
     kill both ghosts was to kill them simultaneously (very difficult) or to deliberately
     crash the system.

     Finally, the system programmers did the latter --- only to find that the bandits
     appeared once again when the system rebooted!  It turned out that these two programs
     had patched the boot-time OS image (the kernel file, in UNIX terms) and had added
     themselves to the list of programs that were to be started at boot time.

     The Robin Hood and Friar Tuck ghosts were finally eradicated when the system staff
     rebooted the system from a clean boot-tape and reinstalled the monitor.  Not long
     thereafter, Xerox released a patch for this problem.

     It is alleged that Xerox filed a complaint with Motorola's management about the
     merry-prankster actions of the two employees in question.  It is not recorded that
     any serious disciplinary action was taken against either of them.

TV Typewriters: A Tale of Hackish Ingenuity

Here is a true story about a glass tty: One day an MIT hacker was in a motorcycle accident
and broke his leg.  He had to stay in the hospital quite a while, and got restless because
he couldn't {hack}.  Two of his friends therefore took a terminal and a modem for it to the
hospital, so that he could use the computer by telephone from his hospital bed.

Now this happened some years before the spread of home computers, and computer terminals
were not a familiar sight to the average person. When the two friends got to the hospital,
a guard stopped them and asked what they were carrying.  They explained that they wanted
to take a computer terminal to their friend who was a patient.

The guard got out his list of things that patients were permitted to have in their rooms:
TV, radio, electric razor, typewriter, tape player, ... no computer terminals.  Computer
terminals weren't on the list, so the guard wouldn't let it in.  Rules are rules, you know.
(This guard was clearly a {droid}.)

Fair enough, said the two friends, and they left again.  They were frustrated, of course,
because they knew that the terminal was as harmless as a TV or anything else on the list...
which gave them an idea.

The next day they returned, and the same thing happened: a guard stopped them and asked
what they were carrying.  They said: "This is a TV typewriter!"  The guard was skeptical,
so they plugged it in and demonstrated it.  "See?  You just type on the keyboard and what
you type shows up on the TV screen."  Now the guard didn't stop to think about how utterly
useless a typewriter would be that didn't produce any paper copies of what you typed; but
this was clearly a TV typewriter, no doubt about it.  So he checked his list: "A TV is all
right, a typewriter is all right ... okay, take it on in!"

Two Stories About `Magic' (by GLS)

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 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

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

A Selection of AI Koans

These are some of the funniest examples of a genre of jokes told at the MIT AI Lab about
various noted hackers.  The original koans were composed by Danny Hillis.  In reading these,
it is at least useful to know that Minsky, Sussman, and Drescher are AI researchers of note,
that Tom Knight was one of the Lisp machine's principal designers, and that David Moon wrote
much of Lisp machine Lisp.

                                 * * *

   A novice was trying to fix a broken Lisp machine by turning the power off and on.

   Knight, seeing what the student was doing, spoke sternly: "You cannot fix a machine by
   just power-cycling it with no understanding of what is going wrong."

   Knight turned the machine off and on.

   The machine worked.

                                 * * *

   One day a student came to Moon and said: "I understand how to make a better garbage
   collector.  We must keep a reference count of the pointers to each cons."

Moon patiently told the student the following story:

     "One day a student came to Moon and said: `I understand how to make a better garbage

[Ed. note: Pure reference-count garbage collectors have problems with circular structures
that point to themselves.]

                                 * * *

In the days when Sussman was a novice, Minsky once came to him as he sat hacking at the PDP-6.

   "What are you doing?", asked Minsky.

   "I am training a randomly wired neural net to play Tic-Tac-Toe" Sussman replied.

   "Why is the net wired randomly?", asked Minsky.

   "I do not want it to have any preconceptions of how to play", Sussman said.

   Minsky then shut his eyes.

   "Why do you close your eyes?", Sussman asked his teacher.

   "So that the room will be empty."

   At that moment, Sussman was enlightened.

                                 * * *

   A disciple of another sect once came to Drescher as he was eating his morning meal.

   "I would like to give you this personality test", said the outsider, "because I want you
   to be happy."

   Drescher took the paper that was offered him and put it into the toaster, saying: "I wish
   the toaster to be happy, too."


This story says a lot about the the ITS ethos.

On the ITS system there was a program that allowed you to see what was being printed on
someone else's terminal.  It spied on the other guy's output by examining the insides of
the monitor system.  The output spy program was called OS.  Throughout the rest of the
computer science (and at IBM too) OS means `operating system', but among old-time ITS
hackers it almost always meant `output spy'.

OS could work because ITS purposely had very little in the way of `protection' that
prevented one user from trespassing on another's areas.  Fair is fair, however.  There
was another program that would automatically notify you if anyone started to spy on your
output.  It worked in exactly the same way, by looking at the insides of the operating
system to see if anyone else was looking at the insides that had to do with your output.
This `counterspy' program was called JEDGAR (a six-letterism pronounced as two syllables:
/jed'gr/), in honour of the former head of the FBI.

But there's more.  JEDGAR would ask the user for `license to kill'.  If the user said yes,
then JEDGAR would actually {gun} the job of the {luser} who was spying.  Unfortunately,
people found that this made life too violent, especially when tourists learned about it.
One of the systems hackers solved the problem by replacing JEDGAR with another program that
only pretended to do its job.  It took a long time to do this, because every copy of JEDGAR
had to be patched.  To this day no one knows how many people never figured out that JEDGAR
had been defanged.

The Story of Mel, a Real Programmer

This was posted to USENET by its author, Ed Nather, on May 21, 1983.

     A recent article devoted to the *macho* side of programming made the bald and
     unvarnished statement:

         Real Programmers write in FORTRAN.

     Maybe they do now,
     in this decadent era of
     Lite beer, hand calculators, and "user-friendly" software
     but back in the Good Old Days,
     when the term "software" sounded funny
     and Real Computers were made out of drums and vacuum tubes,
     Real Programmers wrote in machine code.
     Not FORTRAN. Not RATFOR.  Not, even, assembly language.
     Machine Code.
     Raw, unadorned, inscrutable hexadecimal numbers.

     Lest a whole new generation of programmers
     grow up in ignorance of this glorious past,
     I feel duty-bound to describe,
     as best I can through the generation gap,
     how a Real Programmer wrote code.
     I'll call him Mel,
     because that was his name.

     I first met Mel when I went to work for Royal McBee Computer Corp.,
     a now-defunct subsidiary of the typewriter company.
     The firm manufactured the LGP-30,
     a small, cheap (by the standards of the day)
     drum-memory computer,
     and had just started to manufacture
     the RPC-4000, a much-improved,
     bigger, better, faster --- drum-memory computer.
     Cores cost too much,
     and weren't here to stay, anyway.
     (That's why you haven't heard of the company, or the computer.)

     I had been hired to write a FORTRAN compiler
     for this new marvel and Mel was my guide to its wonders.
     Mel didn't approve of compilers.

     "If a program can't rewrite its own code",
     he asked, "what good is it?"

     Mel had written,
     in hexadecimal,
     the most popular computer program the company owned.
     It ran on the LGP-30
     and played blackjack with potential customers
     at computer shows.
     Its effect was always dramatic.
     The LGP-30 booth was packed at every show,
     and the IBM salesmen stood around
     talking to each other.
     Whether or not this actually sold computers
     was a question we never discussed.

     Mel's job was to re-write
     the blackjack program for the RPC-4000.
     (Port?  What does that mean?)
     The new computer had a one-plus-one
     addressing scheme,
     in which each machine instruction,
     in addition to the operation code
     and the address of the needed operand,
     had a second address that indicated where, on the revolving drum,
     the next instruction was located.

     In modern parlance,
     every single instruction was followed by a GO TO!
     Put *that* in Pascal's pipe and smoke it.

     Mel loved the RPC-4000
     because he could optimize his code:
     that is, locate instructions on the drum
     so that just as one finished its job,
     the next would be just arriving at the "read head"
     and available for immediate execution.
     There was a program to do that job,
     an "optimizing assembler",
     but Mel refused to use it.

     "You never know where it's going to put things",
     he explained, "so you'd have to use separate constants".

     It was a long time before I understood that remark.
     Since Mel knew the numerical value
     of every operation code,
     and assigned his own drum addresses,
     every instruction he wrote could also be considered
     a numerical constant.
     He could pick up an earlier "add" instruction, say,
     and multiply by it,
     if it had the right numeric value.
     His code was not easy for someone else to modify.

     I compared Mel's hand-optimized programs
     with the same code massaged by the optimizing assembler program,
     and Mel's always ran faster.
     That was because the "top-down" method of program design
     hadn't been invented yet,
     and Mel wouldn't have used it anyway.
     He wrote the innermost parts of his program loops first,
     so they would get first choice
     of the optimum address locations on the drum.
     The optimizing assembler wasn't smart enough to do it that way.

     Mel never wrote time-delay loops, either,
     even when the balky Flexowriter
     required a delay between output characters to work right.
     He just located instructions on the drum
     so each successive one was just *past* the read head
     when it was needed;
     the drum had to execute another complete revolution
     to find the next instruction.
     He coined an unforgettable term for this procedure.
     Although "optimum" is an absolute term,
     like "unique", it became common verbal practice
     to make it relative:
     "not quite optimum" or "less optimum"
     or "not very optimum".
     Mel called the maximum time-delay locations
     the "most pessimum".

     After he finished the blackjack program
     and got it to run
     ("Even the initializer is optimized",
     he said proudly),
     he got a Change Request from the sales department.
     The program used an elegant (optimized)
     random number generator
     to shuffle the "cards" and deal from the "deck",
     and some of the salesmen felt it was too fair,
     since sometimes the customers lost.
     They wanted Mel to modify the program
     so, at the setting of a sense switch on the console,
     they could change the odds and let the customer win.

     Mel balked.
     He felt this was patently dishonest,
     which it was,
     and that it impinged on his personal integrity as a programmer,
     which it did,
     so he refused to do it.
     The Head Salesman talked to Mel,
     as did the Big Boss and, at the boss's urging,
     a few Fellow Programmers.
     Mel finally gave in and wrote the code,
     but he got the test backwards,
     and, when the sense switch was turned on,
     the program would cheat, winning every time.
     Mel was delighted with this,
     claiming his subconscious was uncontrollably ethical,
     and adamantly refused to fix it.

     After Mel had left the company for greener pa$ture$,
     the Big Boss asked me to look at the code
     and see if I could find the test and reverse it.
     Somewhat reluctantly, I agreed to look.
     Tracking Mel's code was a real adventure.

     I have often felt that programming is an art form,
     whose real value can only be appreciated
     by another versed in the same arcane art;
     there are lovely gems and brilliant coups
     hidden from human view and admiration, sometimes forever,
     by the very nature of the process.
     You can learn a lot about an individual
     just by reading through his code,
     even in hexadecimal.
     Mel was, I think, an unsung genius.

     Perhaps my greatest shock came
     when I found an innocent loop that had no test in it.
     No test.  *None*.
     Common sense said it had to be a closed loop,
     where the program would circle, forever, endlessly.
     Program control passed right through it, however,
     and safely out the other side.
     It took me two weeks to figure it out.

     The RPC-4000 computer had a really modern facility
     called an index register.
     It allowed the programmer to write a program loop
     that used an indexed instruction inside;
     each time through,
     the number in the index register
     was added to the address of that instruction,
     so it would refer
     to the next datum in a series.
     He had only to increment the index register
     each time through.
     Mel never used it.

     Instead, he would pull the instruction into a machine register,
     add one to its address,
     and store it back.
     He would then execute the modified instruction
     right from the register.
     The loop was written so this additional execution time
     was taken into account ---
     just as this instruction finished,
     the next one was right under the drum's read head,
     ready to go.
     But the loop had no test in it.

     The vital clue came when I noticed
     the index register bit,
     the bit that lay between the address
     and the operation code in the instruction word,
     was turned on ---
     yet Mel never used the index register,
     leaving it zero all the time.
     When the light went on it nearly blinded me.

     He had located the data he was working on
     near the top of memory ---
     the largest locations the instructions could address ---
     so, after the last datum was handled,
     incrementing the instruction address
     would make it overflow.
     The carry would add one to the
     operation code, changing it to the next one in the instruction set:
     a jump instruction.
     Sure enough, the next program instruction was
     in address location zero,
     and the program went happily on its way.

     I haven't kept in touch with Mel,
     so I don't know if he ever gave in to the flood of
     change that has washed over programming techniques
     since those long-gone days.
     I like to think he didn't.
     In any event,
     I was impressed enough that I quit looking for the
     offending test,
     telling the Big Boss I couldn't find it.
     He didn't seem surprised.

     When I left the company,
     the blackjack program would still cheat
     if you turned on the right sense switch,
     and I think that's how it should be.
     I didn't feel comfortable
     hacking up the code of a Real Programmer.

This is one of hackerdom's great heroic epics, free verse or no.  In a few spare images it
captures more about the esthetics and psychology of hacking than all the scholarly volumes
on the subject put together.  For an opposing point of view, see the entry for {real

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