maggotbox

<abuse>

/mag'*t-boks/ An even more derogatory term than Macintrash.

[Jargon File]

Last updated: 1995-01-25

MAGIC

An early system on the Midac computer.

[Listed in CACM 2(5):16 (May 1959)].

[Jargon File]

Last updated: 1995-01-25

magic

1. As yet unexplained, or too complicated to explain; compare automagically and (Arthur C.) Clarke's Third Law:

 Any sufficiently advanced technology is
 indistinguishable from magic.

"TTY echoing is controlled by a large number of magic bits." "This routine magically computes the parity of an 8-bit byte in three instructions."

2. Characteristic of something that works although no one really understands why (this is especially called black magic).

3. (Stanford) A feature not generally publicised that allows something otherwise impossible or a feature formerly in that category but now unveiled.

Compare wizardly, deep magic, heavy wizardry.

For more about hackish "magic" see Magic Switch Story.

4. magic number.

[Jargon File]

Last updated: 2001-03-19

magic bullet

<jargon>

(Or "silver bullet" from vampire legends) A term widely used in software engineering for a supposed quick, simple cure for some problem. E.g. "There's no silver bullet for this problem".

Last updated: 1999-01-13

magic cookie

1. Something passed between routines or programs that enables the receiver to perform some operation; a capability ticket or opaque identifier. Especially used of small data objects that contain data encoded in a strange or intrinsically machine-dependent way. E.g. on non-Unix operating systems with a non-byte-stream model of files, the result of "ftell" may be a magic cookie rather than a byte offset; it can be passed to "fseek", but not operated on in any meaningful way. The phrase "it hands you a magic cookie" means it returns a result whose contents are not defined but which can be passed back to the same or some other program later.

2. An in-band code for changing graphic rendition (e.g. inverse video or underlining) or performing other control functions. Some older terminals would leave a blank on the screen corresponding to mode-change magic cookies; this was also called a glitch (or occasionally a "turd"; compare mouse droppings).

See also cookie.

[Jargon File]

Last updated: 1995-01-25

magic number

<jargon, programming>

1. In source code, some non-obvious constant whose value is significant to the operation of a program and that is inserted inconspicuously in-line (hard-coded), rather than expanded in by a symbol set by a commented "#define". Magic numbers in this sense are bad style.

2. A number that encodes critical information used in an algorithm in some opaque way. The classic examples of these are the numbers used in hash or CRC functions or the coefficients in a linear congruential generator for pseudorandom numbers. This sense actually predates, and was ancestral to, the more common sense 1.

3. Special data located at the beginning of a binary data file to indicate its type to a utility. Under Unix, the system and various applications programs (especially the linker) distinguish between types of executable file by looking for a magic number. Once upon a time, these magic numbers were PDP-11 branch instructions that skipped over header data to the start of executable code; 0407, for example, was octal for "branch 16 bytes relative". Nowadays only a wizard knows the spells to create magic numbers. MS DOS executables begin with the magic string "MZ".

*The* magic number, on the other hand, is 7+/-2. The paper cited below established the number of distinct items (such as numeric digits) that humans can hold in short-term memory. Among other things, this strongly influenced the interface design of the phone system.

["The magical number seven, plus or minus two: some limits on our capacity for processing information", George Miller, in the "Psychological Review" 63:81-97, 1956].

[Jargon File]

Last updated: 2003-07-02

Magic Paper

An early interactive symbolic mathematics system.

[Sammet 1969, p. 510].

Last updated: 1995-01-25

magic smoke

<electronics, humour>

A substance trapped inside integrated circuit packages that enables them to function (also called "blue smoke"; this is similar to the archaic "phlogiston" hypothesis about combustion). Its existence is demonstrated by what happens when a chip burns up - the magic smoke gets let out, so it doesn't work any more.

See Electing a Pope, smoke test.

Usenetter Jay Maynard tells the following story:

"Once, while hacking on a dedicated Zilog Z80 system, I was testing code by blowing EPROMs and plugging them in the system then seeing what happened. One time, I plugged one in backward. I only discovered that *after* I realised that Intel didn't put power-on lights under the quartz windows on the tops of their EPROMs - the die was glowing white-hot. Amazingly, the EPROM worked fine after I erased it, filled it full of zeros, then erased it again. For all I know, it's still in service. Of course, this is because the magic smoke didn't get let out."

Compare the original phrasing of Murphy's Law.

[Jargon File]

Last updated: 1995-01-25

Magic Switch Story

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 labelled 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 diked 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".

GLS

Last updated: 1995-02-22

Magma

<symbolic mathematics, tool>

A program used for heavy duty algebraic computation in many branches of mathematics. Magma, developed by John Cannon and associates at the University of Sydney, succeeded Cayley. It runs at several hundred sites.

E-mail: <[email protected]>.

http://maths.usyd.edu.au:8000/u/magma/.

[W. Bosma, J. Cannon and C. Playoust, The Magma algebra system I: The user language, J. Symb. Comp., 24, 3/4, 1997, 235-265].

Last updated: 2000-12-21

Magma2

<language>

A language that allows programmability of the control environment, e.g. recursion, backtracking, coroutines, nondeterminism, etc. Magma2 was the successor to MagmaLISP.

["Magma2: A Language Oriented Toward Experiments in Control", Franco Turini, ACM TOPLAS 6(4):468-486 (Oct 1984)].

Last updated: 1995-07-30

MagmaLISP

<language>

The predecessor of Magma2.

["MagmaLISP: A Machine Language for Artificial Intelligence", C. Mantagero et al, Proc 4th Intl Joint Conf Artif Intell, 1975, pp. 556-561].

Last updated: 1995-07-30

magnetic disk

<storage>

A flat rotating disc covered on one or both sides with magnetisable material. The two main types are the hard disk and the floppy disk.

Data is stored on either or both surfaces of discs in concentric rings called "tracks". Each track is divided into a whole number of "sectors". Where multiple (rigid) discs are mounted on the same axle the set of tracks at the same radius on all their surfaces is known as a "cylinder".

Data is read and written by a disk drive which rotates the discs and positions the read/write heads over the desired track(s). The latter radial movement is known as "seeking". There is usually one head for each surface that stores data. To reduce rotational latency it is possible, though expensive, to have multiple heads at different angles.

The head writes binary data by magnetising small areas or "zones" of the disk in one of two opposing orientations. It reads data by detecting current pulses induced in a coil as zones with different magnetic alignment pass underneath it.

In theory, bits could be read back as a time sequence of pulse (one) or no pulse (zero). However, a run of zeros would give a prolonged absence of signal, making it hard to accurately divide the signal into individual bits due to the variability of motor speed. Run Length Limited is one common solution to this clock recovery problem.

High speed disks have an access time of 28 milliseconds or less, and low-speed disks, 65 milliseconds or more. The higher speed disks also transfer their data faster than the slower speed units.

The disks are usually aluminium with a magnetic coating. The heads "float" just above the disk's surface on a current of air, sometimes at lower than atmospheric pressure in an air-tight enclosure. The head has an aerodynamic shape so the current pushes it away from the disk. A small spring pushes the head towards the disk at the same time keeping the head at a constant distance from the disk (about two microns).

Disk drives are commonly characterised by the kind of interface used to connect to the computer, e.g. ATA, IDE, SCSI.

See also winchester. Compare magnetic drum, compact disc, optical disk, magneto-optical disk.

Suchanka's PC-DISK library.

Last updated: 2007-06-14

Magnetic Ink Character Recognition

<business, printer>

(MICR) A character recognition system using special ink and characters which can be magnetised and read automatically.

MICR is used almost exclusively in the banking industry where it is used to print details on cheques to enable automatic processing.

Last updated: 1995-04-13

magnetic stripe

<storage>

A black stripe, printed on the back of a credit card or similar, that stores a machine-readable copy of the information on the card. The stripe contains iron particles about 500 nanometers long that can be magnetised like magnetic tape. The data can be read by swiping the card through a card reader.

Last updated: 2007-06-04

magnetic tape

<storage>

(Or "magtape", "tape" - paper tape is now obsolete) A data storage medium consisting of a magnetisable oxide coating on a thin plastic strip, commonly used for backup and archiving.

Early industry-standard magnetic tape was half an inch wide and wound on removable reels 10.5 inches in diameter. Different lengths were available with 2400 feet and 4800 feet being common. DECtape was a variation on this "round tape".

In modern magnetic tape systems the reels are much smaller and are fixed inside a cartridge to protect the tape and for ease of handling ("square tape" - though it's really rectangular). Cartridge formats include QIC, DAT, and Exabyte.

Tape is read and written on a tape drive (or "deck") which winds the tape from one reel to the other causing it to move past a read/write head. Early tape had seven parallel tracks of data along the length of the tape allowing six bit characters plus parity written across the tape. A typical recording density was 556 characters per inch. The tape had reflective marks near its end which signaled beginning of tape (BOT) and end of tape (EOT) to the hardware.

Data is written to tape in blocks with inter-block gaps between them. Each block is typically written in a single operation with the tape running continuously during the write. The larger the block the larger the data buffer required in order to supply or receive the data written to or read from the tape. The smaller the block the more tape is wasted as inter-block gaps. Several logical records may be combined into one physical block to reduce wastage ("blocked records"). Finding a certain block on the tape generally involved reading sequentially from the beginning, in contrast to magnetic disks. Tape is not suitable for random access. The exception to this is that some systems allow tape marks to be written which can be detected while winding the tape forward or rewinding it at high speed. These are typically used to separate logical files on a tape.

Most tape drives now include some kind of data compression. There are several algorithms which provide similar results: LZ (most), IDRC (Exabyte), ALDC (IBM, QIC) and DLZ1 (DLT).

See also cut a tape, flap, Group Code Recording, spool, macrotape, microtape, Non Return to Zero Inverted, Phase Encoded.

Last updated: 1997-04-05

magnetic tape drive

<storage>

(Or "tape drive") A peripheral device that reads and writes magnetic tape.

Last updated: 1996-05-25

magneto-optical disk

<hardware, storage>

(MO) A plastic or glass disk coated with a compound (often TbFeCo) with special optical, magnetic and thermal properties. The disk is read by bouncing a low-intensity laser off the disk. Originally the laser was infrared, but frequencies up to blue may be possible giving higher storage density. The polarisation of the reflected light depends on the polarity of the stored magnetic field.

To write, a higher intensity laser heats the coating up to its Curie point, allowing its magnetisation to be altered in a way that is retained when it has cooled.

Although optical, they appear as hard drives to the operating system and do not require a special filesystem (they can be formatted as FAT, HPFS, NTFS, etc.).

The initial 5.25" MO drives, introduced at the end of the 1980s, were the size of a full-height 5.25" hard drive (like in IBM PC XT) and the disks looked like a CD-ROM enclosed in an old-style cartridge

In 2006, a 3.5" drive has the size of 1.44 megabyte diskette drive with disks about the size of a regular 1.44MB floppy disc but twice the thickness.

Storage FAQ.

Last updated: 2006-07-25

magneto-optical drive

magneto-optical disk

magnetostrictive delay line

<storage, history>

An early storage device that used tensioned wires of nickel alloy carrying longitudinal waves produced and detected electromagnetically.

They had better storage behaviour than mercury delay lines.

[H. Epstein and O.B. Stram, "A High Performance Magnetostriction-Sonic Delay Line," Transactions, Institute of Radio Engineers, Professional Group on Ultrasonic Engineering, 1957, pp. 1-24].

Last updated: 2002-11-08

MAGNUM

A database language for DEC-10's, used internally by Tymshare, Inc.. MAGNUM was designed in the late 1970's by Dale Jordan, Rich Strauss and Dave McQuoid originally, and was written in BLISS-10. It was the world's first commercial relational database. It was in the process of being written in 1976.

Last updated: 1995-01-25

Magritte

A constraint language for interactive graphical layout by J. Gosling. It solves constraints using algebraic transformations.

["Algebraic Constraints", J. Gosling, PhD Thesis, TR CS-83-132, CMU, May 1983].

Last updated: 1994-10-20

Nearby terms:

MadalineMADCAPMADTRANmaggotboxMAGICmagicmagic bulletmagic cookie

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