diff
/dif/ 1. A change listing, especially giving differences between (and additions
to) different versions of a piece of source code or documentation (the term is
often used in the plural "diffs"). "Send me your diffs for the Jargon File!"
Compare vdiff.
2. Specifically, such a listing produced by the diff Unix command, especially
when used as input to the patch utility (which actually performs the
modifications). This is a common method of distributing patches and source
updates.
3. To compare (whether or not by use of automated tools on machinereadable
files).
See also vdiff, mod.
[Jargon File]
(19950210)
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Difference Engine » difference equation »
differential backup
Difference Engine
<computer, history> Charles Babbage's design for the first automatic
mechanical calculator. The Difference Engine was a special purpose device
intended for the production of mathematical tables. Babbage started work on the
Difference Engine in 1823 with funding from the British Government. Only
oneseventh of the complete engine, about 2000 parts, was built in 1832 by
Babbage's engineer, Joseph Clement. This was demonstrated successfully by
Babbage and still works perfectly. The engine was never completed and most of
the 12,000 parts manufactured were later melted for scrap.
It was left to Georg and Edvard Schuetz to construct the first working devices
to the same design which were successful in limited applications. The Difference
Engine No. 2 was finally completed in 1991 at the Science Museum, London, UK and
is on display there.
The engine used gears to compute cumulative sums in a series of registers: r[i]
:= r[i] + r[i+1]. However, the addition had the side effect of zeroing r[i+1].
Babbage overcame this by simultaneously copying r[i+1] to a temporary register
during the addition and then copying it back to r[i+1] at the end of each cycle
(each turn of a handle).
Difference Engine at the Science Museum.
(19970929)
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» difference equation » differential backup »
differential driver
difference equation
A relation between consecutive elements of a sequence. The first difference is
D u(n) = u(n+1)  u(n)
where u(n) is the nth element of sequence u. The second difference is
D2 u(n) = D (D u(n))
= (u(n+2)  u(n+1))  (u(n+1)  u(n))
= u(n+2)  2u(n+1) + u(n)
And so on. A recurrence relation such as
u(n+2) + a u(n+1) + b u(n) = 0
can be converted to a difference equation (in this case, a second order
linear difference equation):
D2 u(n) + p D u(n) + q u(n) = 0
and vice versa. a, b, p, q are constants.
(19950210)
Nearby terms:
die horribly « diff « Difference Engine «
difference equation » differential backup »
differential driver » differential line
differential backup
<operating system> A kind of backup that copies all files that have
changed since the last full backup. Each differential backup will include all
files in previous differential backups since the full backup so to restore a
version of a file, you only need to search the full backup and the relevant
differential backup.
Some systems support differential backup by associating an "Archive" flag with
each file and setting this flag whenever the file is modified to indicate that
it should be included in the next backup. A differential backup does not change
this flag, whereas an incremental backup resets it.
(20040305)
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differential backup » differential driver »
differential line » DiffieHellman
differential driver
<hardware> An electronic device (commonly an integrated circuit),
containing two amplifiers, used to drive a differential line.
(19950314)
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differential backup « differential driver »
differential line » DiffieHellman » digerati
differential line
<hardware> A kind of electrical connection using two wires, one of which
carries the normal signal (V) and the other carries an inverted version the
signal (V). A differential amplifier at the receiver subtracts the inverted
signal from the normal signal to yield a signal proportional to V. This
subtraction is intended to cancel out any noise induced in the wires, on the
assmption that the same level of noise will have been induced in both wires.
Twisted pair wiring is often used to try to ensure that this is the case.
The two wires might be connected at the receiver to separate analogue to digital
converters and the subtraction performed digitally.
The RS422 serial line standard specifies differential drivers and receivers,
whereas the earlier RS232 standard does not.
Opposite: single ended.
(19950308)
Nearby terms:
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differential driver « differential line »
DiffieHellman » digerati » digest
DiffieHellman
<cryptography> A publickey encryption key exchange algorithm.
FAQ.
(19990315)
Nearby terms:
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differential line « DiffieHellman » digerati
» digest » Digex
