## 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 machine-readable files).

[Jargon File]

Last updated: 1995-02-10

## 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 one-seventh 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.

Last updated: 1997-09-29

## 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.

Last updated: 1995-02-10

## differential backup

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.

Last updated: 2004-03-05

## differential driver

<hardware>

An electronic device (commonly an integrated circuit), containing two amplifiers, used to drive a differential line.

Last updated: 1995-03-14

## 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 RS-422 serial line standard specifies differential drivers and receivers, whereas the earlier RS-232 standard does not.

Opposite: single ended.

Last updated: 1995-03-08

## Diffie-Hellman

FAQ.

Last updated: 1999-03-15

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