two-binary, one-quaternary

<communications>

(2B1Q) A physical layer encoding used for Integrated Services Digital Network basic rate interface. 2B1Q represents two bits (2B - a "dibit") using one of four signal levels (1Q - a "quadratude"). The first bit of the dibit is indicated by polarity: positive indicates a binary 1 and negative indicates a 0. The second half of the dibit is indicated by voltage magnitude: 1 Volt indicates a binary 1 and 3 Volts indicates binary 0.

Last updated: 2003-01-10

twonkie

/twon'kee/ The software equivalent of a Twinkie (a variety of sugar-loaded junk food, or (in gay slang) the male equivalent of "chick"); a useless "feature" added to look sexy and placate a marketroid.

Compare Saturday-night special.

The term may also be related to "The Twonky", title menace of a classic SF short story by Lewis Padgett (Henry Kuttner and C. L. Moore), first published in the September 1942 "Astounding Science Fiction" and subsequently much anthologised.

[Jargon File]

Last updated: 1994-10-20

two-phase commit

<database>

A technique for maintaining integrity in distributed databases. Where a system uses two or more database, a transaction among the distibuted database should be atomic ("all or nothing"). This is done by handling the transaction in two phases. First the databases prepare the transaction, confirm that it is possible to process it, and lock the relevant record.

Once all the required databases confirm that the transaction is viable, the system instructs them all to commit it - i.e. to make it permanent. If it is not possible to process it, the system will instruct the databases to rollback (undo) the transaction.

Last updated: 2000-02-28

twos complement

<data>

A system used in some computers to represent negative numbers in binary. Each bit of the number is inverted (zeros are replaced with ones and vice versa), as for ones complement, but then one (000...0001) is added (ignoring overflow). This avoids the two representations for zero found in ones complement by using all ones to represent -1. 

 ...
 000...00011 = +3
 000...00010 = +2
 000...00001 = +1
 000...00000 =  0
 111...11111 = -1
 111...11110 = -2
 111...11101 = -3
 ...
This representation simplifies the logic required for addition and subtraction, at the expense of a little extra complexity for negation.

Last updated: 1994-10-31

two-to-the-N

An amount much larger than N but smaller than infinity. "I have 2-to-the-N things to do before I can go out for lunch" means you probably won't show up.

Numbers of the form two-to-the-N are very important in computing because they represent the value of bit N of a binary number (counting from 0) and the number of things you can count with an N bit number.

[Jargon File]

Last updated: 1994-12-06

two-valued logic

<logic>

(Commonly known as "Boolean algebra") A mathematical system concerning the two truth values, TRUE and FALSE and the functions AND, OR, NOT. Two-valued logic is one of the cornerstones of logic and is also fundamental in the design of digital electronics and programming languages.

The term "Boolean" is used here with its common meaning - two-valued, though strictly Boolean algebra is more general than this.

Boolean functions are usually represented by truth tables where "0" represents "false" and "1" represents "true". E.g.: 

 A | B | A AND B
 --+---+--------
 0 | 0 |    0
 0 | 1 |    0
 1 | 0 |    0
 1 | 1 |    1
This can be given more compactly using "x" to mean "don't care" (either true or false): 

 A | B | A AND B
 --+---+--------
 0 | x |    0
 x | 0 |    0
 1 | 1 |    1
Similarly: 

        A | NOT A       A | B | A OR B
        --+------       --+---+--------
        0 |  1          0 | 0 |   0
        1 |  0          x | 1 |   1
                        1 | x |   1
Other functions such as XOR, NAND, NOR or functions of more than two inputs can be constructed using combinations of AND, OR, and NOT. AND and OR can be constructed from each other using DeMorgan's Theorem: 

 A OR B   =  NOT ((NOT A) AND (NOT B))
 A AND B	 =  NOT ((NOT A) OR (NOT B))
In fact any Boolean function can be constructed using just NOR or just NAND using the identities: 

 NOT A  =  A NOR A
 A OR B  =  NOT (A NOR B)
and DeMorgan's Theorem.

Last updated: 2003-06-18

Nearby terms:

Twitter ♦ twm ♦ two-binary, one-quaternary ♦ twonkie ♦ two-phase commit

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