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What is ASCII?

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THE notion of assigning specific number values to letters dates back as least as far as 700 BC. According to an inscription from his reign, Sargon II, an Assyrian king, built a perimeter wall around his capital whose length (16,280 cubits) was the same as the numeric sum of the letters of his name. Such numerological assignments, as in the later Hebrew gematria, were often used for purposes of divination. More recently, associating numbers with specific characters has proved necessary to allow automated telegraph printers (teleprinters) and then computers to represent text. The most widely used mapping between numbers and letters was that approved 50 years ago this month, on June 17th 1963, by the American Standards Association (ASA). It is the American Standard Code for Information Interchange, better known as ASCII. What is it, and where did it come from?

ASCII's spiritual predecessor might have been ancient Assyrian, but it is a direct descendent of Baudot code, a scheme patented by Émile Baudot in 1874 for use in telegraphy. (The term "baud", for modulations per second, used to denote modem transmission speeds, is derived from his name.) Telegraph operators would hold down different chords on a piano-like keyboard with five keys to generate 32 possible values (numbered 0 to 31), each of which corresponded to a particular character. The advantage of this approach was that Baudot codes could be teleprinted by machinery at the other end, rather than relying on a human operator to receive and transcribe messages in the dots and dashes of Morse code. The piano-like keyboard was later replaced with an alphanumeric keyboard, which saved the operator from having to memorise chords. Baudot's system evolved into the Murray code, and then became the International Telegraph Alphabet No. 2 (ITA2). In theory it could represent only 32 distinct symbols, but Baudot's scheme allowed each code to stand for two characters, using two of the symbols to switch to and from an alternative character set that included numerals and punctuation marks. The Baudot code corresponding to the I, III and V keys was a T or an exclamation mark, for example. (Today we would call it a five-bit code and label that sequence as 10101 in binary.)

The problem with this approach was that whenever a control character was lost in transmission, the resulting stream of text (until the next control character) was gibberish. ITA2 also lacked support for lower-case letters and basic mathematical symbols. Several alternative schemes were devised, including IBM's six-bit BCDIC and the seven-bit Fieldata system, developed for military use. But there was no single standard, prompting the ASA to step in and define a new standard to replace ITA2 and address its deficiencies. Its boffins decided on a seven-bit, 128-symbol standard that extended the range of control characters, aided sorting (by ordering letters alphabetically) and allowed numerical characters to be used more easily in calculations. Support for lower-case letters was added in 1967. ASCII's supremacy was assured in 1968, when President Lyndon B. Johnson mandated that all computers purchased by the United States government had to use the new scheme. And it has remained in widespread use ever since, providing a common standard between different types of computers, without which widespread computer networking would have been unnecessarily difficult. (It might seem obvious that all computers should represent the letter A in the same way, but until the adoption of ASCII, they did not.)

Yet ASCII, too, has its flaws. In particular it does not support the various characters required in non-English languages. As a result, several variations of ASCII emerged to support accents and non-Roman characters. To address these problems an entirely new standard, Unicode, was devised starting in the late 1980s. It uses between 8 and 32 bits to represent each character, theoretically providing support for 4.3 billion unique characters, symbols or glyphs—though in practice it is limited to 1.1m symbols, of which more than 100,000 have so far been defined, including the cuneiform characters of Sargon's time. In 2007 the dominant form of Unicode (called UTF-8) surpassed American ASCII and a Western European variant as the most widely used encoding on the web, and UTF-8 is now used on more than 75% of web pages. As it celebrates its 50th birthday, then, ASCII is in decline, as it is gradually replaced by Unicode. But in a sense it lives on: the first 128 characters of Unicode precisely match those of ASCII, providing backwards compatibility, while also preserving aspects of even older codes in digital amber.



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