Extended ASCII

The term extended ASCII (EASCII or high ASCII) refers to eight-bit or larger character encodings that include the standard seven-bit ASCII characters, plus additional characters. The use of the term is sometimes criticized, because it can be mistakenly interpreted to mean that the ASCII standard has been updated to include more than 128 characters or that the term unambiguously identifies a single encoding, both of which are not the case.

There are many extended ASCII encodings (more than 220 DOS and Windows codepages). EBCDIC ("the other" major 8-bit character code) likewise developed many extended variants (more than 186 EBCDIC codepages) over the decades.

ASCII was designed in the 1960s for teleprinters and telegraphy, and some computing. Early teleprinters were electromechanical, having no microprocessor and just enough electromechanical memory to function. They fully processed one character at a time, returning to an idle state immediately afterward. They were typewriter-derived impact printers, and could only print a fixed set of glyphs, which were cast into a metal type element or elements. Seven-bit ASCII improved over prior five- and six-bit codes: it added the lowercase letters of the English alphabet and more symbols, for a total of 95 printing characters; it encoded the letters in alphabetical order rather than keyboard position; and it used the extra bits to address all characters rather than use control characters to select banks of characters.

ASCII encodes only 95 carefully selected printable characters (94 glyphs and one space), which include the English alphabet (uppercase and lowercase), digits, and 31 punctuation marks and symbols: all of the symbols on a standard US typewriter plus a few; enough to communicate text information. The number of printable characters was deliberately kept small, to keep teleprinters and line printers inexpensive. Some popular peripherals only implemented a 64-printing-character subset: Teletype Model 33 could not transmit "a" through "z" or five less-common symbols (` { | } and ~). and when they received such characters they instead printed "A" through "Z" (forced all caps) and five other mostly-similar symbols (@ [ \ ] and ^).

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