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The C0 and C1 control code or control character sets define control codes for use in text by computer systems that use ASCII and derivatives of ASCII. The codes represent additional information about the text, such as the position of a cursor, an instruction to start a new line, or a message that the text has been received.
There were quite a few control characters defined (33 in ASCII, and the ECMA-48 standard adds 32 more). This was because early terminals had very primitive mechanical or electrical controls that made any kind of state-remembering API quite expensive to implement, thus a different code for each and every function looked like a requirement.
The control code ranges 0x00–0x1F ("C0") and 0x7F originate from the 1967 edition of US-ASCII.The standard ISO/IEC 2022 (ECMA-35) defines extension methods for ASCII, including a secondary "C1" range of 8-bit control codes from 0x80 to 0x9F, equivalent to 7-bit sequences of ESC with the bytes 0x40 through 0x5F.
ASCII was incorporated into the Unicode (1991) character set as the first 128 symbols, so the 7-bit ASCII characters have the same numeric codes in both sets. This allows UTF-8 to be backward compatible with 7-bit ASCII, as a UTF-8 file containing only ASCII characters is identical to an ASCII file containing the same sequence of characters.
For example, the character "a" has a binary ASCII code of 110 0001. This code would be converted to 000 0001, corresponding to the ASCII character with id 1 (the SOH Character). The table at C0 and C1 control codes § C0 controls shows the ASCII control characters, with the "Caret notation" column showing a caret (^), followed by the character ...
For the control-codes outside of the range 1–26, the notation extends to the adjacent, non-alphabetic ASCII characters. Often a control character can be typed on a keyboard by holding down the Ctrl and typing the character shown after the caret.
The block contains all the letters and control codes of the ASCII encoding. It ranges from U+0000 to U+007F, contains 128 characters and includes the C0 controls, ASCII punctuation and symbols, ASCII digits, both the uppercase and lowercase of the English alphabet and a control character.
C character classification is a group of operations in the C standard library that test a character for membership in a particular class of characters; such as alphabetic, control, etc. Both single-byte, and wide characters are supported.