C0 controls

The C0 controls are the control characters (code positions 0-31 decimal) which are part of the ASCII standard (as adopted in 1967; the earlier 1963 version had somewhat different control character assignments). They are also part of a number of other character sets derived from ASCII, and are in the Unicode set at the same code positions they have in ASCII. The official definition gives particular meanings for each of the characters, though they have also been used in a number of other ways, and some of them are rarely used these days. Some character sets (including Unicode) have a second set of controls called the C1 controls in the equivalent positions with the 8th bit set (128-159 decimal), but those are rarely used in their control meanings (the Windows character sets have printable characters there).

Various platform-specific character sets also assign graphical renditions to these characters, and it is sometimes necessary to "poke" them directly into screen memory to use them in their graphical meanings, as opposed to "printing" them using standard output methods, which causes them to be interpreted as control characters and activate non-graphical functionality.

In addition to this contiguous block of control characters, there is also the DEL character at position 127 decimal (7F hex), and the 1963 ASCII standard also used position 126 for ESC and 124 for ACK.