ASCII

Definition

ASCII, the American Standard Code for Information Interchange, is a 7-bit character encoding with 128 values from 0 to 127. It assigns codes to basic Latin letters, digits, punctuation, spaces, and control characters.

Values 32 through 126 are printable characters. Values 0 through 31 and 127 are controls such as line feed, carriage return, and delete; their effect depends on the surrounding system.

How it works

Each character has a numeric code. Uppercase A is decimal 65, hexadecimal 41, and binary 01000001. These are three notations for the same value.

ASCII defines characters and control codes, while binary and hexadecimal merely display their numbers. A sequence of ASCII codes becomes text only when a program interprets those values with the ASCII character mapping.

Practical example

The text Hi has ASCII decimal codes 72 105, hexadecimal bytes 48 69, and binary bytes 01001000 01101001. Converting any of those notations to the same byte values recovers the two characters.

ASCII cannot directly represent Cyrillic, most accented letters, or emoji. Such text needs a broader character encoding, most commonly Unicode encoded as UTF-8.

ASCII, UTF-8, and extended ASCII

UTF-8 preserves ASCII exactly for values 0 through 127, so ordinary ASCII text is also valid UTF-8. Characters beyond that range use additional bytes in UTF-8.

“Extended ASCII” is not one universal encoding. The phrase can refer to several incompatible 8-bit code pages, so the exact code page must be known before bytes from 128 through 255 can be decoded reliably.