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ASCII

A 7-bit character code from the 1960s with 128 values: letters, digits, punctuation and control codes like the line feed. Most modern encodings, UTF-8 included, keep these first 128 values the same.

The 128 codes

ASCII numbers its characters 0 to 127, which fits in seven bits. Codes 0 to 31 and 127 are control codes, like tab (9), line feed (10), carriage return (13) and delete (127). Codes 32 to 126 are the printable characters: the space, punctuation, the digits starting at 48, uppercase letters at 65 and lowercase at 97.

The layout is deliberate. Lowercase sits 32 codes after uppercase, which is a single bit (0x20), so a program can change case by flipping one bit. The digits 0 to 9 use the codes 0x30 to 0x39, so the low four bits of a digit's code are the digit itself.

The spare bit

ASCII uses seven bits of an 8-bit byte, so the top bit is free. Vendors used it for 128 more characters, and those 8-bit code pages, like ISO 8859-1 (Latin-1) and Windows-1252, keep ASCII in the lower half and add accented letters and symbols above it. They aren't ASCII. A file labeled ASCII that contains an é is really in a code page or in UTF-8, and opening it with the wrong one gives mojibake.

UTF-8 keeps the lower half identical and uses the top bit as a signal: a byte of 128 or more is part of a longer character. That's why any ASCII file is already a valid UTF-8 file.

ASCII inside other formats

Many formats are built on ASCII even when they carry other text. RFC 3986 says a URL is composed from a limited set of characters, which is why everything else gets percent encoding. Base64 picks its alphabet from US-ASCII, so its output is plain ASCII text, one byte per character: 44 bytes for a 32-byte hash. Base32 and hex do the same, and JSON's structural characters like {, }, : and , are all ASCII too.

The practical rule: treat ASCII as the first 128 characters of UTF-8, and you rarely need to think about it as a separate encoding.

Work it out yourselfOpen the Text Encoding Converter

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Related terms

UTF-8The most common way to store Unicode text as bytes. It writes each code point as one to four bytes, and plain ASCII text stays one byte per character, so any ASCII file is already valid UTF-8.Unicode code pointThe number Unicode gives to a character, written as U+ and hex digits, like U+0041 for A. Code points run from U+0000 to U+10FFFF, and an encoding such as UTF-8 turns each one into bytes.MojibakeGarbled text like é where é belongs. It happens when bytes saved in one encoding are read as another, most often UTF-8 read as Latin-1 or Windows-1252.Base64 encodingAn encoding that writes binary data as 64 printable ASCII characters. It's used to embed binary data in JSON, URLs, and email, and it adds about 33% to the size.

Sources

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