What does the American standard for information interchange mean?

Answer 1

You mean ASCII, the American Standard Code for Information Interchange.

ASCII is a 7-bit encoding that is supported by all computer systems as standard. This makes it possible to send plain-text data between any two systems and know that it will interpreted correctly.

Each encoding maps to a unique character (a glyph) or a control code, such that 0x41 through 0x5A represents all the capital letters in the English alphabet, 'A' through 'Z', while 0x61 to 0x7A represents all the lower case letters, 'a' through 'z'. The digits '0' though '9' are represented by 0x30 through 0x39. Common punctuation is also supported, as is whitespace, such as 0x20 for a single space and 0x09 for tab. Non-printing characters such as carriage return and line feed are also supported. there are 127 glyphs and control codes in the ASCII character set.

Since ASCII uses only 7-bits, this conveniently leaves 1 bit free in an 8-bit byte. When this bit is on, the encoding maps to an extended ASCII table of 128 additional characters. This set of characters is system-dependant, but allows a complete set of 256 unique characters, where the first 127 are always the same.

All English-based text can therefore be encoded in 7-bit ASCII alone. Non-English language support typically requires a much broader range of encodings, whether through the extended ASCII character set or through UNICODE encodings. However, ASCII is a subset of UNICODE so no matter how many bytes are used to represent an encoding, 7-bit ASCII data can always be interpreted correctly. The only additional information required is a byte-order mark (BOM) which determines whether the encoding is little-endian or big-endian, as well as the type of UNICODE employed in the encoding (UTF8, UTF16, UTF32 or UTF64).

UTF8 is the most common form of UNICODE encoding in use today because it is a variable-length encoding and has no overhead with respect to ASCII-encoded data. If bit 8 is off, the encoding is simply interpreted as a 7-bit ASCII character, otherwise it is interpreted as an escape sequence, the value of which determines how many subsequent bytes are used to represent the full encoding. This could be anything from 1 to 6 additional bytes per character, each of which maps directly to a character in one of the UNICODE character sets, all of which incorporate the ASCII set.