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Code 39 (also known as Alpha39, Code 3 of 9, Code 3/9, Type 39, USS Code 39, or USD-3) is a variable length, discrete barcode symbology defined in ISO/IEC 16388:2007. The Code 39 specification defines 43 characters, consisting of uppercase letters (A through Z), numeric digits (0 through 9) and a number of special characters ...
The standard requires operations to convert between basic formats and external character sequence formats. [57] Conversions to and from a decimal character format are required for all formats. Conversion to an external character sequence must be such that conversion back using round to nearest, ties to even will recover the original number.
ASCII reserves the first 32 code points (numbers 0–31 decimal) and the last one (number 127 decimal) for control characters. These are codes intended to control peripheral devices (such as printers ), or to provide meta-information about data streams, such as those stored on magnetic tape.
In HTML and XML, a numeric character reference refers to a character by its Universal Character Set/Unicode code point, and uses the format: &#xhhhh;. or &#nnnn; where the x must be lowercase in XML documents, hhhh is the code point in hexadecimal form, and nnnn is the code point in decimal form.
For codes from 0 to 127, the original 7-bit ASCII standard set, most of these characters can be used without a character reference. Codes from 160 to 255 can all be created using character entity names. Only a few higher-numbered codes can be created using entity names, but all can be created by decimal number character reference.
In 1973, ECMA-35 and ISO 2022 [17] attempted to define a method so an 8-bit "extended ASCII" code could be converted to a corresponding 7-bit code, and vice versa. [18] In a 7-bit environment, the Shift Out would change the meaning of the 96 bytes 0x20 through 0x7F [a] [20] (i.e. all but the C0 control codes), to be the characters that an 8-bit environment would print if it used the same code ...
A code point is a value or position of a character in a coded character set. [9] A code space is the range of numerical values spanned by a coded character set. [9] [11] A code unit is the minimum bit combination that can represent a character in a character encoding (in computer science terms, it is the word size of the character encoding).
To this end a series of encodings registered with the IANA add the C0 control set (control characters mapped to bytes 0 to 31) from ISO 646 and the C1 control set (control characters mapped to bytes 128 to 159) from ISO 6429, resulting in full 8-bit character maps with most, if not all, bytes assigned.