1

Scope

This International Standard specifies the structure of 8-bit codes and 7-bit codes which provide for the coding of character sets. The code elements used in the structure are common to both the 8-bit and 7-bit codes. The codes use a veriety of techniques for extending the capabilities of elementary 8-bit and 7-bit codes. Greater emphasis is given to 8-bit codes in this edition of the Standard than in previous editions because they are now more widely used.

The use of common elements in the 8-bit and 7-bit code structure enables any specific conforming 8-bit code to be transformed into an equivalent 7-bit code, and vice versa, in a simple and direc t fashion.

ISO/IEC 4873 conforms to the 8-bit code structure specified here, and ISO/IEC 646 conforms to the 7-bit code structure specified here.

Note - The coded character set specified in ISO/IEC 10646-1 has a different structure not in accordance with this International Standard.

The code structure facilities specified here include various means of extending the number of control functions and graphic characters available in a code. They also include techniques to construct and formalize the definition of specific codes, and to provide a coded identification of the structure and of the constituent elements of such specific codes.

Specific codes may also be identified by means of object identifiers in accordance with ISO 8824, Abstract Syntax Notation One (ASN.1). The form of such object identifiers is specified in annex A.

Individual character sets and control functions intended for use with these 8-bit and 7-bit codes are assumed to be registered in the ISO International Register of Coded Character Sets to be Us ed with Escape Sequences, in accordance with ISO 2375 (see annex B). The register includes details to relate individual character sets and control functions with their coded representations, and also with the associated coded identifications of such character sets.

The principles established in this International Standard may be utilized to form supplementary code structure facilities. For example ISO/IEC 6429 has followed such a procedure to formulate some parameterized control functions.

The use of uniform code structure techniques for the 8-bit and 7-bit codes specified here has the advantage of: - permitting uniform provision for code structure in the design of information processing systems, - providing standardized methods of calling into use agreed sets of characters, - allowing the interchange of data between environments that utilise 8-bit and 7-bit codes respectively, - reducing the risk of conflict between systems required to inter-operate.

When two systems with different levels of implementation of code st ructure facilities are required to communicate with one another, they may do so using the code structure facilities that they have in common.

The codes specified here are designed to be used for data that is processed sequentially in a forward direction. Use of these codes in strings of data which are processed in some other way, or which are included in data formatted for fixed-length record processing, may have undesirable results or may require additional special tratment to ensure correct interpretation.

Note - Since the previous edition (1986) of this International Standard the text has been completely rearranged and rewritten to make the Standard more convenient to use as a reference document. It is now arranged in three main sections as follows: 1 General 2 Character Sets and Codes 3 Code Identification and Escape Sequences.