Overview
ISO/IEC 4873:1991 specifies an 8‑bit code for information interchange derived from and compatible with the 7‑bit ISO/IEC 646 coded character set. It defines the structure, invocation rules, conformance criteria, and options for creating specific versions and levels of the 8‑bit code. The standard is intended for general information interchange in 8‑bit environments across data processing systems, input/output devices and communication channels.
Key topics and technical requirements
- Code structure: The 8‑bit code is organized into code table positions (xx/yy) mapped to the 256 bit combinations (00/00–15/15). It divides the space into sets: C0 (control), GL/GO (graphic), DELETE, C1 (control), G1, G2, G3 (graphic).
- Character positions and notation: Bits b8..b1 are used with weights 128..1; positions are identified by hexadecimal-like column/row notation (xx/yy).
- Control and graphic characters:
- SPACE = 02/00; DELETE = 07/15.
- ESCAPE (control) = 01/11, used to form escape sequences for code extension.
- GO set covers 02/01–07/14 (94 spacing graphic characters).
- C0 and C1 restrictions: Normative Annex A specifies restrictions applicable to the C0 and C1 control sets; Annex D notes unused bit combinations (00/14 and 00/15).
- Invocation and shifts: The standard specifies how sets are invoked and how locking- and single‑shift functions (LS1R, LS2R, LS3R, SS2, SS3) operate to bring G1/G2/G3 into use (see Annex B for shift functions).
- Levels and versions: Defines Levels 1–3 and mechanisms to define versions (character repertoire and mapping). Claims of conformance must identify the adopted version/level.
- Conformance and device requirements:
- Originating devices must be able to supply any character sequence from the adopted version and transmit coded representations.
- Receiving devices must interpret any coded representations conforming to the version and present distinguishable characters to users.
- Prohibitions: Use of control functions (e.g., backspace, carriage return) to encode composite characters is prohibited (Annex C).
Practical applications and users
- Used by system integrators, protocol designers, font and terminal implementers, and device manufacturers who need a standardized 8‑bit character encoding for:
- Data interchange between computers and peripherals.
- File storage formats, transmission channels, and legacy system interoperability.
- Defining national or application-specific 8‑bit versions that extend the basic Latin repertoire (accents, special letters, non‑Latin alphabets).
- Useful in migration and interoperability work where compatibility with ISO/IEC 646 and ISO 2022 code‑extension techniques is required.
Related standards
- ISO/IEC 646 (7‑bit coded character set)
- ISO 2022 (code extension techniques)
- ISO/IEC 6429 (control functions)
- ISO/IEC 10367 (standardized graphic character sets)
- ISO International Register (ISO‑IR) for escape sequence registrations
Keywords: ISO/IEC 4873, 8-bit code, information interchange, ISO 2022, character sets, C0 C1, G1 G2 G3, ESCAPE, conformity, code extension.
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