Code point

In character encoding terminology, a code point or code position is any of the numerical values that make up the codespace.[1][2] Many code points represent single characters but they can also have other meanings, such as for formatting.[3]

Not to be confused with point code.

For example, the character encoding scheme ASCII comprises 128 code points in the range 0hex to 7Fhex, Extended ASCII comprises 256 code points in the range 0hex to FFhex, and Unicode comprises 1,114,112 code points in the range 0hex to 10FFFFhex. The Unicode code space is divided into seventeen planes (the basic multilingual plane, and 16 supplementary planes), each with 65,536 (= 216) code points. Thus the total size of the Unicode code space is 17 × 65,536 = 1,114,112.

Definition

The notion of a code point is used for abstraction, to distinguish both:

  • the number from an encoding as a sequence of bits, and
  • the abstract character from a particular graphical representation (glyph).

This is because one may wish to make these distinctions to:

  • encode a particular code space in different ways, or
  • display a character via different glyphs.

For Unicode, the particular sequence of bits is called a code unit – for the UCS-4 encoding, any code point is encoded as 4-byte (octet) binary numbers, while in the UTF-8 encoding, different code points are encoded as sequences from one to four bytes long, forming a self-synchronizing code. See comparison of Unicode encodings for details. Code points are normally assigned to abstract characters. An abstract character is not a graphical glyph but a unit of textual data. However, code points may also be left reserved for future assignment (most of the Unicode code space is unassigned), or given other designated functions.

The distinction between a code point and the corresponding abstract character is not pronounced in Unicode, but is evident for many other encoding schemes, where numerous code pages may exist for a single code space.

History

The concept of a code point is part of Unicode's solution to a difficult conundrum faced by character encoding developers in the 1980s.[4] If they added more bits per character to accommodate larger character sets, that design decision would also constitute an unacceptable waste of then-scarce computing resources for Latin script users (who constituted the vast majority of computer users at the time), since those extra bits would always be zeroed out for such users.[5] The code point avoids this problem by breaking the old idea of a direct one-to-one correspondence between characters and particular sequences of bits.

See also

References
  1. Glossary of Unicode Terms
  2. "The Unicode® Standard Version 11.0 – Core Specification" (PDF). Unicode Consortium. 30 June 2018. p. 22. Archived from the original (pdf) on 19 September 2018. Retrieved 25 December 2018. On a computer, abstract characters are encoded internally as numbers. To create a complete character encoding, it is necessary to define the list of all characters to be encoded and to establish systematic rules for how the numbers represent the characters. The range of integers used to code the abstract characters is called the codespace. A particular integer in this set is called a code point. When an abstract character is mapped or assigned to a particular code point in the codespace, it is then referred to as an encodedcharacter.
  3. "The Unicode® Standard Version 11.0 – Core Specification" (PDF). Unicode Consortium. 30 June 2018. p. 23. Archived from the original (pdf) on 19 September 2018. Retrieved 25 December 2018. Format: Invisible but affects neighboring characters; includes line/paragraph separators
  4. Constable, Peter (13 June 2001). "Understanding Unicode™ - I". NRSI: Computers & Writing Systems. Archived from the original (html) on 16 September 2010. Retrieved 25 December 2018. By the early 1980s, the software industry was starting to recognise the need for a solution to the problems involved with using multiple character encoding standards. Some particularly innovative work was begun at Xerox. The Xerox Star workstation used a multi-byte encoding that allowed it to support a single character set with potentially millions of characters.
  5. Mark Davis, Ken Whistler (23 March 2001). "Unicode Technical Standard #10 UNICODE COLLATION ALGORITHM". Unicode Consortium. Archived from the original (html) on 25 August 2001. Retrieved 25 December 2018. 6.2 Large Weight ValuesCS1 maint: uses authors parameter (link)
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