Null-terminated string: Difference between revisions

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Null-terminated strings were produced by the <code>.ASCIZ</code> directive of the [[PDP-11]] [[assembly language]]s and the <code>ASCIZ</code> directive of the [[MACRO-10]] macro assembly language for the [[PDP-10]]. These predate the development of the C programming language, but other forms of strings were often used.

At the time C (and the languages that it was derived from) was developed, memory was extremely limited, so using only one byte of overhead to store the length of a string was attractive. The only popular alternative at that time, usually called a "Pascal string" (a more modern term is "[[String (computer science)#Length-prefixed|length-prefixed]]"), used a leading ''byte'' to store the length of the string. This allows the string to contain NUL and made finding the length of an already stored string need only one memory access (O(1) [[constant time|(constant) time]]), but limited string length to 255 characters (on a machine using 8-bit bytes). C designer [[Dennis Ritchie]] chose to follow the convention of null-termination to avoid the limitation on the length of a string and because maintaining the count seemed, in his experience, less convenient than using a terminator.<ref>{{cite conference | first = Dennis M. | last = Ritchie | date = April 1993 | ___location = Cambridge, MA | title = The development of the C language | conference = Second History of Programming Languages conference | url = https://www.bell-labs.com/usr/dmr/www/chist.html }}</ref><ref>{{cite book | first = Dennis M. | last = Ritchie | article = The development of the C language | title = History of Programming Languages | edition = 2 | editor-first1= Thomas J. | editor-last1= Bergin, Jr. | editor-first2 = Richard G. | editor-last2 = Gibson, Jr. | publisher = ACM Press | ___location = New York | via = Addison-Wesley (Reading, Mass) | year = 1996 | isbn = 0-201-89502-1 }}</ref>

This had some influence on CPU [[instruction set]] design. Some CPUs in the 1970s and 1980s, such as the [[Zilog Z80]] and the [[Digital Equipment Corporation|DEC]] [[VAX]], had dedicated instructions for handling length-prefixed strings. However, as the null-terminated string gained traction, CPU designers began to take it into account, as seen for example in IBM's decision to add the "Logical String Assist" instructions to the [[IBM ES/9000 family|ES/9000]] 520 in 1992 and the vector string instructions to the [[IBM z13 (microprocessor)|IBM z13]] in 2015.<ref name=pop>[http://publibfp.dhe.ibm.com/epubs/pdf/a227832c.pdf IBM z/Architecture Principles of Operation]</ref>