Editing ASCII (section)

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The inherent ambiguity of many control characters, combined with their historical usage, created problems when transferring "plain text" files between systems. The best example of this is the [[newline]] problem on various [[operating system]]s. Teletype machines required that a line of text be terminated with both "carriage return" (which moves the printhead to the beginning of the line) and "line feed" (which advances the paper one line without moving the printhead). The name "carriage return" comes from the fact that on a manual [[typewriter]] the carriage holding the paper moves while the typebars that strike the ribbon remain stationary.  The entire carriage had to be pushed (returned) to the right in order to position the paper for the next line.

DEC operating systems ([[OS/8]], [[RT-11]], [[RSX-11]], [[RSTS/E|RSTS]], [[TOPS-10]], etc.) used both characters to mark the end of a line so that the console device (originally Teletype machines) would work. By the time so-called "glass TTYs" (later called CRTs or "dumb terminals") came along, the convention was so well established that [[backward compatibility]] necessitated continuing to follow it. When [[Gary Kildall]] created [[CP/M]], he was inspired by some of the command line interface conventions used in DEC's RT-11 operating system.

Until the introduction of PC DOS in 1981, [[IBM]] had no influence in this because their 1970s operating systems used EBCDIC encoding instead of ASCII, and they were oriented toward punch-card input and line printer output on which the concept of "carriage return" was meaningless. IBM's PC DOS (also marketed as [[MS-DOS]] by Microsoft) inherited the convention by virtue of being loosely based on CP/M,<ref>{{cite web|url=http://dosmandrivel.blogspot.com/2007/08/is-dos-rip-off-of-cpm.html|title=Is DOS a Rip-Off of CP/M?|author=Tim Paterson|date=August 8, 2007|website=DosMan Drivel|author-link=Tim Paterson|access-date=April 19, 2018|archive-url=https://web.archive.org/web/20180420075137/http://dosmandrivel.blogspot.com/2007/08/is-dos-rip-off-of-cpm.html|archive-date=April 20, 2018|url-status=live}}</ref> and [[Windows]] in turn inherited it from MS-DOS.

Requiring two characters to mark the end of a line introduces unnecessary complexity and ambiguity as to how to interpret each character when encountered by itself. To simplify matters, [[plain text]] data streams, including files, on [[Multics]] used line feed (LF) alone as a line terminator.<ref>{{cite conference |url=http://www.multicians.org/jhs-jfo-terminals.pdf |title=Technical and human engineering problems in connecting terminals to a time-sharing system |author-last1=Ossanna |author-first1=J. F. |author-link1=Joe Ossanna |author-last2=Saltzer |author-first2=J. H. |author-link2=Jerry Saltzer |date=November 17–19, 1970 |publisher=[[AFIPS]] Press |book-title=Proceedings of the November 17–19, 1970, [[Fall Joint Computer Conference]] (FJCC) |pages=355–362 |quote=Using a "new-line" function (combined carriage-return and line-feed) is simpler for both man and machine than requiring both functions for starting a new line; the American National Standard X3.4-1968 permits the line-feed code to carry the new-line meaning. |access-date=January 29, 2013 |archive-url=https://web.archive.org/web/20120819085101/http://www.multicians.org/jhs-jfo-terminals.pdf |archive-date=August 19, 2012 |url-status=live }}</ref>{{rp|357}} The tty driver would handle the LF to CRLF conversion on output so files can be directly printed to terminal, and NL (newline) is often used to refer to CRLF in [[UNIX]] documents. [[Unix]] and [[Unix-like]] systems, and [[Amiga]] systems, adopted this convention from Multics.  On the other hand, the original [[Macintosh OS]], [[Apple DOS]], and [[ProDOS]] used carriage return (CR) alone as a line terminator; however, since Apple later replaced these obsolete operating systems with their Unix-based [[macOS]] (formerly named OS X) operating system, they now use line feed (LF) as well. The Radio Shack [[TRS-80]] also used a lone CR to terminate lines.

Computers attached to the [[ARPANET]] included machines running operating systems such as TOPS-10 and [[TENEX (operating system)|TENEX]] using CR-LF line endings; machines running operating systems such as Multics using LF line endings; and machines running operating systems such as [[OS/360]] that represented lines as a character count followed by the characters of the line and which used EBCDIC rather than ASCII encoding.  The [[Telnet]] protocol defined an ASCII "Network Virtual Terminal" (NVT), so that connections between hosts with different line-ending conventions and character sets could be supported by transmitting a standard text format over the network. Telnet used ASCII along with CR-LF line endings, and software using other conventions would translate between the local conventions and the NVT.<ref name="RFC-158">{{cite IETF |title=TELNET Protocol |rfc=158 |pages=4–5 |author-first=T. |author-last=O'Sullivan |date=1971-05-19 |publisher=[[Internet Engineering Task Force]] (IETF) |access-date=2013-01-28}}</ref> The [[File Transfer Protocol]] adopted the Telnet protocol, including use of the Network Virtual Terminal, for use when transmitting commands and transferring data in the default ASCII mode.<ref name="RFC-542">{{cite IETF |title=File Transfer Protocol |rfc=542 |author-first=Nancy J. |author-last=Neigus |date=1973-08-12 |publisher=[[Internet Engineering Task Force]] (IETF) |access-date=2013-01-28}}</ref><ref name="RFC-765">{{cite IETF |title=File Transfer Protocol |rfc=765 |author-first=Jon |author-last=Postel |author-link=Jon Postel |date=June 1980 |publisher=[[Internet Engineering Task Force]] (IETF) |access-date=2013-01-28}}</ref> This adds complexity to implementations of those protocols, and to other network protocols, such as those used for E-mail and the World Wide Web, on systems not using the NVT's CR-LF line-ending convention.<ref>{{cite web |url=https://www.mercurial-scm.org/wiki/EOLTranslationPlan |title=EOL translation plan for Mercurial |publisher=Mercurial |access-date=2017-06-24 |archive-url=https://web.archive.org/web/20160616235536/https://www.mercurial-scm.org/wiki/EOLTranslationPlan |archive-date=June 16, 2016 |url-status=live }}</ref><ref>{{cite web |title=Bare LFs in SMTP |url=http://cr.yp.to/docs/smtplf.html |author-first=Daniel J. |author-last=Bernstein |author-link=Daniel J. Bernstein |access-date=2013-01-28 |archive-url=https://web.archive.org/web/20111029013105/http://cr.yp.to/docs/smtplf.html |archive-date=October 29, 2011 |url-status=live }}</ref>