TECO (/ˈtiːkoʊ/^[1]), short forText Editor & Corrector,^[2][3][4] is both a character-orientedtext editor and aprogramming language,^[5][6] that was developed in 1962 for use onDigital Equipment Corporation computers, and has since become available onPCs andUnix.Dan Murphy developed TECO while a student at theMassachusetts Institute of Technology (MIT).^[5]

According to Murphy, the initial acronym wasTape Editor and Corrector because "punched paper tape was the only medium for the storage of program source on our PDP-1. There was no hard disk, floppy disk, magnetic tape (magtape), or network."^[5] By the time TECO was made available for general use, the name had become "Text Editor and Corrector",^[4] since even the PDP-1 versionby then supported other media.^[5] It was subsequently modified by many other people^[7] and is a direct ancestor ofEmacs, which was originally implemented in TECO macros.^[8][9][10]

TECO is not only an editor but also aninterpretedprogramming language for text manipulation. Arbitrary programs (called "macros") for searching and modifying text give it great power. Unlikeregular expressions, however, the language wasimperative, though some versions had an "or" operator in string search.

TECO does not really havesyntax; each character in a program is an imperative command, dispatched to its corresponding routine. That routine may read further characters from the program stream (giving the effect of string arguments), change the position of the "program counter" (giving the effect of control structures), or push values onto a value stack (giving the effect of nested parentheses). But there is nothing to prevent operations like jumping into the middle of a comment, since there is no syntax and no parsing.

TECO ignorescase andwhitespace (excepttab, which is an insertion command).^[11]

A satirical essay on computer programming, "Real Programmers Don't Use Pascal", suggested that a common game for TECO fans was to enter their name as a command sequence, and then try to work out what would happen. The same essay in describing TECO coined theacronymYAFIYGI, meaning "You Asked For It You Got It" (in contrast toWYSIWYG).

TheEMACS editor originally started byDavid A. Moon andGuy L. Steele Jr. was implemented in TECO as a set of Editor MACroS. TECO became more widely used following aDigital Equipment Corporation (DEC)PDP-6 implementation developed at MIT'sProject MAC in 1964. This implementation continuously displayed the edited text visually on aCRT screen, and was used as an interactive online editor. Later versions of TECO were capable of driving full-screen mode on various DECRS-232 video terminals such as theVT52 orVT100.

TECO was available for several operating systems and computers, including thePDP-1 computer, thePDP-8 (under OS/8),^[12][13] theIncompatible Timesharing System (ITS) on the PDP-6 andPDP-10, andTOPS-10 andTOPS-20 on thePDP-10. A version of TECO was provided with all DEC operating systems; the version available forRT11 was able to drive the GT40 graphics display while the version available forRSTS/E was implemented as a multi-userrun-time system and could be used as the user's complete operating environment; the user never actually had to exit TECO. The VTEDIT (Video Terminal Editor) TECO macro was commonly used onRSTS/E andVAX systems with terminals capable of direct-cursor control (e.g.VT52 andVT100) to provide a full-screen visual editor similar in function to the contemporaneously developedEmacs.

TECO continues to be included inOpenVMS by VSI, and is invoked with theEDIT/TECO command.^[14]

A descendant of the version DEC distributed for the PDP-10 is still available on the Internet, along with several partial implementations for theMS-DOS/Microsoft Windows environment.

TECO was originally developed at MIT^[15] in around 1963 byDaniel L. Murphy for use on twoPDP-1 computers, belonging to different departments, both housed in MIT's Building 26.^[16] On these machines, the normal development process involved the use of aFriden Flexowriter to prepare source code offline on a continuous strip of punched paper tape. Programmers of the bigIBM mainframes customarily punched theirsource code oncards, usingkey punches which printed human-readabledot-matrix characters along the top of every card at the same time as they punched each machine-readable character. Thus IBM programmers could read, insert, delete, and move lines of code by physically manipulating the cards in the deck. Punched paper tape offered no such amenities, leading to the development of online editing.

An early editor for the PDP-1 was named "Expensive Typewriter". Written by Stephen D. Piner, it was the most rudimentary imaginable line-oriented editor, lacking even search-and-replace capabilities. Its name was chosen as a wry poke at an earlier, rather bloated, editor called "Colossal Typewriter". Even in those days, online editing could save time in the debugging cycle. Another program written by the PDP-1hackers wasExpensive Desk Calculator, in a similar vein.

The original stated purpose of TECO was to make more efficient use of the PDP-1. As envisioned in the manual, rather than performing editing "expensively" by sitting at aconsole, one would simply examine the faulty text and prepare a "correction tape" describing the editing operations to be performed on the text. One would efficiently feed the source tape and the correction tape into the PDP-1 via its high-speed (200 characters per second) reader. Running TECO, it immediately would punch an edited tape with its high-speed (60 characters per second) punch. One could then immediately proceed to load and run the assembler, with no time wasted in online editing.

TECO's sophisticated searching operations were motivated by the fact that the offline Flexowriter printouts were not line-numbered. Editing locations therefore needed to be specified by context rather than by line number. The various looping and conditional constructs (which made TECOTuring-complete) were included in order to provide sufficient descriptive power for the correction tape. The terse syntax minimized the number of keystrokes needed to prepare the correction tape.

The correction tape was a program, and required debugging just like any other program. The pitfalls of even the simplest global search-and-replace soon became evident. In practice, TECO editing was performed online just as it had been with Expensive Typewriter (although TECO was certainly a more feature-complete editor than Expensive Typewriter, so editing was much more efficient with TECO). The original PDP-1 version had no screen display. The only way to observe the state of the text during the editing process was to type in commands that would cause the text (or portions thereof) to be typed out on the console typewriter.

By 1964, a special Version of TECO (TECO-6) had been implemented on thePDP-6 at MIT. That version supported visual editing, using a screen display that showed the contents of the editing buffer in real time, updating as it changed.^[17] Amongst the creators of TECO-6 wereRichard Greenblatt andStewart Nelson.^[18]

At MIT, TECO development continued in the fall of 1971.^{[citation needed]} Carl Mikkelsen had implemented a real-time edit mode loosely based on the TECO-6 graphic console commands, but working with the newly installedDatapoint-3300 CRT text displays.^[19] The TECO buffer implementation, however, was terribly inefficient for processing single character insert or delete functions—editing consumed 100% of the PDP-10. WithRichard Greenblatt's support, in summer of 1972 Carl reimplemented the TECO buffer storage and reformed the macros as native PDP-10 code.^{[citation needed]} As entering the real-time mode was by typingcntl+R, this was known as control-R mode. At the same time, Rici Liknaitski added input-time macros (cntl+]), which operated as the command string was read rather than when executed.^{[citation needed]} Read-time macros made the TECO auxiliary text buffers, called Q-registers, more useful.^{[citation needed]} Carl expanded the Q-register name space. With read-time macros, a large Q-register name space, and efficient buffer operations, the stage was set for binding each key to a macro.^[20] These edit macros evolved intoEmacs.^[21]

The VMS implementation has a long history - it began as TECO-8, implemented in PDP-8 assembly. This code was translated into PDP-11 assembly to produce TECO-11. TECO-11 was used in early versions of VAX/VMS in PDP-11 compatibility mode. It was later translated from PDP-11 assembly into VAX assembly to produce TECO32. TECO32 was then converted with the VEST and AEST binary translation utilities to make it compatible with OpenVMS on theAlpha andItanium respectively.^[22][23][24]

The OS/8CCLMUNG command invoked TECO to read and execute the specified .TE TECO macro. Optional command line parameters gave added adaptability.^[25]

During and shortly following the years ofthe punched card era, there were source programs that had begun aspunched card-based. Comments were often a series of lines that included single marginal asterisks and top/bottom full lines of asterisks. Once the cards were transferred online, it was difficult to realign the marginal stars. TECO aimed to solve this problem.^[26][27]

The obscurity of the TECO programming language is described in the following quote from "Real Programmers Don't Use Pascal", a letter from Ed Post to Datamation, July 1983:

It has been observed that a TECO command sequence more closely resembles transmission line noise than readable text. One of the more entertaining games to play with TECO is to type your name in as a command line and try to guess what it does. Just about any possible typing error while talking with TECO will probably destroy your program, or even worse - introduce subtle and mysterious bugs in a once working subroutine.^[28]

According to Craig Finseth, author ofThe Craft of Text Editing,^[29] TECO has been described as a "write-only" language, implying that once a program is written in TECO, it is extremely difficult to comprehend what it did without appropriate documentation.

Despite its syntax, the TECO command language was tremendously powerful, and clones are still available forMS-DOS and forUnix.

TECO commands are characters (including control-characters), and the prompt is a single asterisk:

*

The escape key displays as a dollar sign, pressed once it delineates the end of a command requiring an argument and pressed twice initiates the execution of the entered commands:

*$$

Given a file named hello.c with the following contents:

intmain(intargc,char**argv){printf("Hello world!\n");return0;}

one could use the following TECO session (noting that the prompt is "*" and "$" is how ESC is echoed) to change "Hello" into "Goodbye":

*EBhello.c$$                     Open file for read/write with backup*P$$                             Read in the first page*SHello$0TT$$                    Search for "Hello" and print the line (pointer placed after searched string)    printf("Hello world!\n");    The line*-5DIGoodbye$0TT$$               Delete five characters before pointer (ie "Hello"), insert "Goodbye", and print the line    printf("Goodbye world!\n");  The updated line*EX$$                            Copy the remainder of the file and exit

These two example programs are a simple interchange sort of the current text buffer, based on the 1st character of each line, taken from the PDP-11 TECO User's Guide.^[12] A "goto" and "structured" version are shown. The second program originally had a bug that prevented the program terminating and the fixed version is used here instead.

!START! j 0aua               ! jump to beginning, load 1st char in register A !!CONT! l 0aub                ! load first char of next line in register B !qa-qb"g xa k -l ga 1uz '     ! if A>B, switch lines and set flag in register Z !qbua                         ! load B into A !l z-."g -l @o/CONT/ '        ! loop back if another line in buffer !qz"g 0uz @o/START/ '         ! repeat if a switch was made on last pass !

<0uz                            ! clear repeat flag !j 0aua l                        ! load 1st char into register A !<0aub                           ! load 1st char of next line into B !qa-qb"g xa k -l ga -1uz '       ! if A>B, switch lines and set flag !qbua                            ! load B into A !l .-z;>                         ! loop back if another line in buffer !qz;>                            ! repeat if a switch was made last pass !

• TECO pocket guide.Digital Equipment Corporation. 1978. Archived fromthe original on 2008-02-07. Retrieved2012-05-24.
• Murphy, Dan (2009)."The Beginnings of TECO"(PDF).IEEE Annals of the History of Computing.31 (4):110–115.doi:10.1109/MAHC.2009.127.S2CID 18805607.
• TECO Historic Archive. 1990.

• Dan Murphy's personal site
• Pete Siemsen's TECO collection
• Tom Almy's TECO page. Includes a TECO based on Pete Siemsen's TECOC and DECUS documentation. There are MS-DOS, Windows (console), Linux, Mac OS X, and OS/2 versions.
• Introduction to the TECO syntax
• TECO Information

This article is based in part on theJargon File, which is in the public domain.

• TECO Manual (OS/8)

• History of software
• Massachusetts Institute of Technology software
• OpenVMS text editors
• Line editor
• Text-oriented programming languages

• All articles with unsourced statements
• Articles with unsourced statements from October 2009
• Wikipedia articles incorporating text from the Jargon File