Text Editor And Corrector Description And Impact History

TECO, noted for its complex syntax, can be considered a general-purpose, interpreted programming language targeted for text manipulation. Almost every character is a command—a 1- or 2-character sequence replaces the usual keywords of more verbose languages—thus any character string is a TECO program, although not necessarily a useful one. One common game used to be mentally working out what the TECO commands corresponding to human names did.

TECO was available for several operating systems and computers, including the PDP-1The PDP-1 P rogrammed D ata P rocessor 1 was the first computer in Digital Equipment's PDP series and was first produced in 1960. It is famous for being the computer most important in the creation of hacker culture, at MIT, BBN and elsewhere. The PDP-1 wa, ITSITS the Incompatible Timesharing System was an early, revolutionary, and influential MIT time-sharing operating system; it was developed principally by the Artificial Intelligence Laboratory at MIT, with some help from Project MAC. ITS development was ini on the PDP-6 and PDP-10The PDP-10 was a computer manufactured by Digital Equipment Corporation (DEC) from the late 1960s on; the name stands for "Programmed Data Processor model 10". It was the machine that made time-sharing common; it looms large in hacker folklore because of, and TOPS-10The TOPS-10 System was a computer operating system from Digital Equipment Corporation (DEC) for the PDP-10 released in 1964. TOPS-10 had an interesting scheduler of having many run queues, unlike OpenVMS for example which has two run queues, and inserts p and TOPS-20The TOPS-20 operating system by DEC the second proprietary OS for the PDP-10 preferred by most PDP-10 hackers over TOPS-10 (that is, by those who were not ITS or WAITS partisans). TOPS-20 began in 1969 as Bolt, Beranek and Newman's TENEX operating system, on the PDP-10The PDP-10 was a computer manufactured by Digital Equipment Corporation (DEC) from the late 1960s on; the name stands for "Programmed Data Processor model 10". It was the machine that made time-sharing common; it looms large in hacker folklore because of. A descendant of the version DEC distributed for the PDP-10 is still available on the Internet, along with several partial implementations for the MS-DOS / Windows environment. A version of TECO was provided with all DEC operating systems; the version available for RT11 was able to drive the GT40 graphics display while the version available for RSTS/E was actually implemented as a multi-user "RunTime System" and could be used as the user's complete operating environment; you never had to actually exit TECO! HP, having bought Compaq/DEC, still provides TECO with VMS.

2 History

TECO was originally developed at MIT circa 1963 for use on two PDP-1 computers, belonging to different departments, both housed in Building 26. On these machines, the normal development process involved the use of a Friden Flexowriter to prepare source code offline on a continuous strip of punched paper tape. Programmers of the big IBM mainframes customarily punched their source code on cards, using key punches which printed human-readable dot-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, and necessity was the mother of online editing.

The first such editor for the PDP-1 was (officially!) named " Expensive Typewriter." It was the most rudimentary imaginable line-oriented editor, lacking even search-and-replace capabilities. Its name reflected distaste for the expense and apparent inefficiency of a programmer's monopolizing an expensive computer for the purpose of editing text.

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

It was all nonsense, of course, because the correction tape was, in fact 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.

3 Example TECO code

ER file $ — open file for read access
[q ... ]q — push ... pop register Q (can hold number, text, or code)
< code > — iteration; there are codes for next, break, continue, etc.
n"X then-code | else-code ' — if-then-else (X is a test type)

4 Example TECO programs

This is 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. A " goto" and " structured" version are shown. Note that TECO ignores case and whitespace (except tab, which is an insertion command).

4.1 Example 1

!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 !

4.2 Example 2

0uz ! clear repeat flag ! 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 !

5 TECO Trivia

Most DEC command languages interpreted the "MAKE filename" command as a command to start TECO and create the named filename. Many (most?) TECOs would respond to "MAKE LOVE" with the message "Not war?".

TECO was considered to be one of the first "Write-only" languages. That is, you could write a program in TECO, but it was unlikely that you could look at an arbitrary TECO program and precisely describe what it did. (More recently, APL and Forth have earned this dubious distinction.)

This article is based on an entry in the Jargon File, which is in the public domain.

6 See also

line editor

7 External links

Pete Siemsen's TECO collection

Text editors

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1 Description and impact