Atlas Autocode
Encyclopedia
Atlas Autocode was a programming language
developed around 1965 at Manchester University for the Atlas Computer. It was developed by Tony Brooker
and Derrick Morris as an improvement on the ALGOL
programming language
s, removing some of Algol's poorer features such as "passing parameters by name" (which with Algol 60 meant not its usual meaning but passing the address of a short subroutine to recalculate the parameter each time it was mentioned). It featured explicitly typed variables, subroutines, and functions. The AA compiler generated range-checking for array accesses, and allowed an array to have dimensions that were determined at run-time (i.e. you could declare an array as
was allowed.
Atlas Autocode included a
s (which comprise a real
part, and an imaginary
part which is a multiple of the square root of -1), partly because of pressure from the electrical engineering
department, as complex numbers are used to represent the behavior of alternating current
. The square root of -1 was represented by
The
programming language. (Imp was an extension of AA and was notable for being used to write the EMAS
operating system
.)
Keywords in AA were distinguishable from other text by being underlined; there was also an "uppercasedelimiters" mode where all uppercase letters (outside strings) were treated as underlined lowercase.
In some versions (but not in the original Atlas version), for the sake of easy typing it was possible to strop keywords by placing a "
There were no reserved words in the language as keywords were identified by underlining, not by recognising reserved character sequences. In the statement
Because of this keyword stropping, it was possible for AA to allow spaces in variable names, such as
The syntax for expressions let the multiplication operator be omitted, e.g.
Atlas Autocode's syntax was influenced by the output device which the author had available, a Friden
Flexowriter. Consequently it allowed symbols like "½" for ".5" and the superscript 2 for "to the power of 2". The flexowriter supported overstriking and therefore AA did as well - up to three characters could be overstruck as a single symbol. For example the character set had no "↑" symbol, so exponentiation was an overstrike of "|" and "*". (The underlining of keywords mentioned above could also be done using overstriking.) The language is described in detail in the Atlas Autocode Reference Manual.
Other Flexowriter characters that were found a use in Atlas Autocode were:
number.
When AA was ported to the English Electric KDF9
computer, the character set was changed to ISO and that compiler has been recovered from an old paper tape by the Edinburgh Computer History Project and is available online, as is a high-quality scan of the original Atlas Autocode manual.
Atlas Autocode's second-greatest claim to fame (after being the progenitor of Imp and EMAS) was that it had many of the features of the original "Compiler Compiler
". A variant of the AA compiler included run-time support for a top-down recursive descent parser
. The style of parser used in the Compiler Compiler was in use continuously at Edinburgh from the 60's until almost the turn of the millennium.
Other Autocode
s were developed for the Titan (computer)
(a prototype Atlas 2) at Cambridge and the Ferranti
Mercury.
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
developed around 1965 at Manchester University for the Atlas Computer. It was developed by Tony Brooker
Tony Brooker
Tony Brooker graduated in Mathematics from Imperial College in 1945 and returned there in 1947 as Assistant Lecturer. His first computer project was the construction of a fast multiplier unit from electro-mechanical relays. This was taken over by Professor K D Tocher and incorporated into ICCE, the...
and Derrick Morris as an improvement on the ALGOL
ALGOL
ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...
programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s, removing some of Algol's poorer features such as "passing parameters by name" (which with Algol 60 meant not its usual meaning but passing the address of a short subroutine to recalculate the parameter each time it was mentioned). It featured explicitly typed variables, subroutines, and functions. The AA compiler generated range-checking for array accesses, and allowed an array to have dimensions that were determined at run-time (i.e. you could declare an array as
integer array Thing (i:j), where i and j were calculated values). Direct machine codingMachine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
was allowed.
Atlas Autocode included a
complex data type which would support complex numberComplex number
A complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the one-dimensional number line to the two-dimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part...
s (which comprise a real
Real number
In mathematics, a real number is a value that represents a quantity along a continuum, such as -5 , 4/3 , 8.6 , √2 and π...
part, and an imaginary
Imaginary number
An imaginary number is any number whose square is a real number less than zero. When any real number is squared, the result is never negative, but the square of an imaginary number is always negative...
part which is a multiple of the square root of -1), partly because of pressure from the electrical engineering
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
department, as complex numbers are used to represent the behavior of alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
. The square root of -1 was represented by
i, which was treated as a fixed complex constant = i.The
complex data type was dropped when Atlas Autocode later morphed into the Edinburgh IMPEdinburgh IMP
Edinburgh IMP is a development of ATLAS Autocode, initially developed around 1966-1969 at Edinburgh University, Scotland. IMP was a general-purpose programming language which was used heavily for systems programming....
programming language. (Imp was an extension of AA and was notable for being used to write the EMAS
Edinburgh Multiple Access System
The Edinburgh Multi-Access System was a mainframe computer operating system developed at the University of Edinburgh, Scotland, during the 1970s...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
.)
Keywords in AA were distinguishable from other text by being underlined; there was also an "uppercasedelimiters" mode where all uppercase letters (outside strings) were treated as underlined lowercase.
In some versions (but not in the original Atlas version), for the sake of easy typing it was possible to strop keywords by placing a "
%" sign in front of them, for example the keyword endofprogramme could be typed as %end %of %programme or %endofprogrammeThere were no reserved words in the language as keywords were identified by underlining, not by recognising reserved character sequences. In the statement
if token=if then result = token, there is both a keyword if and a variable named if.Because of this keyword stropping, it was possible for AA to allow spaces in variable names, such as
integer previous value. Spaces were not significant and were removed prior to parsing in a trivial pre-lexing stage called "line reconstruction". What the compiler would see in the above example would be "iftoken=ifthenresult=token".The syntax for expressions let the multiplication operator be omitted, e.g.
3a was treated as 3*a, and a(i+j) was treated as a*(i+j) if a was not an array. In ambiguous usages, the longest possible name was taken, for example ab was not treated as a*b, whether or not a and b had been declared.Atlas Autocode's syntax was influenced by the output device which the author had available, a Friden
Friden, Inc.
Friden Calculating Machine Company was an American manufacturer of typewriters and electronic calculators. It was founded by Carl Friden in San Leandro, California in 1934. Friden electromechanical calculators were robust and popular....
Flexowriter. Consequently it allowed symbols like "½" for ".5" and the superscript 2 for "to the power of 2". The flexowriter supported overstriking and therefore AA did as well - up to three characters could be overstruck as a single symbol. For example the character set had no "↑" symbol, so exponentiation was an overstrike of "|" and "*". (The underlining of keywords mentioned above could also be done using overstriking.) The language is described in detail in the Atlas Autocode Reference Manual.
Other Flexowriter characters that were found a use in Atlas Autocode were:
α in floating-point numbers, e.g. 3.56α-7 for modern 3.56e-7 ; β to mean "the second half of an Atlas memory word"; π for the mathematical piPi
' is a mathematical constant that is the ratio of any circle's circumference to its diameter. is approximately equal to 3.14. Many formulae in mathematics, science, and engineering involve , which makes it one of the most important mathematical constants...
number.
When AA was ported to the English Electric KDF9
English Electric KDF9
KDF9 was an early British computer designed and built by English Electric, later English Electric Leo Marconi, EELM, later still incorporated into ICL. It first came into service in 1964 and was still in use in 1980 in at least one installation...
computer, the character set was changed to ISO and that compiler has been recovered from an old paper tape by the Edinburgh Computer History Project and is available online, as is a high-quality scan of the original Atlas Autocode manual.
Atlas Autocode's second-greatest claim to fame (after being the progenitor of Imp and EMAS) was that it had many of the features of the original "Compiler Compiler
Compiler-compiler
A compiler-compiler or compiler generator is a tool that creates a parser, interpreter, or compiler from some form of formal description of a language and machine...
". A variant of the AA compiler included run-time support for a top-down recursive descent parser
Recursive descent parser
A recursive descent parser is a top-down parser built from a set of mutually-recursive procedures where each such procedure usually implements one of the production rules of the grammar...
. The style of parser used in the Compiler Compiler was in use continuously at Edinburgh from the 60's until almost the turn of the millennium.
Other Autocode
Autocode
Autocode is the name of a family of "simplified coding systems", later called programming languages, devised in the 1950s and 1960s for a series of digital computers at the Universities of Manchester and Cambridge...
s were developed for the Titan (computer)
Titan (computer)
Titan was the prototype of the Atlas 2 computer developed by Ferranti and the University of Cambridge Mathematical Laboratory in Cambridge, England...
(a prototype Atlas 2) at Cambridge and the Ferranti
Ferranti
Ferranti or Ferranti International plc was a UK electrical engineering and equipment firm that operated for over a century from 1885 until it went bankrupt in 1993. Known primarily for defence electronics, the Company was once a constituent of the FTSE 100 Index but ceased trading in 1993.The...
Mercury.
External links
- Atlas Autocode Reference Manual
- Programming In Atlas Autocode - Edinburgh University Computer Unit Report #1 (1965). (Original scan)