occam is a concurrent programming language that builds on the Communicating Sequential Processes (CSP) process algebra,^[1] and shares many of its features. A programming paradigm is a fundamental style of Computer programming. Parallel computing is a form of computation in which many instructions are carried out simultaneously operating on the principle that large problems can often A software developer is a person or organization concerned with facets of the software development process wider than design and coding a somewhat broader scope of INMOS Ltd was a British semiconductor company founded by Iann Barron, based in Bristol and incorporated in November 1978 In Computer science, occam-π (or occam-pi) is the name of a variant of the occam developed by the Kent Retargetable occam Compiler ( KRoC In Computer science, Communicating Sequential Processes ( CSP) is a formal language for describing Patterns of Interaction in Concurrent Ease is a general purpose parallel Programming language, designed by Steven Ericsson-Zenith of Yale University. Concurrent computing is the concurrent (simultaneous execution of multiple interacting computational tasks A programming language is an Artificial language that can be used to write programs which control the behavior of a machine particularly a Computer. In Computer science, Communicating Sequential Processes ( CSP) is a formal language for describing Patterns of Interaction in Concurrent It is named after William of Ockham of Occam's Razor fame. William of Ockham (also Occam, Hockham, or any of several other spellings ˈɒkəm (c Occam's razor (sometimes spelled Ockham's razor) is a principle attributed to the 14th-century English Logician and Franciscan Friar,

occam is an imperative procedural language (such as Pascal). In Computer science, imperative programming is a Programming paradigm that describes computation in terms of statements that change a program state Procedural programming can sometimes be used as a synonym for Imperative programming (specifying the steps the program must take to reach the desired state but can also Pascal is an influential imperative and procedural Programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small It was developed by David May and others at INMOS, advised by Tony Hoare, as the native programming language for their transputer microprocessors, but implementations for other platforms are available. (Michael David May, born February 24 1951 is a British Computer scientist. INMOS Ltd was a British semiconductor company founded by Iann Barron, based in Bristol and incorporated in November 1978 Sir Charles Antony Richard Hoare ( Tony Hoare or CAR Hoare, born January 11, 1934) is a British computer scientist, probably A transputer was a pioneering Concurrent computing Microprocessor design of the 1980s from INMOS, a British semiconductor company A microprocessor incorporates most or all of the functions of a Central processing unit (CPU on a single Integrated The most widely known version is occam 2; the occam 2 programming manual was put together by Steven Ericsson-Zenith and others at INMOS. Steven Ericsson-Zenith, born July 6th 1966 is a British /American Computer scientist. INMOS Ltd was a British semiconductor company founded by Iann Barron, based in Bristol and incorporated in November 1978

Overview

In the following examples indentation and formatting are critical for parsing the code: expressions are terminated by the end of the line, lists of expressions need to be on the same level of indentation (this feature, also known as the off-side rule, is also found in other languages). A computer Programming language is said to adhere to the off-side rule if in it the scope of declarations (a Statement block) is expressed by their Indentation

Communication between processes work through named channels. One process outputs data to a channel via ! while another one inputs data with ?. Input and output will block until the other end is ready to accept or offer data. Examples (c is a variable):

 keyboard ? c

 screen ! c

SEQ introduces a list of expressions that are evaluated sequentially. This is not implicit as it is in most other programming languages. Example:

 SEQ   x := x + 1   y := x * x

PAR begins a list of expressions that may be evaluated concurrently. Example:

 PAR   p()   q()

ALT specifies a list of guarded commands. The guards are a combination of a boolean condition and an input expression (both optional). Each guard for which the condition is true and the input channel is ready is successful. One of the successful alternatives is selected for execution. Example:

 ALT   count1 < 100 & c1 ? data     SEQ       count1 := count1 + 1       merged ! data   count2 < 100 & c2 ? data     SEQ       count2 := count2 + 1       merged ! data   status ? request     SEQ       out ! count1       out ! count2

This will read data from channels c1 or c2 (whichever is ready) and pass it into a merged channel. If countN reaches 100, reads from the corresponding channel will be disabled. A request on the status channel is answered by outputting the counts to out.

Language revisions

occam 1

occam 1^[2] (released 1983) was a preliminary version of the language. This supported only the VAR data type, which was an integral type corresponding to the native word length of the target architecture, and arrays of only one dimension.

occam 2

occam 2^[3] is an extension produced by INMOS Ltd in 1987 that adds floating-point support, functions, multi-dimensional arrays and more data types such as varying sizes of integers (INT16, INT32) and bytes. In Computing, floating point describes a system for numerical representation in which a string of digits (or Bits represents a Real number.

With this revision, occam became a language capable of expressing useful programs, whereas occam 1 was more suited to examining algorithms and exploring the new language (though it should be noted that the occam 1 compiler was written in occam 1, so there is an existence proof that reasonably sized codes could usefully be written in occam 1, despite its limitations).

occam 2. 1

occam 2. 1^[1] was the last of the series of occam language developments contributed by INMOS. Defined in 1994, it was influenced by an earlier proposal for an occam 3 language (also referred to as "occam91" during its early development) created by Geoff Barrett at INMOS in the early 1990s. A revised Reference Manual describing occam 3 was distributed for community comment^[4], but the language was never fully implemented in a compiler.

occam 2. 1 introduced several new features to occam 2, including:

• Named data types (DATA TYPE x IS y)
• Named Records
• Packed Records
• Relaxation of some of the type conversion rules
• New operators (e. g. BYTESIN)
• Channel retyping and channel arrays
• Ability to return fixed-length array from function.

For a full list of the changes see Appendix P of the INMOS occam 2.1 Reference Manual.

occam-π

occam-π^[5] is the common name for the occam variant implemented by later versions of KRoC, the Kent Retargetable occam Compiler. In Computer science, occam-π (or occam-pi) is the name of a variant of the occam developed by the Kent Retargetable occam Compiler ( KRoC KRoC, the Kent Retargetable occam Compiler, is an occam implementation that is based on the INMOS occam 2 The addition of the symbol "π" (pi) to the occam name is an allusion to the fact that KRoC occam includes several ideas inspired by the Pi-calculus. Pi (uppercase &Pi, lower case &pi) is the sixteenth letter of the Greek alphabet. In Theoretical computer science, the \pi-calculus is a Process calculus originally developed by Robin Milner, Joachim Parrow and It contains a significant number of extensions to the occam 2. 1 compiler, for example:

• Nested protocols
• Run-time process creation
• Mobile channels, data, and processes
• Recursion
• Protocol inheritance
• Array constructors
• Extended rendezvous

See also

• Concurrent programming languages

References

Further reading

• (2007) Communicating Process Architectures 2007 - WoTUG-30. IOS Press, 513 pages. ISBN 978-1-58603-767-3.   [1]
• (2006) Communicating Process Architectures 2006 - WoTUG-29. IOS Press, 391 pages. ISBN 978-1-58603-671-3.   [2]
• (2005) Communicating Process Architectures 2005 - WoTUG-28. IOS Press, 405 pages. ISBN 978-1-58603-561-7.   [3]
• Kerridge, Jon; (ed. ) (1993). Transputer and Occam Research: New Directions. IOS Press, 253 pages. ISBN 0-8247-0711-7.  
• Roscoe, A W; C A R Hoare (1986). Prof A W "Bill" Roscoe is Director of Oxford University Computing Laboratory and a Professor of Computing Science. Sir Charles Antony Richard Hoare ( Tony Hoare or CAR Hoare, born January 11, 1934) is a British computer scientist, probably The Laws of Occam Programming. Programming Research Group, Oxford University.  

External links

• The occam Archive
• The occam-π programming language
• WoTUG: occam (Information and Implementations)
• The Transterpreter, an occam virtual machine
• KRoC - Kent Retargetable occam Compiler
• occam tutorial
• Internet Parallel Computing Archive: occam (Documentation and implementations but no longer maintained)

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL. The Free On-line Dictionary of Computing ( FOLDOC) is an online searchable encyclopedic Dictionary of Computing subjects The GNU Free Documentation License ( GNU FDL or simply GFDL) is a Copyleft License for free documentation designed by the Free Software