Editing ALGOL 68 (section)

===Implementation specific extensions===
ALGOL 68R from [[Royal Radar Establishment|RRE]] was the first ALGOL 68 subset implementation, running on the [[ICT 1900 series|ICL 1900]]. Based on the original language, the main subset restrictions were ''definition before use'' and no parallel processing. This compiler was popular in [[United Kingdom|UK]] universities in the 1970s, where many [[computer science]] students learnt ALGOL 68 as their first programming language; the compiler was renowned for good error messages.

ALGOL 68RS<sup>(RS)</sup> from [[Royal Signals and Radar Establishment|RSRE]] was a portable compiler system written in ALGOL 68RS (bootstrapped from ALGOL 68R), and implemented on a variety of systems including the [[ICL 2900 Series|ICL 2900]]/[[ICL Series 39|Series 39]], [[Multics]] and [[VAX|DEC VAX/VMS]]. The language was based on the Revised Report, but with similar subset restrictions to ALGOL 68R. This compiler survives in the form of an Algol68-to-C compiler.

In ALGOL 68S<sup>(S)</sup> from [[Carnegie Mellon University]] the power of parallel processing was improved by adding an orthogonal extension, ''eventing''. Any variable declaration containing keyword '''EVENT''' made assignments to this variable eligible for parallel evaluation, i.e. the right hand side was made into a procedure which was moved to one of the processors of the [[C.mmp]] multiprocessor system. Accesses to such variables were delayed after termination of the assignment.

[[University of Cambridge|Cambridge]] [[ALGOL 68C]]<sup>(C)</sup> was a portable compiler that implemented a subset of ALGOL 68, restricting operator definitions and omitting garbage collection, flexible rows and formatted transput.

Algol 68 Genie<sup>(G)</sup> by M. van der Veer is an ALGOL 68 implementation for today's computers and operating systems.

"Despite good intentions, a programmer may violate portability by inadvertently employing a local extension.  To guard against this, each implementation should provide a PORTCHECK pragmat option.  While this option is in force, the compiler prints a message for each construct that it recognizes as violating some portability constraint."<ref>{{Cite web |url=http://www.fh-jena.de/~kleine/history/languages/Algol68-RR-HardwareRepresentation.pdf |title=The Report on the Standard Hardware Representation for Algol 68 |first1=Wilfred J. |last1=Hansen |first2=Hendrik |last2=Boom |access-date=2005-08-27  |archive-date=2014-01-02  |archive-url=https://web.archive.org/web/20140102201013/http://www.fh-jena.de/~kleine/history/languages/Algol68-RR-HardwareRepresentation.pdf |url-status=dead }}</ref>