ALGOL 68C

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The ALGOL68C computer programming language compiler was developed for the CHAOS OS for the CAP capability computer at Cambridge University in 1971 by Stephen Bourne and Mike Guy as a dialect of ALGOL 68. Other early contributors were Andrew Birrell and Ian Walker.

A very early predecessor of this compiler was used by Guy and Bourne to write the first life game programs on the PDP-7 with a DEC 340 display (see Scientific American article) "For long-lived populations such as this one Conway sometimes uses a PDP-7 computer with a screen on which he can observe the changes. The program was written by M. J. T. Guy and S. R. Bourne. Without its help some discoveries about the game would have been difficult to make." Scientific American 223 (October 1970): 120-123.

Subsequent work was done on the compiler after Bourne left Cambridge University in 1975. Garbage collection was added and the code base is still running on an emulated OS/MVT using Hercules.

ALGOL68C "compiler" generated ZCODE output, that could then be either compiled into the local machine code by a ZCODE translator or run interpreted. ZCODE is a register based intermediate language.

Contents 1 Algol 68C and Unix 2 ALGOL 68C Extensions to Algol 68 2.1 The ENVIRON and USING clauses. 2.1.1 Example of ENVIRON clause 2.1.2 Example of USING clause 3 Restrictions to the language from the standard ALGOL 68 4 External links

[edit] Algol 68C and Unix

Stephen Bourne subsequently reused ALGOL 68's reverent if ~ then ~ else ~ fi, case ~ in ~ out ~ esac and for ~ while ~ do ~ od clauses in the common Unix Bourne shell.

After Cambridge, Bourne spent nine years at Bell Labs with the Seventh Edition Unix team. As well as developing the Bourne shell, he ported ALGOL 68C to Unix on the DEC PDP-11-45. The Unix debugger called "adb", written to replace "cdb", contained a stack backtrace that was used to debug programs written in ALGOL68C. Here is an extract from the Unix 7th edition adb manual pages:

NAME
      adb - debugger
SYNOPSIS
      adb [-w] [ objfil [ corfil ] ]
COMMANDS
      $modifier
             Miscellaneous  commands.   The  available modifiers
             are:
      a      ALGOL 68 stack  backtrace.   If  address  is
             given  then it is taken to be the address of
             the current frame (instead of r4).  If count
             is  given  then  only the first count frames
             are printed.

[edit] ALGOL 68C Extensions to Algol 68

Below is a sampling of some notable extensions:

• CODE ... EDOC clause - for embedding ZCODE
• Automatic op:= for any operator, eg *:= and +:=
• UPTO, DOWNTO and UNTIL in loop-clauses;
• displacement operator (:=:=)
• ANDF, ORF and THEF syntactic elements.
• separate compilation - ENVIRON clause and USING clause
• scopes not checked
• bounds in formal-declarers

[edit] The ENVIRON and USING clauses.

These clauses are kind of the inverse of the #include found in the C programming language, or import found in Python. The purpose of the ENVIRON mechanism is to allow a program source to be broken into manageable sized pieces. Note that it is only necessary to parse the shared source file once, unlike a #include found in the C programming language where the include file needs to be parsed for each source file that includes it.

[edit] Example of ENVIRON clause

A file called mylib.a68:

BEGIN
   INT dim = 3; # a constant #
   INT a number := 120; # a variable #
   ENVIRON EXAMPLE1;
   MODE MATRIX = [dim, dim]REAL; # a type definition #
   MATRIX m1;
   a number := ENVIRON EXAMPLE2;
   print((a number))
END

[edit] Example of USING clause

A file called usemylib.a68:

USING EXAMPLE2 FROM mylib
BEGIN
  MATRIX m2; # example only #
  print((a number)); # declared in mylib.a68 #
  print((2 UPB m1)); # also declared in mylib.a68 #
  ENVIRON EXAMPLE3;  # ENVIRONs can be nested #
  666
END

[edit] Restrictions to the language from the standard ALGOL 68

• no algol68 FLEX and variable length arrays.
• MODE STRING implemented without FLEX.
• The PAR parallel clause was not implemented.
• nonstandard transput.
• others...

A translator/compiler for ALGOL 68C was available for the PDP-10 and System/360 as well as a number of other computers.

[edit] External links

• Cambridge Algol 68: on the historical roster of computer languages - includes 10+ publication references.
• A TRANSPORTATION OF ALGOL68C - PJ Gardner, University of Essex - March 1977 (From 370 to DECsystem-10)

	Preceding:	ALGOL 68
	Subsequent:	C » C++, Bourne shell » Bash