ALGOL 68C

Popular Culture

A very early predecessor of this compiler was used by Guy and Bourne to write the first Game of Life 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.

Various Liverpool Software Gazette issues detail the Z80 implementation. The compiler required about 120Kb of memory to run, hence the Z80's 64Kb memory is actually too small to run the compiler. So ALGOL 68C programs for the Z80 had to be cross compiled from ALGOL 68C running on the larger CAP capability computer or an IBM System/370 mainframe.

Algol 68C and Unix

Stephen Bourne subsequently reused ALGOL 68's if ~ then ~ else ~ fi, case ~ in ~ out ~ esac and for ~ while ~ do ~ od clauses in the common Unix Bourne shell, but with in's syntax changed, out removed, and od replaced with done (to avoid conflict with the od utility).

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 and included a special option in his Unix debugger "adb" to obtain a stack backtrace for programs written in ALGOL68C. Here is an extract from the Unix 7th edition adb manual pages:

NAME adb - debuggerSYNOPSIS 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.

ALGOL 68C extensions to Algol 68

Below is a sampling of some notable extensions:

Automatic op:= for any operator, e.g. *:= 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

CODE ... EDOC clause - for embedding ZCODE

The ENVIRON and USING clauses

Separate compilation in ALGOL 68C is done using the ENVIRON and USING clauses. The ENVIRON saves the complete environment at the point it appears. A separate module written starting with a USING clause is effectively inserted into the first module at the point the ENVIRON clause appears.

ENVIRON and USING are useful for a top-down style of programming, in contrast to the bottom-up style implied by traditional library mechanisms.

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. 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.

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

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 # 666END

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 IBM System/360 as well as a number of other computers.