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|Developer(s)||Stephen Bourne, Michael Guy, Andrew D. Birrell, Ian Walker, Chris Cheney, et al.|
|Initial release||circa 1970|
1.3039 / March 3, 2013
|Written in||ALGOL 68|
|Operating system||IBM 360, 370, etc., mainframes (or emulations) running MVT or MVS|
ALGOL 68C is an imperative computer programming language, a dialect of ALGOL 68, that was developed by Stephen R. Bourne and Michael Guy to program the Cambridge Algebra System (CAMAL). The initial compiler was written in the Princeton Syntax Compiler (PSYCO, by Edgar T. Irons) that was implemented by J. H. Mathewman at Cambridge.
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[clarification needed] on an emulated OS/MVT using Hercules.
The ALGOL 68C compiler generated output in ZCODE, a register-based intermediate language, which could then be either interpreted or compiled to a native executable. This ability to interpret or compile ZCODE encouraged the porting of ALGOL 68C to many different computing platforms. Aside from the CAP computer, the compiler was ported to systems including Conversational Monitor System (CMS), TOPS-10, and Zilog Z80.
Various Liverpool Software Gazette issues detail the Z80 implementation. The compiler required about 120 KB of memory to run; hence the Z80's 64 KB memory is actually too small to run the compiler. So ALGOL 68C programs for the Z80 had to be cross-compiled from the larger CAP computer, or an IBM System/370 mainframe computer.
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 Version 7 Unix (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 Advanced Debugger (adb) to obtain a stack backtrace for programs written in ALGOL 68C. Here is an extract from the Unix 7th edition 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.
ALGOL 68C extensions to ALGOL 68
Below is a sampling of some notable extensions:
- Automatic op:= for any operator, e.g.
- displacement operator (
- separate compilation -
- scopes not checked
- bounds in formal-declarers
EDOCclause - for embedding ZCODE
Separate compilation in ALGOL 68C is done using the
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.
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.
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
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
Restrictions to the language from the standard ALGOL 68
- No ALGOL 68 FLEX and variable length arrays
MODE STRINGimplemented without FLEX
- The PAR parallel clause was not implemented
- Nonstandard transput
- Birrell, Andrew D. (December 1977). "System Programming in a High Level Language" (PDF). Dissertation submitted for the degree of Doctor of Philosophy. University of Cambridge. Retrieved 2007-04-22.
- "Item Detail: Digital Equipment Corporation Model 340". Australian Computer Museum Society (ACMS). AceWare Web Hosting. Retrieved 2020-04-17.
- Gardner, Martin (October 1970). "The fantastic combinations of John Conway's new solitaire game "life"" (PDF). Mathematical Games. Scientific American. No. 223. pp. 120–123.
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.
- "The Modular Manual Browser: Adb". UnixDev.net. Archived from the original on 2016-03-03. Retrieved 2020-04-17.
- Bourne, Stephen R.; Birrell, Andrew D.; Walker, Ian (1975). ALGOL 68C reference manual. Cambridge University Computer Laboratory.
- Official website
- Cambridge ALGOL 68: on the historical roster of computer languages – includes 10+ publication references.
- A Transportation of ALGOL 68C – PJ Gardner, University of Essex – March 1977 (From 370 to DECsystem-10)
- Running ALGOL 68C on MVS - how to install ALGOL 68C on an emulated MVS system