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Content derived from Wikipedia article on BCPL Programming Language

BCPL

From Wikipedia, the free encyclopedia

BCPL (Basic Combined Programming Language) is a computer programming language designed by Martin Richards of the University of Cambridge in 1966; it was originally intended for use in writing compilers for other languages. Although not widely used now, it was very influential, because it was highly portable (see below), and because it inspired and influenced Dennis Ritchie's widely-used C programming language.

BCPL was a response to difficulties with its predecessor CPL, created during the early 1960s; Richards created BCPL by "removing those features of the full language which make compilation difficult". The first compiler implementation, for the IBM 7094 under CTSS, was written while Richards was visiting Project MAC at MIT in the spring of 1967. The language was first described in a paper presented to the 1969 Spring Joint Computer Conference.

The language is clean, powerful, and portable. It therefore proved possible to write small and simple compilers for it; reputedly some compilers could be run in 16 kilobytes. In addition, the Richards compiler, itself written in BCPL, was easily portable. BCPL was therefore a popular choice for bootstrapping a system.

A major reason for the compiler's portability lay in its structure. It was split into two parts: the front end parsed the source and generated O-code for a virtual machine, and the backend took the O-code and translated it into the code for the target machine. Only 1/5th of the compiler's code needed to be rewritten to support a new machine, a task that usually took between 2 and 5 man-months. Soon afterwards this structure became fairly common practice, cf. Pascal or Java, but the Richards BCPL compiler was the first to define a virtual machine for this purpose.

The language is unusual in having only one data type: a word, a fixed number of bits, usually chosen to align with the architecture's machine word and of adequate capacity to represent any valid storage address. This choice later proved to be a significant problem when BCPL was used on machines in which the smallest addressable item was not a word, but a byte.

The interpretation of any value was determined by the operators used to process the values. (For example, + added two values together treating them as integers; ! indirected through a value, effectively treating it as a pointer.) In order for this to work, the implementation provided no type checking. The Hungarian Notation was developed to help programmers avoid inadvertent type errors.

The mismatch between BCPL's word orientation and byte-oriented hardware was addressed in a number of ways. One was the provision of standard library routines for packing and unpacking words into byte strings. Later, two language features the bit field selection operator and the infix byte indirection operator (denoted by the '%' character) were added.

BCPL is reputedly the language in which the original hello world program was written. The first MUD was also written in BCPL [1].

Several operating systems were written partially or wholly in BCPL (for example, TRIPOS and significant parts of AmigaOS including Kickstart and the earliest versions of AmigaDOS). BCPL was also the initial language used in the seminal Xerox PARC Alto project, the first modern personal computer; among many other influential projects, the ground-breaking Bravo document preparation system was written in BCPL.

By 1970, implementations existed for the Honeywell 635 and 645, the IBM 360, the TX-2, the CDC 6400, the Univac 1108, the PDP-9, the KDF 9 and the Atlas 2. In 1979 implementations existed for at least 25 architectures; in 2001 it sees little use.

The philosophy of BCPL can be summarised by quoting from the book BCPL, the language and its compiler:

The philosophy of BCPL is not one of the tyrant who thinks he knows best and lays down the law on what is and what is not allowed; rather, BCPL acts more as a servant offering his services to the best of his ability without complaint, even when confronted with apparent nonsense. The programmer is always assumed to know what he is doing and is not hemmed in by petty restrictions.

The design, and philosophy, of BCPL strongly influenced B, which in turn influenced C. C is now the language of choice for systems programming.

Examples

These complete and compilable examples are from Martin Richards' BCPL distribution.

Printing factorials:

GET "libhdr"

LET start() = VALOF

{ FOR i = 1 TO 5 DO writef("fact(%n) = %i4*n", i, fact(i))

RESULTIS 0

}

AND fact(n) = n=0 -> 1, n*fact(n-1)

Counting solutions to the N queens problem:

GET "libhdr"

GLOBAL { count:200; all:201 }

LET try(ld, row, rd) BE TEST row=all

THEN count := count + 1

ELSE { LET poss = all & ~(ld | row | rd)

UNTIL poss=0 DO

{ LET p = poss & -poss

poss := poss - p

try(ld+p << 1, row+p, rd+p >> 1)

}

LET start() = VALOF

{ all := 1

FOR i = 1 TO 12 DO

{ count := 0

try(0, 0, 0)

writef("Number of solutions to %i2-queens is %i5*n", i, count)

all := 2*all + 1

}

RESULTIS 0

}

Sources

Martin Richards, The BCPL Reference Manual (Memorandum M-352, Project MAC, Cambridge, July, 1967)

Martin Richards, BCPL - a tool for compiler writing and systems programming (Proceedings of the Spring Joint Computer Conference, Vol 34, pp 557-566, 1969)

Martin Richards, Arthur Evans, Robert F. Mabee, The BCPL Reference Manual (MAC TR-141, Project MAC, Cambridge, 1974)

Martin Richards, C. Whitby-Strevens, BCPL, the language and its compiler (Cambridge University Press, 1980) ISBN 0-521-28681-6

External links

Martin Richards' BCPL distribution

Martin Richards's BCPL Reference Manual by Dennis M. Ritchie also includes some fascinating commentary from him about BCPL's influence on C

BCPL entry in the Jargon File

HTML formatter (syntax coloring) for BCPL (Example formatted program)

Retrieved from http://en.wikipedia.org/wiki/BCPL

End of Wikipedia content, http://en.wikipedia.org/wiki/BCPL

Web Resources for BCPL

BCPL Reference Manual by Martin Richards

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