Computer History
Tracing the History of the Computer - BCPL (Basic Combined Programming Language)

BCPL (Basic Combined Programming Language) is a computer programming language that was 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 Dennis Ritchie would later develop the widely-used C programming language from BCPL.

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. 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.