The language is lean, powerful, and portable. It proved possible to write small and simple compilers for it and was therefore a popular choice for bootstrapping a system. Reputedly some compilers could be run in 16 kilobytes. Several operating systems were written partially or wholly in BCPL (for example, Tripos or Amiga Kickstart). A major cause of its portability lay in the form of the compiler. It was split into two parts. The front end parsed the source and generated O-Code for a virtual machine; the backend took the O-code and translated it into the code for the target machine. Soon afterwards this 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). The interpretation of any value was determined by the operators used to process the values (so, + 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.
It is reputedly the language in which the original hello world program was written.
In 1979 implementations existed for at least 25 architectures; in 2001 it sees little use. Its successor, C, is now the language of choice for systems programming.
The philosophy of BCPL can summarised by quoting from the book (BCPL, the language and its compiler):