Report of session on systems programming languages

Abstract

Discussion in this session centered around two major topics: experience using a particular language for systems programming and features which are required or desirable in a systems programming language. In general, as one might expect, the users of a particular language were happy with that language and felt they had made the right choice, even though they were not completely satisfied. There seemed to be general agreement that a higher level language than machine language (assembly language) should be used for systems programming. There was considerable difference of opinion as to how high level the language should be. There was a similar difference of opinion as to what features are desirable, and even necessary, in a systems programming language. Although the session was organized so as to discuss the two major topics separately, there was considerable discussion of desirable and necessary language features mixed in with the discussion of experience with specific languages. The session opened with a report by Bob Freiburghouse on the use of PL/I for implementation of Multics. "In general the project was successful. We're not about to switch to some other programming language. But we would like to enforce more discipline. I think we would like to recognize and enforce a subset of all this. He listed a number of key features, the most important of which was external procedures and external variables. Other important features included structured data with controlled packing, recursion, block structure, high level control statements, type checking, pointers and dynamic storage allocation, and % include. A few extensions were made to the language such as pointer valued functions, functions to construct and take apart pointers, P and V operations, and a notation for referencing Multics segments. In addition, a support routine called the binder was implemented. Using this routine one can combine a number of procedure and data segments into a single module. References to this module can be made using only designated names, all other names originally associated with the segments in the module will be hidden, that is, undefined outside the module.