A facility for the downward extension of a high-level language

This paper presents a method whereby a high-level language can be extended to provide access to all the capabilities of the underlying hardware and operating system of a machine. In essence, it is a facility that allows a user to make special purpose extensions to a language without requiring the compiler to be modified for each extension. Extensions are specified in an assembler-like language that is used at compile time to produce executable code to be combined with compiler-generated code. This facility has been implemented in a systems-programming language and was designed to provide access to facilities not directly available in the language. The way in which the facility was implemented calls for a minimum of user-visible language changes and is well suited for generating code sequences for any language. The facility provides the extension writer access to information available in the high-level language during compilation, permits the selective generation of user-defined code sequences depending on the context in which they are being used, provides for the integration of this code with compiler-generated code, and provides for the generation of user-understandable error messages when an extension is used incorrectly.