System programming language

A system programming language usually refers to a programming language used for system programming; such languages are designed for writing system software, which usually requires different development approaches when compared with application software.

System software is computer software designed to operate and control the computer hardware, and to provide a platform for running application software. System software includes software categories such as operating systems, utility software, device drivers, compilers, and linkers.

In contrast with application languages, system programming languages typically offer more-direct access to the physical hardware of the machine: an archetypical system programming language in this sense was BCPL. System programming languages often lack built-in input/output (I/O) facilities because a system-software project usually develops its own I/O mechanisms or builds on top of basic monitor I/O or screen management facilities. The distinction between languages used for system programming and application programming became blurred over time with the widespread popularity of PL/I, C and Pascal.

The earliest system software was written in assembly language primarily because there was no alternative, but also for reasons including efficiency of object code, compilation time, and ease of debugging. Application languages such as FORTRAN were used for system programming, although they usually still required some routines to be written in assembly language.

Mid-level languages "have much of the syntax and facilities of a higher level language, but also provide direct access in the language (as well as providing assembly language) to machine features." The earliest of these was ESPOL on Burroughs mainframes in about 1960, followed by Niklaus Wirth's PL360 (initially written on a Burroughs system as a cross compiler), which had the general syntax of ALGOL 60 but whose statements directly manipulated CPU registers and memory. Other languages in this category include MOL-360 and PL/S.

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