VISTA Structured Assembler

摘要

VISTA is a structured assembly language for the VIPER microprocessor chip. Thoughthe syntax of VISTA and the appearance of VISTA programs are reminiscent of a high-level language, the actual statements are VIPER machine instructions. VISTA provides the clarity and much of the convenience of a true high-level language without the need for a complex and possibly untrustworthy compiling program. This Report describes the VISTA language by means of an annotated example, and gives instructions for using the VISTA translator. A formal grammar of VISTA, and descriptions of VIPER and the VIPER Object Program format, are included. VISTA is a high-level assembly language: its statements are essentially VIPER machine instructions (Ker 87) embedded in a syntax resembling that of Algol 68. While it does not hide the structure of the underlying machine in the way that a true high-level language would, it does perform many of the routine housekeeping functions which can otherwise distract the programmer and make mistakes more likely. The advantage of a relatively simple language like VISTA is that it does not need a complex compiler. Not only is the VISTA translator very much smaller and simpler than a typical compiler, but also its outputs are similar enough to the inputs to be checked easily. The outputs are printable, and annotated in such a way that mechanical checking is possible. VISTA is not an inherently safe language, in the sense that some specialized languages (Cur 84) might be, though it does limit the programmer to structures which are reasonably free of complications.