Systems programming with JSP: example—a VDU controller

Reviewer: John R. Levine

Jackson Structured Programming (JSP) is a methodology which, is an oversimplified sense, says that the structure of a program must model the data on which the program operates. Sanden argues that although JSP has in the past mostly been used to design COBOL application programs, it is equally applicable to systems programming. He does this by showing an example: the program in a terminal which interprets a data stream that contains intermixed text and escape sequences. First the program deals with the simplest case. Then it is extended to deal with blocked input, the blocking boundaries being unrelated to the content of the data stream. This means that there are two data structures, the stream and the blocking, so there should in concept be two programs structured as coroutines. Most programming languages don't deal well with coroutines, so one program is embedded in the other as a subroutine. This all seemed pretty self-evident to me, but on reflection, I think it is valuable anyway. The isomorphism between data and the corresponding program is obvious and inevitable, but that doesn't mean that there haven't been a lot of instances where my programs would have been much better had I kept that in mind from the start. Similarly, the idea of developing a program as cooperating routines and mapping that into subroutines only as necessary is often useful and too infrequently practiced. System programmers would do well to overcome their disdain of anything with a whiff of COBOL and spend the 20 minutes reading and remembering this one.