B72-4 System/360 Job Control Language

Since IBM's introduction of the System/360 and its operating system OS/360, probably no other aspect of that vast collection of programs and publications-the IBM OS Bibliography alone runs to 100 pages-has caused more heated discussions or been as poorly understood by its users as the job control language JCL. It is the one feature of the operating system that all users must employ: it is the gateway to the goodies of OS. JCL is not a programming language in the usual sense; it is not translated into machine instructions. Nor is it a command language, wherein each statement causes a specific action to take place. JCL is a job description language; it is a language used by the programmer to describe to the machine's operating system the work he has for it to do. To the System/360 operating system, a job is completely described by specifying a sequence of programs to be executed and, for each program, a list of external data sets required. Because there are so many possible combinations ofprograms and data, programs with all the various constraints applicable to them, data sets that can reside on any of a number of different devices, and often in different ways on the same device, because of all these possibilities and because JCL avoids making assumptions about what the programmer intends, the job descriptions expressed in JCL often require considerable detail. This requirement for details that are often meaningless to the programmer is one of the factors which, as Gary DeWard Brown says in the Preface of Sysfem/360 Job Control Language, make JCL "... hard to learn and not always easy to use." Another factor is the language itself; it is very low level as computer languages go. That is to say, it is not like any language used in disciplines other than computer science. One would expect that while each of the myriad special-purpose programming languages caters to its own community of users, the one computer language that they all must use would be as close as possible to the natural language that they all use, English. Instead, just the opposite is true. JCL is syntactically closest to the language used to write individual System/360 machine instructions. This assembly language is intimately related to the computer itself rather than to any application of the computer; so if it is the "work language" for anyone, it is the computer professional, not the accountant, physicist, or sociologist. Still, with the millions invested in OS/360 programs, you can be sure that come hardware relocation or high water, OS will be around for a good while. So if, as Harry W. Cadow of IBM states in OS/360 Job Control Language, JCL is the manager of OS, then we had better try to become IBM eomputer "professionals," at least to the extent that we are conversant, if not fluent, in this language. Up to now, the only help we had in learning JCL was the official IBM manual.' But