Editing Job Control Language (section)

==Motivation==
Originally, mainframe systems were oriented toward [[batch processing]]. Many batch jobs require setup, with specific requirements for [[main storage]], and dedicated devices such as [[magnetic tape]]s, private disk volumes, and printers set up with special forms.<ref>McQuillen, ''System/360–370 Assembler Language'', pp. 22–24.</ref> JCL was developed as a means of ensuring that all required resources are available before a job is scheduled to run. For example, many systems, such as [[Linux]] allow identification of required datasets to be specified on the [[Command-line interface|command line]], and therefore subject to substitution by the [[Shell (computing)|shell]],  or generated by the program at run-time.

On these systems the [[operating system]] [[job scheduler]] has little or no idea of the requirements of the job. In contrast, JCL explicitly specifies all required datasets and devices. The scheduler can pre-allocate the resources prior to releasing the job to run. This helps to avoid "[[Deadlock (computer science)|deadlock]]", where job A holds resource R1 and requests resource R2, while concurrently running job B holds resource R2 and requests R1. In such cases the only solution is for the [[computer operator]] to terminate one of the jobs, which then needs to be restarted. With job control, if job A is scheduled to run, job B will not be started until job A completes or releases the required resources.