Your company's core applications, such as payroll, are usually performed through batch processing, which involves executing one or more batch jobs in a sequential flow. The job entry subsystem (JES) helps z/OS® receive jobs, schedule them for processing, and determine how job output is processed.

Batch processing is the most fundamental function of z/OS. Many batch jobs are run in parallel and JCL is used to control the operation of each job. Correct use of JCL parameters (especially the DISP parameter in DD statements) allows parallel, asynchronous execution of jobs that may need access to the same data sets.

An initiator is a system program that processes JCL, sets up the necessary environment in an address space, and runs a batch job in the same address space. Multiple initiators (each in an address space) permit the parallel execution of batch jobs.

z/OS shares with the job entry subsystem (JES) the management of jobs and resources. JES receives jobs into the system, schedules them for processing by z/OS, and controls their output processing. JES is the manager of the jobs waiting in a queue. It manages the priority of the jobs and their associated input data and output results. The initiator uses the statements in the JCL records to specify the resources required of each individual job after it is released (dispatched) by JES.

IBM® provides two kinds of job entry subsystems: JES2 and JES3. In many cases, JES2 and JES3 perform similar functions.

During the life of a job, both JES and the z/OS base control program control different phases of the overall processing. Jobs are managed in queues: Jobs that are waiting to run (conversion queue), currently running (execution queue), waiting for their output to be produced (output queue), having their output produced (hard-copy queue), and waiting to be purged from the system (purge queue).