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JCL is used in a mainframe environment to act as a communication between a program (Example: COBOL, Assembler or PL/I) and the operating system. In a mainframe environment, programs can be executed in batch and online mode. Example of a batch system can be processing the bank transactions through a VSAM (Virtual Storage Access Method) file and applying it to the corresponding accounts. Example of an online system can be a back office screen used by staffs in a bank to open an account. In batch mode, programs are submitted to the operating system as a job through a JCL.
Batch and Online processing differ in the aspect of input, output and program execution request. In batch processing, these aspects are fed into a JCL which is in turn received by the Operating System.
A job is a unit of work which can be made up of many job steps. Each job step is specified in a Job Control Language (JCL) through a set of Job Control Statements.
The Operating System uses Job Entry System (JES) to receive jobs into the Operating System, to schedule them for processing and to control the output.
Job processing goes through a series of steps as given below: