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What is a JES?
z/OS JES2 Introduction
MVS™ uses a job entry subsystem (JES) to receive jobs into the operating system, schedule jobs for processing by MVS, and control job output processing. JES2 (job entry subsystem 2) is descended from HASP (Houston automatic spooling priority), which is defined as a computer program that provides supplementary job management, data management, and task management functions such as: scheduling, control of job flow, and spooling. HASP persists within JES2 as the prefix string for most module names and for all messages sent by JES2 to the operator.
JES2 is a functional extension of the HASP II program that receives jobs into the system and processes all output data produced by the job. JES2 is the component of MVS that provides the necessary functions to get jobs into, and output out of, the MVS system. It is designed to provide efficient spooling, scheduling, and management facilities for the MVS operating system. (See Spool data sets and spooling for a full definition of spooling.)
By separating job processing into a number of tasks, MVS operates more efficiently. At any point in time, the computer system resources are busy processing the tasks for individual jobs, while other tasks are waiting for those resources to become available. In its most simple view, MVS divides the management of jobs and resources between the JES and the base control program of MVS. In this manner, JES2 manages jobs before and after running the program; the base control program manages them during processing. 1 Figure 1 presents a diagram of the relationship between JES2 and MVS.
Figure 1. Relationship of JES2 to the Base Control Program
1 JES3, in contrast, manages jobs throughout the entire process cycle (before, during, and after running the programs).
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