JES2

The Job Entry Subsystem (JES) is a component of IBM's MVS (OS/VS2 R2 through z/OS) operating systems that receives jobs into the computer system, schedules them for processing, and controls their output processing. There is also a JES in OS/VS1, often referred to as JES1. While MVS/370's immediate predecessor, SVS, retained the spooling facilities of OS/360, most installations used the optional ASP Version 3 or HASP II Version 4, sometimes incorrectly referred to as HASP4; ASP V3 is available for OS/360 and HASP II V4 has been quite easily retrofitted to OS/360, even on systems which did not, and cannot support the System/370 instruction set (there are a very limited number of non-privileged S/370 instructions, and no privileged S/370 instructions, and each such instance can be replaced by a number of S/360 instructions which performs the equivalent function).

In MVS, JES is a task that runs under MVS and provides the necessary functions to get jobs in to, and jobs out of, the MVS operating system, and to control the scheduling of, and, indeed, their execution. It is designed to provide efficient spooling, scheduling, and management facilities for the MVS operating system. 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, the JES manages jobs before (i.e., during the reading and scheduling phases) and after the completion of running the program (i.e., the printing, punching and purgeing phases); the base control program manages them during processing, usually without any specific knowledge of JES.

There are three job entry subsystems in MVS; Master, JES2 and JES3. The Master subsystem is used during system initialization and for starting system tasks that must run outside of the control of the primary JES; in particular, it is used to start the primary JES.

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