Job Entry Subsystem 2/3
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IBM's MVS and z/OS operating systems use a job entry subsystem (JES) to receive jobs into the operating system, schedule them for processing by MVS or z/OS, and to control their output processing.
There are two versions of the job entry subsystem concept, JES2 and JES3. The two systems have very little in common, and in fact jobs written to run on one usually require small Job Control Language changes before they can be run on the other.
JES2 (Job Entry Subsystem 2) is descended from HASP, the Houston Automated Spooling Program, built by IBM for NASA in the mid 1960s. JES3 (Job Entry Subsystem 3) is similarly descended from the Attached Support Processor (ASP), which was IBM's initially-preferred system for OS/360 "unit record I/O".
JES3 has more network style dependency than JES2; as networking and inter-system dependencies have developed, this has become more practical than the single platform environment and single task processes that JES2 addresses.
HASP 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 remains within JES2 subsystem as the prefix of most module names and the prefix of all messages sent by JES 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.
So, what does all that mean? Simply stated, a JES is a task that runs under MVS which provides the necessary functions to get jobs into, and output out of, the MVS system, and to control the scheduling of their execution. It is designed to provide efficient spooling, scheduling, and management facilities for the MVS operating system. But, none of this explains why MVS needs a JES. Basically, 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 and after running the program; the base control program manages them during processing.
