Job Entry Subsystems - IBM Mainframe

To process a job like the one we have seen, an important component of MVS called the Job Entry Subsystem is used. Simply put, the Job Entry Subsystem is the MVS component that keeps track of jobs that enter the system, presents them to MVS for processing, and sends their spooled output to the correct destination, normally a printer.

To fully understand MVS Job Entry Subsystems, you must understand that the original versions of OS did not provide a Job Entry Subsystem. On those systems, an operating system component called the job scheduler provided a relatively crude form of job entry and spooling. Because the MVS job scheduler was inadequate for most installation's needs, a program called HASP, which stood for Houston Automatic Spooling Program, was used at most installations. HASP, which itself ran as a user job, duplicated many of the functions of the job scheduler, but it provided more efficient operation and comprehensive control than the job scheduler alone. Although HASP became the norm at most installations, some shops used another program, called ASP, or Asymmetric Multiprocessing System. ASP was designed particularly for installations that used more than one processor. For those shops, it provided excellent control of job processing in a multiprocessing environment. ASP, like HASP, ran as a user job and duplicated many functions of the job scheduler.

When MVS was announced, IBM integrated the functions performed by HASP and ASP into the operating system by providing the Job Entry Subsystem. Because both HASP and ASP had strong footholds at large IBM shops, yet were incompatible with one another, two versions of JES was announced—JES2 and JES3. Basically, JES2 is the equivalent of HASP, and JES3 is the equivalent of ASP. The job scheduler itself was dropped, and many operating system components related to job scheduling were rewritten to accommodate JES2 and JES3. As a result, the overhead inherent with HASP/ASP processing is eliminated under JES2/JES3.

Although you should know whether your shop uses JES2 or JES3, it does not really matter in most cases. For typical jobs that do not have unusual processing requirements that depend on the unique characteristics of JES2 or JES3, both Job Entry Subsystems provide essentially the same functions:

  • They let you enter a job into the system
  • They decide when your job should be processed
  • They pass your job to MVS for processing
  • They deliver your job's printed output to the correct destination

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