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, developed by the programmers of IBM as self-initiative and eventually owned and supported 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".^{[citation needed]} In the 1970s a notable installation of ASP was at Princeton University controlling an IBM 360/91 mainframe.

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, 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 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.

[edit] History

In particular, the I/O queueing of all the OS/360 family left a lot to be desired. This was addressed by two field-developed packages: Houston Automated Spooling Package (HASP), and ASP. ASP stood for Attached Support Processor, which was something like a 360/40 controlling a 360/65 or greater processor through a magnetic tape channel controller processor. In effect, ASP required the purchase of a second computer. In contrast, HASP needed only one computer, such as a 360/65 alone, and MFT to start with.

HASP was developed by IBM Federal Systems Division contractors at the Johnson Space Center in Houston. In MVS, HASP became the standard part of system, Job Entry Subsystem 2 (JES2). It was many years before the HASP labels were removed from the JES2 source, and the messages issued by JES2 are still prefixed with $HASP.

ASP was developed to provide efficient use of multiple OS/360 systems working in conjunction, and allowed one central system to distribute jobs to multiple connected systems. ASP evolved from the design of the 7094/7044 Direct Coupled System, using data channel to data channel communication. It was primarily targeted at large government agencies and defense contractors that might have as many as six 360/65s all being scheduled and managed by a separate ASP machine. A variant was a single large machine with the ASP functions running on the same machine. ASP became JES3.