Evolution Of Z / OS (zero downtime operating system)

Primary Control Program(PCP):

➽ It was the first release of OS version by IBM in the year 1965. It had limited capabilities and it doesn't offer multi
programming, no priority scheduling and no spooling facility.

It was an unreliable system and became obsolete in the year 1969.

➽ Multiprogramming concept was introduced with the introduction of MFT and MVT.

MFT: Multi programming with fixed number of tasks.

MVT: Multiprogramming with variable number of tasks. It was the first reliable system and both MVT and MFT provide limited spooling
capability.

HASP(Houston Automatic Spooling Priority):

A group of IBM employees developed a package that is superimposed on MFT or MVT and provides good spooling capabilities.

ASP(Attached Support Processor):

It is another package developed for MVT that would allow several mainframes to work together under one operating system
in a master or slave relationship.

OS/VS OR System/370:

In this virtual storage capabilities were provided and new hardware VS1 and VS2.

VS1 was basically an MFT system in a virtual environment. VS2 has two versions SVS(Single Virtual Storage) and MVS (Multiple Virtual Storage)

➽ In 1987 only MVS was left behind and MVS along with HAS was named as JES2(Job entry subsystem) and MVS along with ASP was named as JES3(Job entry subsystem).

➽ MVS provides reliability, integrity and performance. The only drawback was the limited virtual storage capability of 16 MB.

MVS / XA:(Multiple Virtual Storage or Extended Architecture)

It was a new version of MVS having storage availability of 2GB.

MVS / ESA:(Enterprise Systems Archittecture)

It was announced in 1988. In this virtual storage was extended upto 16TB.

Z / OS (zero downtime operating system)

It is the latest version.


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