MVS Version Names - Make sense of the versioning confusion

MVS Version Names

MVS 3.8 from the TK4 distribution is what we know best. But often there is a misguided understanding that later versions are improvements and therefore better versions of MVS. That is not always true, and often not how IBM released software.

In the beginning
After the OS/360 (MFT and MVT) start, IBM released OS/VS2 rel 1, as the next version. That name, OS/VS2 stuck to this very day. Every subsequent release was an OS/VS2 operating system. OS/VS r1 was also called SVM, for Single Virtual Memory.

Quickly after IBM renamed that to SVS, Single Virtual Storage. Why the change from "memory" to "storage"? Because IBM had decided to switch with the last S/370 mainframes from core memory to solid state storage instead. SVS was mainly geared at solving real memory fragementation.

MVS
In the mid 1970s, (1973 to be exact) IBM annoucned MVS. This very important new operarting systems, now allowed each application to have its own full 16MB address space and therefore do away with all the limitations of MVT and SVS.

The early MVS releases were painful. Within 8 years, IBM released over 100,000 PTFs (or patches) for MVS and the important JES2, JES3, VTAM, TSO and other subsystems.

MVS/SE (for system extensions) was released in the late 1970s. All prior operating systems for the S/360 and S/370 lines of mainframes were free for customers of IBM. MVS/SE was the first product that came with a license fee. MVS/SE just worked, and was much liked by customers. And so this version of MVS lingered on for many years after.

It is at this point that a significant split happened in the OS/360 line of operating systems. MVS/SE became MVS/SE1 with many advancements and support for microcode in the 3033 and 3031 line of mainframes. The free of charge OS/VS2 MVS became MVS 3.8 and here it stopped. IBM ceased to make new improvements to this line with the latest release which was MVS 3.8j with PUT tape level 8505.

It is important to understand that MVS 3.8j continued to be used by IBM and customers for the purposes of generating a new MVS installation for years thereafter. And so MVS 3.8j became the carrier system used for SYSGENs.

MVS/SP1
All surviving descendants of MVS/SE follow from MVS/SE1 and MVS/SE2 (announced in 1979).

From 1980 on, all subsequent versionf of MVS/SE2 became known as MVS/SP (for system product). So, MVS/SE2 was simply renamed to MVS/SP.

Several months after the first MVS/SP version was announced, IBM made MVS/SP2 avaialble, with new technology to alleviate real memory pressure and keep the 16MB address space and common virtual memory areas co-exist. While keeping the standard locking API for MVS (Enqueue/Dequeue), IBM moved this API component (with a big impact on overall system thrutput) into its own seperate control address space, called GRS. Also, the operator console handling was significantly improved as the blinkenlights panels disappeared and all console work was exclusively done on CRT consoles as we know them today.

MVS/SP3
In November 1980, IBM released MVS/SP3 which had support for the latest and most powerful mainframes (the 308x family). MVS/SP3 was the most mature version of MVS with very scalable subsystems for the handling of thousands of connected terminal in VTAM and NCP, for work with CICS, IMS DB/DC and DB2. It was not uncommong to see a single 3081D mainframe with 32MB of memory handle 8000 CICS terminals, 20 millions CICS transactions a day, and 300-400 TSO terminals, with batch, production and test, all on one machine. And keep response times under 1 second. Truly unthinkable as of today. Busy MVS/SP3 systems were regularly seeing rates of 500-900 pages per second beign moved from disk to real memory, while keeping reliable thruput rates.

MVS/SP2 (MVS/XA)
In 1981 IBM announced a new far-reaching version of MVS with a confusing version name: MVS/SP2.1.0, aka MVS/XA. MVS/XA brought vastly improved I/O handling and 31bit addressing. Therefore all previous version of MVS were generally called MVS/370 to signify 24bit addressing.

at the same time the MVS/SP3 from a year before was renamed MVS/SP1.3.

And so, MVS/SP2 was really the first version of MVS/XA.

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