Over the years, processing power continued to grow. Yet, even with the use of multiprogramming techniques, the processor still had more capacity to execute programs than it was able to contain in storage.
In 1972, IBM announced System/370 hardware, which contained dynamic address translation, known as the DAT box, and provided virtual storage capability.
Shmuel's note (10/31/2002). DAT was not in the original announcement. 370/145 included the hardware, which was used by the "DOS Emulation" feature, and the DAT upgrade was just a new floppy. DAT for 370/155 was a $200K upgrade and for 370/165 a $400K upgrade.
To utilize the new hardware, OS/360 had to be transformed into a virtual storage operating system. Since customers had already made a significant investment in OS/360 applications, ensuring compatibility was of prime concern. Many customers were proponents of MFT. Others favored MVT. In order to satisfy both groups, MFT was replaced by OS/VS1, and MVT was replaced by OS/VS2. Both OS/VS1 and OS/VS2 were announced and delivered in 1972. To provide for orderly installation and migration of hardware and software, OS/360 Release 21 was updated to run on System/370, although it could not utilize the new hardware capability.
Shmuel's note. OS/360 had support for new features of the S/370, e.g., TOD clock.
OS/VS1 provided a single virtual storage address space system, while OS/VS2 allowed multiple virtual storage address spaces. However, the first release was restricted to a single virtual storage address space and became known as OS/VS2 SVS. The following release, made available in July 1974, contained multiple virtual storage, address space support and was named OS/VS2 MVS Release 2. Both OS/VS1 and OS/VS2 SVS supported a total of 16MB of virtual storage. Because the OS/VS2 MVS release supported multiple virtual storage address spaces, each of which provided 16MB, most people assumed it would be years before additional storage would be required.
The MVS release incorporated two job entry subsystems: JES2 and JES3, whose roots are found in two field-developed programs, HASP and ASP. HASP assisted in job scheduling. ASP supported a small front-end processor to handle I/O for the larger mainframes. JES2 and JES3 functions superseded the OS/360 readers and writers. OS/VS1 supported only one job entry subsystem, JES1, which was essentially a HASP look-alike.
Shmuel's note. JES2 and JES3 did not supersede the OS/360 readers and writers; the Reader/Interpreter became the Converter/Interpreter and the Writer became the External Writer, components that are still there in OS/390.
Shmuel's note. JES1 was much more tightly integrated into the operating system than HASP was, e.g., it was controlled by standard commands instead of having its own commands with a leading escape character.
VSAM and VTAM were developed to support virtual storage, and were subsequently incorporated into the virtual storage operating system.
Shmuel's note. The only things in VSAM and VTAM that relates to virtual storage are the names; in fact, there was a retrofit to run VSAM in OS/360 (< bletch >). If you know the dates it would be nice to include them.
Mark Butler's note (September 2003). I have some input for you. I was involved with that retrofit. It was done at ITEL Corp. in the 1974-1976 timeframe and, as I remember, was limited to OS/MVT. I initiated and headed up another project starting before that one (in 1973) that retrofitted all of DOS/VS Releases 28-33 to S/360's. It was called ITEL Basic Control Mode (BCM) DOV/VS. It ran on S/360-30's and above. Both projects used a S/370 instruction simulator that we had previously developed to enable us to run DOS Release 27 (S/370-only release and stop-gap to DOS/VS Rel 28) on S/360's. The new S/370 Move Long (MVCL) instruction that started to be used in DOS/VS Release 28-29 Power spooler program was one of the most difficult to emulate for both compatibility (it was interruptable) and performance reasons. We also had IBM 3330-compatible DASDs (ITEL 7330's) that could interface to selector channels on 360-40's and above (complete with fully functional Rotational Position Sensing - RPS). All these solutions were very cost-effective during a down economic time when DP shops needed more horsepower but couldn't afford the new S/370's from IBM. These brought some, or all, of the new S/370 "goodies" (like 3330's and VSAM) to the S/360 platform.
Following OS/VS2 MVS Release 3 in March 1975, a series of enhancements, known as Selectable Units (SUs), were released individually. These allowed customers to tailor their operating systems by selecting only those functions which they needed. Over the next four years, approximately 60 SUs were made available. While SUs offered a great deal of flexibility, they also created a geometric increase in the number of software combinations that had to be installed, tracked and serviced. Consequently, at our customers' request, IBM reversed its course and began rolling the SUs into MVS Release 3 maintenance releases. The last SU was shipped in 1979. Out of the SU experience came a new installation process, an integrated product offering called IPO, which packaged many of the products that users needed.
OS/VS2 MVS packaging followed the same philosophy as OS/360, with each release containing a base control program, data access methods, teleprocessing access methods, system utilities and an assembler. With OS/VS2 MVS Release 3.8, there was little separate pricing for operating system code, although the trend was beginning. Some of the SUs were individually priced, including two performance enhancements to the base control program, MVS/SE1, which introduced TSO logical swap and MVS/SE2.
Shmuel's note. MVS/SE R1 preceded OS/VS2 3.8; "By the pricking of my thumbs, SU7 this way comes." MVS/SE R2 required, in addition to OS/VS2 R3.8, SU64.
Early on, IBM made a commitment to provide availability and recoverability for MVS through ongoing enhancements such as functional recovery routines, channel-check, handlers, automatic CPU recovery, improved dumping services, faster IPLs, and dynamic parmlib.