MVS
Multiple Virtual Storage, more commonly called MVS, is the most commonly used operating system on the System/370, System/390 and IBM Z IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unrelated to IBM's other mainframe operating system lines, e.g., VSE, VM, TPF.
Overview
First released in 1974, MVS was extended by program products with new names multiple times, retaining the term MVS in the nomenclature:- first to MVS/SE,
- next to MVS/SP Version 1,
- next to MVS/XA,
- next to MVS/ESA,
- to OS/390 for the System/390 systems, and
- finally to z/OS. IBM added UNIX support in MVS/SP V4.3 and has obtained POSIX and UNIX™ certifications at several different levels from IEEE, X/Open and The Open Group. The MVS core remains fundamentally the same operating system. By design, programs written for MVS run on z/OS without modification.
Evolution of MVS
provides multiprogramming: several memory partitions, each of a fixed size, are set up when the operating system is installed and when the operator redefines them. For example, there could be a small partition, two medium partitions, and a large partition. If there were two large programs ready to run, one would have to wait until the other finished and vacated the large partition. OS/360 R19 added MFT sub-tasking, the ability for a job to dynamically create new tasks with the ATTACH macro.OS/360 MVT was an enhancement that further refined memory use. Instead of using fixed-size memory partitions, MVT allocates memory to regions for job steps as needed, provided enough contiguous physical memory is available. This is a significant advance over MFT's memory management, but has some weaknesses: if a job allocates memory dynamically, the programmers has to estimate the job's maximum memory requirement and pre-define it for MVT. A job step that contains a mix of small and large programs wastes memory while the small programs run. Most seriously, memory can become fragmented, i.e., the memory not used by current jobs could be divided into uselessly small chunks between the areas used by current jobs, and the only remedy was to wait until some current jobs finished before starting any new ones.
In the early 1970s IBM sought to mitigate these difficulties by introducing virtual memory, which allowed programs to request address spaces larger than physical memory. The original implementations had a single virtual address space, shared by all jobs. OS/VS1 is OS/360 MFT within a single virtual address space; OS/VS2 SVS was OS/360 MVT within a single virtual address space. So OS/VS1 and SVS in principle had the same disadvantages as MFT and MVT, but the impacts are less severe because jobs and operators could request much larger partitions with a 2 KiB granularity or regions with a 4 KiB granularity, and the requests came out of a 16MiB address space even if physical storage was smaller. As in OS/360 MVT, TSO users in SVS are assigned to a TSO region during login processing and competed with other users assigned to the same region, with essentially the same swap-in and swap-out logic as TSO in MVT.
In the mid-1970s IBM introduced MVS, which not only supported virtual storage that was larger than the available real storage, as did SVS, but also allowed an indefinite number of applications to run in different address spaces. Two concurrent programs might try to access the same virtual memory address, but the virtual memory system redirected these requests to different areas of physical memory. Each of these address spaces consisted of three areas: an operating system, an application area unique for each application, and a shared virtual area used for various purposes, including inter-job communication. IBM promised that application areas would always be at least 8MB. This made MVS the perfect solution for business problems that resulted from the need to run more applications.
MVS maximized processing potential by providing multiprogramming and multiprocessing capabilities. Like its MVT and OS/VS2 SVS predecessors, MVS supported multiprogramming; program instructions and associated data are scheduled by a control program and given processing cycles. Unlike a single-programming operating system, these systems maximize the use of the processing potential by dividing processing cycles among the instructions associated with several different concurrently running programs. This way, the control program does not have to wait for the I/O operation to complete before proceeding. By executing the instructions for multiple programs, the computer is able to switch back and forth between active and inactive programs.
Early editions of MVS are among the first of the IBM OS series to support multiprocessor configurations, though the M65MP variant of OS/360 running on 360 Models 65 and 67 had provided limited multiprocessor support. The 360 Model 67 had also hosted the multiprocessor capable TSS/360, MTS and CP-67 operating systems. Because multiprocessing systems can execute instructions simultaneously, they offer greater processing power than single-processing system. As a result, MVS was able to address the business problems brought on by the need to process large amounts of data.
Multiprocessing systems are either loosely coupled, which means that each computer has access to a common workload, or tightly coupled, which means that the computers share the same real storage and are controlled by a single copy of the operating system. MVS retained both the loosely coupled multiprocessing of Attached Support Processor and the tightly coupled multiprocessing of OS/360 Model 65 Multiprocessing. In tightly coupled systems, two CPUs shared concurrent access to the same memory and peripherals, providing greater processing power and a degree of graceful degradation if one CPU failed. In loosely coupled configurations each of a group of processors had its own memory and operating system but shared peripherals and the operating system component JES3 allowed managing the whole group from one console. This provided greater resilience and let operators decide which processor should run which jobs from a central job queue. MVS JES3 gave users the opportunity to network together two or more data processing systems via shared disks and Channel-to-Channel Adapters. This capability eventually became available to JES2 users as Multi-Access SPOOL.
MVS originally supported 24-bit addressing. As the underlying hardware progressed, it supported 31-bit and now 64-bit addressing. The most significant motives for the rapid upgrade to 31-bit addressing were the growth of large transaction-processing networks, mostly controlled by CICS, which ran in a single address space—and the DB2 relational database management system needed more than 8MB of application address space to run efficiently.
The main user interfaces to MVS are: Job Control Language, which was originally designed for batch processing but from the 1970s onwards was also used to start and allocate resources to long-running interactive jobs such as CICS; and TSO, the interactive time-sharing interface, which was mainly used to run development tools and a few end-user information systems. ISPF is a TSO application for users on 3270-family terminals, which allows the user to accomplish the same tasks as TSO's command line but in a menu and form oriented manner, and with a full screen editor and file browser. TSO's basic interface is command line, although facilities, such as ISPF, were added later for form-driven interfaces.
MVS took a major step forward in fault-tolerance, built on the earlier STAE facility, that IBM called software recovery. IBM decided to do this after years of practical real-world experience with MVT in the business world. System failures were now having major impacts on customer businesses, and IBM decided to take a major design jump, to assume that despite the very best software development and testing techniques, that 'problems WILL occur.' This profound assumption was pivotal in adding great percentages of fault-tolerance code to the system and likely contributed to the system's success in tolerating software and hardware failures. Statistical information is hard to come by to prove the value of these design features, but IBM has, in many dimensions, enhanced these fault-tolerant software recovery and rapid problem resolution features, over time.
This design specified a hierarchy of error-handling programs, in system mode, called Functional Recovery Routines, and in user mode, called "ESTAE" that are invoked in case the system detected an error. Each recovery routine made the 'mainline' function reinvokable, captured error diagnostic data sufficient to debug the causing problem, and either 'retried' or 'percolated'.
Thus, with each error the system captured diagnostic data, and attempted to perform a repair and keep the system up. The worst thing possible was to take down a user address space in the case of unrepaired errors. Though it was an initial design point, it was not until the most recent MVS version, that recovery program was not only guaranteed its own recovery routine, but each recovery routine now has its own recovery routine. This recovery structure was embedded in the basic MVS control program, and programming facilities are available and used by application program developers and 3rd party developers.
Practically, the MVS software recovery made problem debugging both easier and more difficult. Software recovery requires that programs leave 'tracks' of where they are and what they are doing, thus facilitating debugging—but the fact that processing progresses despite an error can overwrite the tracks. Early data capture at the time of the error maximizes debugging, and facilities exist for the recovery routines to do this.
IBM included additional criteria for a major software problem that required IBM service. If a mainline component failed to initiate software recovery, that was considered a valid reportable failure. Also, if a recovery routine failed to collect significant diagnostic data such that the original problem was solvable by data collected by that recovery routine, IBM standards dictated that this fault was reportable and required repair. Thus, IBM standards, when rigorously applied, encouraged continuous improvement.
IBM continued to support the major serviceability tool Dynamic Support System that it had introduced in OS/VS1 and OS/VS2 Release 1. This interactive facility could be invoked to initiate a session to create diagnostic procedures, or invoke already-stored procedures. The procedures trapped special events, such as the loading of a program, device I/O, system procedure calls, and then triggered the activation of the previously defined procedures. These procedures, which could be invoked recursively, allowed for reading and writing of data, and alteration of instruction flow. Program Event Recording hardware was used.
IBM dropped support for DSS with Selectable Unit 7, an update to OS/VS2 Release 3.7 required by the program product OS/VS2 MVS/System Extensions, Program Number 5740-XEl. The User group SHARE passed a requirement that IBM reinstate DSS, and IBM provided a PTF to allow use of DSS after MVS/SE was installed.
IBM again dropped support for DSS with SU64, an update to OS/VS2 Release 3.8 required by Release 2 of MVS/SE.
Program-Event Recording exploitation was performed by the enhancement of the diagnostic SLIP command with the introduction of the PER support in SU 64/65.
Multiple copies of MVS could share the
same machine if that machine was controlled by VM/370. In this case VM/370 was the real operating system, and regarded the "guest" operating systems as applications with unusually high privileges. As a result of later hardware enhancements one instance of an operating system could also occupy a Logical Partition instead of an entire physical system.
Multiple MVS instances can be organized and collectively administered in a structure called a systems complex or sysplex, introduced in September, 1990. Instances interoperate through a software component called a Cross-system Coupling Facility and a hardware component called a Hardware Coupling Facility. Multiple sysplexes can be joined via standard network protocols such as IBM's proprietary Systems Network Architecture or, more recently, via TCP/IP. The z/OS operating system also has native support to execute POSIX and Single UNIX Specification applications. The support began with MVS/SP V4R3, and IBM has obtained UNIX 95 certification for z/OS V1R2 and later.
The system is typically used in business and banking, and applications are often written in COBOL. COBOL programs were traditionally used with transaction processing systems like IMS and CICS. For a program running in CICS, special EXEC CICS statements are inserted in the COBOL source code. A preprocessor replaces those EXEC CICS statements with the appropriate COBOL code to call CICS before the program is compiled — not altogether unlike SQL used to call DB2. Applications can also be written in other languages such as C, C++, Java, assembly language, FORTRAN, BASIC, RPG, and REXX. Language support is packaged as a common component called "Language Environment" or "LE" to allow uniform debugging, tracing, profiling, and other language independent functions.
MVS systems are traditionally accessed by 3270 terminals or by PCs running 3270 emulators. However, many mainframe applications these days have custom web or GUI interfaces. The z/OS operating system has built-in support for TCP/IP. System management, done in the past with a 3270 terminal, is now done through the Hardware Management Console and, increasingly, Web interfaces. Operator consoles are provided through 2074 emulators, so you are unlikely to see any S/390 or zSeries processor with a real 3270 connected to it.
The native character encoding scheme of MVS and its peripherals is EBCDIC, but the TR instruction made it easy to translate to other 7- and 8-bit codes. Over time IBM added hardware-accelerated services to perform translation to and between larger codes, hardware-specific service for Unicode transforms and software support of, e.g., ASCII, ISO/IEC 8859, UTF-8, UTF-16, and UTF-32. The software translation services take source and destination code pages as inputs.