OS/VS2 (SVS)

Operating System/Virtual Storage 2 (Single Virtual Storage)

Developer	IBM
Written in	Assembler (XF),PL/S(nee BSL)
OS family	OS/360 and successors
Source model	Open
Initial release	1972(1972)
Latest release	Release 1.7
Marketing target	IBM mainframe computers
Platforms	S/370
License	none
Preceded by	OS/360
Succeeded by	OS/VS2 (MVS)

Single Virtual Storage(SVS)refers to Release 1 ofOperating System/Virtual Storage 2(OS/VS2); it is the successor system to theMVToption ofOperating System/360.OS/VS2 (SVS) was a stopgap measure pending the availability ofMVS,although IBM provided support and enhancements to SVS long after shipping MVS.

SVS provides asingle16MiBaddress spacewhich is shared by all tasks in the system, regardless of the size of physical memory.

Differences from MVT

OS/360 used theInterval Timerfeature for providing time of day and for triggering time-dependent events. The support forS/370made limited use of new timing facilities, but retained a dependency on theInterval Timer.SVS uses theTOD Clock,Clock ComparatorandCPU Timerexclusively.

OS/360 loads error recovery and transient SVC routines fromSYS1.SVCLIBinto smalltransient areas.SVS loads these routine fromSYS1.LPALIBinto thePageable Link Pack Area(PLPA) during an IPL with the Create LPA (CLPA) option; there are no transient areas.

In the wake of theApplied Data ResearchlawsuitIBM decided todevelop chargeable versions of several applications, mostly language processors,although it's not clear whether the lawsuit was actually the deciding factor. As a result, SVS does not include asort/merge programor any language processor other than the new Assembler (XF) (replacing Assembler (F)) which is required for thesystem generationprocess.

Authorized Program Facility(APF) is a new facility that limited use of certain dangerous services to programs that areauthorized,that is link edited withAC(1)and were loaded from the link list, LPA, orSYS1.SVCLIB.InMVSIBM enhanced the facility to allow the installation to designate additional data sets as authorized.

Because the Reader/Interpreter in SVS runs in pageable storage, there is much less benefit to the Automatic SYSIN Batching (ASB) Reader, and SVS does not include it. OS/360 has a facility calledDirect SYSOUT(DSO) whereby specific output classes can be diverted to data sets on tape instead of normalSPOOLdatasets. AsDASDprices dropped, the facility dropped from use, and SVS does not provide it.

OS/360 provides limited interactive facilities inConversational Remote Job Entry(CRJE),Graphic Job Processing(GJP),Interactive Terminal Facility(ITF) andSatellite Graphic Job Processing(SGJP) prior to theTime Sharing Option(TSO), but IBM did not carry those forward to SVS. TSO continues to provide equivalent facilities, except that it does not support use of a2250as a terminal. Use of 2250 from a batch job usingGraphics Access Method(GAM) andGraphics Subroutine Package(GSP) remains supported. OS/360 includes a batch debugging facility namedTESTRAN;it is clumsier than the equivalent facility inIBSYS/IBJOB,and was not used much. With the advent of TSOTESTRANbecame even less relevant, and SVS does not include it.

Dynamic Support System(DSS) was a new OS/VS debugging facility for system software.It remained available until Selectable Unit 64 and MVS/System Extensions Release 2.

The storage key facility ofSystem/360andSystem/370keeps track of when a page frame has been modified. TheMachine Check Handler(MCH) in SVS can correct a parity or ECC error in an unmodified page by unassigning the damaged page frame and marking the page table entry to cause a pagein operation into a newly assigned page table. This replaces the special handling of refreshable transientSVCroutines in OS/360.

SVS expands the size of theError Recovery Procedure(ERP) transient area.

None of the processors on which SVS runs have an equivalent to the 2361 Large Core Storage (LCS), and thus there is no need for Hierarchy support, which SVS does not provide. SVS also dropped support for some obsolete I/O equipment.

In OS/360 load modules can be permanently loaded atInitial Program Load(IPL) time into an area of real storage known as theLink Pack Area(LPA). In SVS the LPA was split into three areas, each of which is searched in turn.

• The installation can specify a list of modules to be loaded into theFixed Link Pack Area(FLPA). These are loaded into V=R storage at IPL time.

• The installation can specify a list of load modules to be loaded into theModified Link Pack Area(MLPA) at IPL time. These modules are subject to normal paging.

• SVS uses a dedicated paging data set to back up thePermanent Link Pack Area(PLPA). In a normal IPL, SVS will simply allow modules in the existing PLPA paging data set to be paged in at need, but the operator can specify the CLPA option to load all of the load modules fromSYS1.LPALIBinto the PLPA and write the new PLPA into the PLPA paging data set.

OS/360 has support for a multiprocessor version of the 360/65. SVS provide no equivalent support; customers wanting to run a multiprocessor System/370 have to useMVS.

OS/360 introducedTelecommunications Access Method(TCAM) as the successor toQueued Telecommunications Access Method(QTAM). SVS does not include QTAM.

SVS does not includeRemote Job Entry(RJE). However, ASP and HASP provide comparable facilities.

Because of the larger (16 MiB) address space that SVS provides, there is less external fragmentation than in MVT, and Rollin/Rollout would provide less of a benefit. SVS does not include it.

In OS/360, transientSVCroutines are loaded into 1 KiB areas known as SVC Transient Areas, and a considerable amount of code is required to manage them. In SVS, all SVC routines are preloaded into virtual storageand there are no SVC Transient Areas.

While SVS retains the SPOOL support ofOS/360,most shops used ASPor HASP,the precursors of JES3 and JES2, respectively.

Storage Management

Storage management in SVS is similar to that in MVT, with a few notable differences. The description below is somewhat simplified; it glosses over some special cases.

SVS has 16MiB of addressable storage in a single address space, regardless of the size of physical memory. Thenucleusand the FLPA areVirtual=Real(V=R), meaning that each virtual address in that area is mapped to the corresponding physical address.

A job step in SVS can request V=R storage; all assigned pages in a V=R region are mapped to the corresponding real page frames.

When a program check occurs with an interrupt code of 16 or 17, SVS checks whether a page has been assigned to the virtual address. If it has, SVS will assign a page frame and read the contents of the page into it. If no page has been assigned, SVS causes an Abnormal End (ABEND) with the same ABEND code (0C4) that MVT would have used for a protection violation.

SVS provides services for page fi xing and unfi xing. When a page is fixed, its page frame is not subject to page stealing. The primary purpose of page fi xing is I/O.

I/O

I/O channels on S/370 (and successors) do not have the ability to do address translation.However, as part of the support for virtual storage operating systems IBM has provided theIndirect Data Address(IDA) feature. AChannel Control Word(CCW) with the IDA bit set points to an IDA list (IDAL) rather than directly to the I/O buffer.

SVS provides a CCW translation service as part of the Execute Channel Program (EXCP) SVC. EXCP will do any necessary page fi xing, allocate storage for IDA lists, translate virtual addresses to real, put the translated addresses into the appropriate IDA words and put the real addresses of the IDA lists into the translated CCW's. When an I/O completes, EXCP reverses the process, freeing storage and translating status back into virtual.

In addition, SVS provides the Execute Channel Program in Real Storage (EXCVR) SVC for privileged applications that do their own paged fi xing and build their own IDA lists.

Independent Component Releases (ICRs)

IBM provided several enhancements to SVS that were not shipped with SVS initially. These included:

• Telecommunications Access Method(TCAM) Release 10
• Virtual Sequential Access Method(VSAM)
• Virtual Telecommunications Access Method(VTAM) Release 2