Whenever you IPL '8/370 on a processor with the virtual machine assist
feature, the feature is available for all V8/370 virtual aachines.
However, the systea operator's SET coamand can make the feature
unavailable to '8/370 and, subsequently, available again for all users.
The foraat of the systea operator's SET command is: SET SASSIST {ON} [[PROC] xx] OFF If you do not know whether or not the virtual machine assist feature
is available to Y8/370, use the class A and E QUERY coamand. For a
coap1ete description of the Class A and E QUERY and SET commands, see
the V8LJ70 If the virtual machine assist feature is available to V8/370 when you log on !onr vir+uAl .Achinp; i+ for your virtual aachine. If your V8/370 directory entry has the SVCOFF option, the SVC handling portion of the assist feature is not available when you log on.
The class G SET co •• and can disable the assist feature (or only disable SVC handling). It can also enable the assist feature, or if the assist
feature is available, enable the svc handling. The format of the co.mand is: SET ASSIST {[ ON] [SVC ] [T8R ]} [NOSVC] [NOT8R] OFF You can use the class G QUERY SET command line to find whether you have
full, partial, or none of the assist feature available. For a co.p1ete
description of the Class G QUERY and SET coamands, see the !8/37Q CP for 2!iU!era1 Certain interrupts aust be handled by V8/370. Consequently, the assist
feature is not available under certain circuastances. V8/370 autoaatica11y turns off the assist feature in a virtual aachine if it: • Has an instruction address stop set. • Traces SVC and prograa interrupts. Since an address stop is recognized by an SVC interrupt, V8/370 must handle SVC interrupts while address stops are set. Whenever you issue
the ADSTOP coaaand, Y8/370 automatically turns off the SVC handling
portion of the assist feature for your virtual aachine. The assist
feature is turned on again after the instruction is encountered and the
address stop reaoved. If you issue the QUERY SET coamand line while an
address stop is in effect, the response will indicate that the SVC handling portion of the assist feature is off. Whenever a virtual aachine issues a TRACE co.aand with the SVC, PRIV, BRANCH, INSTRUCT, or ALL operands, the virtual assist feature is
autoaatica11y turned off for that virtual aachine. The assist feature
is turned on again when the tracing is completed. If the QUERY SET command line is issued while SVCs or program interrupts are being
traced, the response will indicate the assist feature is off. 1-36 IBM V"/370 System Logic and Problem DeterBination--Voluae 1

VM/370 EXTENDED CONTROL-PROGRAM SUPPORT (ECPS) VM/370 Extended Control-Program Support (ECPS) improves the performance
of the processor when executing V!/370 beyond the improvement attained by the virtual machine assist feature described above. See V!/370 and System Guide for a list of the processors on
which ECPS is available. ECPS consists of three parts: CP assist,
expanded virtual machine assist, and virtual interval timer assist. I detailed description of ECPS is provided in "Appendix I. V!/370 Extended Control-program Support." £f Assist
The CP assist part of ECPS assists various routines that are frequently used by V!/370. Because these routines are assisted by the hardware without involving V!/310. performance of the V!/370 system is improved.
The high-use paths of the following functions are assisted: • Get Free Space (D!KFRE) • Release Free Space (DKKFRE) • Untranslate CSI (DMKUNT) • Free CCI Storage (D!KUNT) • Locate' irtual I/O Control Block (DKKSCI) • Locate Real I/O Control Block (D!KSCN) • Lock a page (D!KPTR) • Unlock a page (D!KPTR) • Common CCW co.mand processing (D!KCCW) • Decode First CCI co •• and (D!KCCW) • Decode following CCW command (D!KCCW) • TRANBRNG subroutine (D!KCCW) • TRANLOCK subroutine (D!KCCW) • Invalidate page table subroutine (D!KV1T) • Invalidate segment table subroutine (D!KVl T) • !ai.n entry to dispatch (D!KDSP) • Dispatch a block or virtual machine (D!KDSP) • svc 8 (LINK) • SVC 12 (RETURN) • Locate changed shared pages (D!KV!l) Expanded virtual machine assist extends the level of handling of the
following privileged instructions: • LPSW • STNS! • STOS! • 55! In addition, expanded virtual machine assist handles
of the following privileged instructions not handled machine assist feature: • PTLB • SIO • SPT • SCKC • STPT • TCH the processing by the virtual
CP Introduction 1-37