3. The virtual machine with the virtual=real option operates in the
preallocated storage area with normal CCW translation in effect until the CP SET NOTRANS ON command is At that time; wiTh several exceptions, all subsequent I/O operations are performed from the virtual CCwsin the virtual=real space without translation. The exceptions occur under any of the following
conditions: • SIO tracing active • First CCW not in the V=R region • I/O operation is a sense command • I/O device is a dial-up terminal • I/O is for a nondedicated device
(spooled unit record console virtual CTCA
or minidisks that are less than a full volume) • I/O device has an alternate path • Pending device status
Any of the above conditions will force CCW translation. Since
minidisks are nondedicated devices, they may be used by programs
running in the V=R region even though CP SET NCTRANS ON is in 4. If the virtual=real machine performs a virtual reset or IPL, then
the normal CCW translation goes into effect until the CP SET NOTRANS ON command is again issued. This permits simulation of an IPL sequence by CP. Only the virtual=real virtual machine can
issue the command. A message is issued if normal translation mode
is entered.
5. A virtual=real machine is not allowed to IPL a named or shared
system. It must IPL by device address. 6. When NOTRANS is in effect for a virtual=real machine. no meaningful
SEEK data is collected by MONITOR operations. AFFINITY This option allows virtual machines that operate on attached processor
systems to select the processor of their choice for program execution.
To select the affinity option, use the directory OPTION statement. or
specify the AFFINITY operand on the class A. B. F. or G SET command.
The directory OPTION statement is described in the The class A. B, and F SET commands are
described in the and the class G SET command is
described in the VML11Q fOf In application, the affinity setting of a virtual machine implies a
preference of operation to either (or neither) processor. Affinity of
operation for a virtual machine means that the program of that virtual
.achine will be executed on the selected or named processor. It does
not imply that supervisory functions and the CP housekeeping functions
associated with that virtual machine will be handled by the same
processor. Part 2. Control Program (CP) 97

In attached processor systeas, all real I/O operations and associated
interrupts are handled by the aain processor. Virtual I/O initiated on
the attached processor that is aapped to real devices must transfer
control to the main processor for real I/O execution. Therefore,
benefits may be realized in a virtual aachine "mix" by relegating those
virtual machines that have a high I/O-to-compute ratio to the main processor, and those virtual machines that have a high compute-to-I/O ratio to the attached processor. Such decisions should be carefully
weiahed as everv virtual machine is in contention with othp.r virtnal machines-for the system.--- ----- -------
A more important use of the affinity setting would be in applications
where there are virtual machine program requirements for special
hardware features that are available on one processor and not the other. Such features could be a performance enhancement such as virtual machine
assist (described later in the text) or a special RPQ that is a
requirement for a particular program's execution. VIRTUAL MACHINE ASSIST FEATURE The virtual machine assist feature is a processor hardware feature that
improves the performance of VM/370. Virtual storage operating systems. which run in problem state under the control of VM/310, use many privileged instructions and SVCs that cause interrupts that V!/310 aust handle. When the virtual machine assist feature is used. many of these
interrupts are intercepted and handled by the processor. Consequently. VM/310 performance is improved.
The Virtual Machine Assist Feature intercepts and handles
interruptions caused by SVCs (other than SVC 16), invalid page
conditions, and several privileged instructions. An SVC 16 is never
handled by the assist feature; it is always handled by CP. The
processing of the following privileged instructions is handled by this
feature:
LRA STCTL RRB ISK SSK IPK STNSM STOSM SSM LPSW SPKA (load real address)
(store control)
(reset reference bit)
(insert storage key)
(set storage key)
(insert PSW key)
(store then AND system mask)
(store then OR system mask) (set systea mask) (load PSW) (set PSW key froa address)
Although the assist feature was designed to improve the performance
of VM/370, virtual machines may see a performance improvement because aore resources are available for virtual machine users. For a list of
processors on which the Virtual Machine Assist Feature is available, see
the 98 IBM 1M/370 System Programmer:s Guide