Virtual storage
Addresses ABSOLUTE PAGE 0 (MODULE DMKPSA) /
/
Virtual Page 1
VIRTUAL=REAL AREA SIZE = 128K BYTES /
/ (Minimum size is 32K bytes.)
OK Virtual Page 0 132K
/ Remainder of CP Resident Nucleus /
/ /
Real storage
Addresses OK 4K
128K
132K (DMKSLC) End of CP Nucleus (DMKCPE) / Dynamic Paging Area /
/ and /
Free Storage PSA for Attached or non-IPL Processor PSA for MAIN or IPL Processor Figure 3. Storage Layout in a VirtuaI=ReaI Machine <-------, < ____ DMKPSA 512K (End of real
storage)
There are several considerations for the virtual=real option that affect overall sys­
tem operation:
1. The area of contiguous storage built for the virtual=real machine must be large
enough to contain the entire addressing space of the largest virtual=real
machine. The virtual=real storage size that a VM/SP system allows is defined
during system generation when the option is selected.
2. The storage reserved for the virtual=real machine can only be used by a virtual
machine with that option specified in the VM/SP directory. It is not available
to other users for paging space, nor for VM/SP usage until released from vir­
tual=real status by a system operator via the CP UNLOCK command. Once released, VM/SP must be loaded again before the virtual=real option can
become active again.
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 issued. At that time, with several exceptions, all
subsequent I/O operations are performed from the virtual CCWs in 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
Performance Guidelines 33

Affinity Pending device status
Any of the above conditions force CCW translation. Since minidisks are non­
dedicated devices, they may be used by programs running in the V=R region
even though CP SET NOTRANS ON is in effect.
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 transiation 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.
7. If an installation defines a V=R area on a 3081 processor, the reliability and
availability of the V=R machine can be improved if the V=R machine issues
the TEST BLOCK instruction to validate storage in the V=R area. Note that
the only two SCPs that issue TEST BLOCK are MVS/SP and VM/SP. The
HSA area on a 3081 processor may reside in the middle of the V =R area;
these two control programs mark the HSA segment as invalid and continue val­
idating storage. Any other system control program, such as OS/VS, validates
storage via the MVCL instruction. When OS/VS encounters the beginning of
the HSA, it assumes that it has reached the end of storage. Therefore, such a
control program running in the V =R area of VM/SP on a 3081 processor may
not have access to the full V =R area.
8. If an installation intends to run in single processor mode on a 3081 processor,
the system operator must issue a VARY OFF PROCESSOR nn VLOG com­
mand.
This option allows virtual machines that operate on attached processor or multi­
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 VM / SP Planning Guide and Reference. The
class A, B, and F SET commands are described in the VM / SP Operator's Guide
and the class G SET command is described in the VM / SP CP Command Reference
for General Users.
In application, the affinity setting of a virtual machine implies a preference of oper­
ation to either (or neither) processor. Affinity of operation for a virtual machine
means that the program of that virtual machine will be executed on the selected or
named processor. It does not imply that supervisory functions and the CP house­
keeping functions associated with that virtual machine will be handled by the same
processor.
In attached processor systems, all real I/O operations and associated interrupts are
handled by the main processor. Virtual I/O initiated on the attached processor
that is mapped to real devices must transfer control to the main processor for real I/O execution. Therefore, benefits may be realized in a virtual machine "mix" by
34 VM/SP System Programmer's Guide