Enabling MVS / System ExtensiollS Support Sillgle Processor Mode
Using the class A SET S370E ON command, the system operator enables the MVS/System Extensions support for all virtual machines. Using the class G SET 370E ON command (or 370E option on the directory OPTION control statement),
the general user enables the support for a particular virtual machine.
When an OS/VS2 MVS Attached Processor (AP) system or an OS/VS2 MVS tightly-coupled Multiprocessing (MP) system runs on a multiprocessor under VM/SP, without using single processor mode, MVS runs in uniprocessor mode.
That is, MVS programs do not execute simultaneously on both processors. There­
fore, MVS does not attain the level of throughput it could attain were it running in
multiprocessor mode.
To improve the throughput of an OS/VS2 MVS AP system or OS/VS2 MVS MP system, run MVS in the V =R machine and use single processor mode. Running in
this mode, MVS has exclusive use of one processor while VM/SP and the V =R
machine (running MVS) use the other processor. In other words, MVS runs on
two processors instead of one. This improves MVS's throughput.
The throughput of an OS/VS2 MVS AP or OS/VS2 MVS MP system running
under VM/SP and using single processor mode is higher than the throughput would
be were single processor mode not used. However, single processor mode may
reduce the throughput of VM/SP and virtual machines not using the V =R area. Single processor mode cannot improve the throughput of a VM/SP attached
processor or multiprocessor system. A VM/SP AP or MP system initialized (by IPL) in the V =R machine with single processor mode on runs in uniprocessor
mode.
Two commands provide operator control of single processor mode. SPMODE, a
class A command, turns single processor mode on or off. QUERY, a class A or G
command, indicates whether single processor mode is on or off.
For detailed instructions on how to turn single processor mode on or off, see
VM / SP Operating Systems in a Virtual Machine.
Dynamic Systeln Control Programming (SCP) Transition to or from Native Mode
There are times when an installation benefits from switching an SCP to or from
native mode. For example, when it is important to obtain the best possible per­
formance from an SCP, switch it to native mode. When there is a need to do dif­
ferent kinds of work simultaneously, switch the SCP from native mode to the VM/SP environment.
Installations have always had the capability to switch an SCP to or from native
mode, but to do so has been time consuming. Switching an SCP to native mode
meant quiescing the SCP and VM/SP and then initial program loading the SCPo To return the SCP to the VM/SP environment meant quiescing the SCP and then
initial program loading VM/SP and the SCPo Dynamic SCP transition to or from native mode enables an operator to dynamically
switch an SCP to or from native mode. Switching to native mode, there is no long- Performance Guidelines 47

er a need to quiesce or reinitialize (via IPL) the SCPo The SCP continues to run
and can do productive work. Switching back to the VM/SP environment, there is
no longer a need to quiesce the SCP or IPL VM/SP or the SCPo Before switching an SCP to or from native mode, an operator must prepare VM/SP and the SCP for the switch: for example, all users except the VM/SP operator and the operator on the V=R machine must logoff VM/SP. Detailed
instructions on preparing the systems and on switching to or from native mode are
in VM/SP Operating Systems in a Virtual Machine. The following discussion high­
lights the switching process and defines precautions that must be observed.
To switch an SCP to native mode, it must be running in the V =R machine. The VM/SP operator then prepares VM/SP and the SCP for the switch. To complete
the switch, the operator issues the QVM command (quiesce VM).
After the switch to native mode is completed, there are two areas of real storage
that must not be altered. Addresses 0-7 contain the restart PSW (program status
word) used to make the transition back to the VM/SP environment. Storage above
the upper limit of the V =R area contains the VM/SP nucleus. Altering either area
may make it impossible to return to the VM/SP environment.
To return the SCP to the VM/SP environment, an operator uses the System/370 restart facility. After stopping the processor, the operator stores the value X'FF'
into the real storage address located eight bytes prior to the address pointed to by
the restart PSW. To complete the switching process, the operator restarts the
processor. Caution: This process does not work unless the SCP was switched to
native mode via the QVM command.
The performance of an SCP switched to native mode depends on the size of the
V =R area. The SCP's performance will be identical to the performance it would
attain were it initialized (via IPL) directly on a hardware configuration identical to
the V =R machine's configuration with a real storage size equal to the storage size
of the V =R area. In other words, the larger the V =R area, the better the SCP per­
forms.
You can can switch to or from native mode using the procedures just described for: OS/VSl running without VM VSl Handshaking OS/VS2 SVS • OS/VS2 MVS 48 VM/SP System Programmer's Guide