Overlapping Areas Other Saved Systems
caused the system to be stored there. That area cannot be the VMSA VE target
area referred to by a VMSA VE enable from another user until the stored system
has been released.
The system programmer, at his option, can specify overlapping DASD areas for
VMSA VE target areas through NAMESYS macro specifications. However, if two
areas overlap, they must start at the same physical cylinder and page. They can
end at different locations if the areas are of different lengths. Overlapping areas
are useful for different environments of the same user, and they are also valid as
VMSAVE target areas for different users. Only one user can be using the area (for IPL or for a VMSA VE target area) at any
one time. In addition, if one user has caused a virtual machine to be stored into an
area, no other user can access that area. The user also cannot issue the SET
VMSA VE command with that area as the VMSA VE target area, until the user who
caused the virtual machine to be stored does the following:
Enables VMSA VE to that area via the SET command, which effectively clears
the area.
Releases the area by issuing a SET VMSA VE command to another area, a SET
VMSA VE OFF, a DEFINE STORAGE, or a normal LOGOFF process. Only when the area has been cleared and released in this manner is it available for
other users.
For overlapping target areas, the user must load a system that has the same name
that it was saved under. This ensures that the page range returned with the load is
the same as the one stored by VMSA VE. Only when the complete page range specified has been saved does the area become
valid and available. If an error occurs in the middle of a save operation, the area is
not valid, and therefore is not retrievable.
The user cannot force a save directly. The MESSAGE command may be used to
ask the operator to force the user off the system. The FORCE command causes an
automatic save, assuming that VMSA VE is enabled. The user can also disconnect
with a READ pending. After 15 minutes the system logs off the user, causing an
automatic save if VMSA VE is enabled.
Systems loaded by name under VM/SP must be saved by the SA VESYS command
under VM/SP. Because of control block changes, systems saved under other
releases of VM/370 are not loaded properly on VM/SP. Conversely, systems
saved on VM/SP will not load properly on a system that does not have this product
installed.
Virtual Machine I/O Management
A real disk device can be shared among multiple virtual machines. Virtual device
sharing is specified in the VM/SP directory entry or by a user command. If speci­
fied by the user, an appropriate password may have to be supplied before gaining
access to the virtual device. A particular virtual machine may be assigned VM/SP 9

read-only or read/write access to a shared disk device. CP checks each virtual
machine input/output operation against the parameters in the virtual machine con­
figuration to ensure device integrity.
Virtual Reserve/Release support can be used to further enhance device integrity
for data on shared minidisks. Reserve/Release operation codes are simulated on a
virtual basis for minidisks, including full-extent minidisks. For details on
Reserve/Release support, refer to the VM / SP System Logic and Problem Determi­
nation Guide, Volume 1.
The virtual machine operating system is responsible for the operation of all virtual
devices associated with it. These virtual devices may be defined in the VM/SP directory entry of the virtual machine, or they may be attached to (or detached
from) the virtual machine's configuration, dynamically, for the duration of the ter­
minal session. Virtual devices may be dedicated, as when mapped to a fully equiv­
alent real device; shared, as when mapped to a minidisk or when specified as a
shared virtual device; or spooled by CP to intermediate direct access storage.
In a real machine, input/output operations are normally initiated when a problem
program requests the operating system to issue a START I/O instruction to a spe­
cific device. Device error recovery is handled by the operating system. In a virtual
machine, the operating system can perform these same functions, but the device
address specified and the storage locations referenced are both virtual. It is the
responsibility of CP to translate the virtual specifications to real.
In addition, the interrupts caused by the input/ output operation are reflected to the
virtual machine for its interpretation and processing. If input/output errors occur, CP records them but does not initiate error recovery operations. The virtual
machine operating system must handle error recovery, but does not record the error
(if SVC 76 is used).
In an attached processor environment, virtual I/O can be initiated by either
processor; however, all real I/O requests must be executed by the main processor,
and all I/O interrupts must be received on the main processor (the processor with I/O capability). Any I/O requests by the attached processor (the processor with­
out I/O capability) are transferred to the main processor.
In a multiprocessor environment, both processors have real I/O capability. If
either processor receives an I/O request, that processor attempts to initiate I/O operations. If none of the online paths from the executing processor to the
required device are available, that processor queues the I/O request on all busy and
scheduled paths to the device; both its own and the alternate paths to the device
from the second processor. If there is no online path from the executing processor,
that processor queues the I/O request on the first online and available path for the
second processor, as well as on all busy or scheduled paths from that processor.
Input! output operations initiated by CP for its own purposes (paging and
spooling), are performed directly and are not subject to translation.
Virtual machines may access data on MSS mass storage volumes using that virtual
machine's standard 3330 device support. MSS cylinder faults, and associated asyn­
chronous interruptions, are transparent to the virtual machine in this situation. 10 VM/SP System Programmer's Guide