Section 4. OS/VS In a Virtual Machine
This section uses the term "OS/VS" as a generic expression. It
represents any or all of the as, OS/VS1, OS/VS2 SVS, and OS/VS2 MVS system control programs unless specified otherwise. When loading OS/VS into a virtual machine running under VM/310, the
terminal becomes the OS/VS operator console, and the user is responsible
for entering all the commands and responses normally required of the
operator.
The three basic technique s to use when running OS/VS in a virtual
machine are: • Batch mode. One user runs as the OS/VS machine (userid OSVS) and
other users (like CMSID1) may submit jobs either through the virtual
card reader, through the system card reader, or via JES remote
stations. • Alternating technique. The IPL command is used to alternate between OS/VS and CMS in a single virtual machine. This method requires only
one directory entry, the one for the as/vs user. Because of the
lengthy 1PL process, it is practical only if an installation bas
created an OS/VS saved system. • Disconnected user (OS/VS2 only). Because 05/VS2 allows a user to
recall a prior system message, he can log on as an OS/VS2 operator
under the OSVS userid, start up his system, and then disconnect. While the OS/VS2 machine continues to run; the user can log on as a CMS user (CMS1D1) and create and submit job streams and check the
resulting output. To check the progress of the operating system,
disconnect from the CMS machine and reconnect to the OSVS virtual
machine via the LOGON command.
Before discussing these three techniques in greater detail, the
user must understand how to: • Generate OS/VS to run in a virtual machine • Create VM/310 directory entries for OS/VS virtual machines • Access the OS/VS system residence volume • Ensure that the proper I/O devices are attached to the as/vs virtual
machine • 1PL and operate as/vs under VM/310 System Generation Recommendations When generating OS/VS to run in a virtual machine, an installation
should have these primary objectives: • To have all commonly used transient routines made residence in
storage • To run all jobs (if possible) as V=R jobs
Section 4. OS/VS in a Virtual Machine 101

To meet these objectives, an installation needs to consider how it
generates both iM/310 and OS/VS. Fer OS/VS2 Release 1 (referred to as Single Virtual Storage or SVS) can use two address spaces: • One address space for V=V jobs (that is, jobs executing even though
some of their pages are not in real storage) • Another address space for SiS and any V=R jobs
By defining the SiS virtual machine large enough for all jobs to run i=R, SVS does not have to alternate between address spaces. It should
perform better under VM/310 than if it had to alternate between address
spaces. i8/310 RECOMMENDATIONS When generating VM/310 for an OS/VS virtual machine, note the following
recommenda tions:
IPL time can be reduced by saving any operating system after the
generated operating system has been load on VMj310. For more information about generating saved systems, refer to the VS1 Release 4 and subsequent releases use VM/VS handshaking. VM/VS handshaking permits instructions issued by VS1 in a virtual machine to be directly by the processor. It also permits VM/310 to
simulate privileged instructions. For further details, refer to the
topic "iM/VS Handshaking for VS1" in this section. OS/VS RECOMMENDATIONS When OS/VS to run in a virtual machine, note the following
recommenda tions:
very often, that improve performance on a real machine have no
effect (or an adverse effect) in a virtual machine. For SEEK separation, which improves performance on the real
machine, is redundant in a virtual machine. VM/310 itself issues a
stand-alone SEEK for all disk I/O. When VM/310 is the primary operating system and another operating system, such as VS1, is running one or two partitions in a virtual
machine under it, generate the other operating system with as few
options as possible, particularly when several virtual machines share
the same system residence volume. 108 IBM VM/370 operating Systems in a Virtual Machine