Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 SYSHRSG=(s,s, ••• ) are the segment numbers designated as shared. The pages in
these segments are set up at load time to be used by any user
that uses this name. All segments to be shared must be
reenterable. PR OTECT = {§Jp} specifies whether CP is to protect shared segments. The
default is ON. To turn off segment protection, specify OFF. For example, a DMKSNT module to create a named C"S system could be
coded as follows: DMKSNTBL CSECT FSTNAME NAMESYS END SYSS1ZE=384K,SYSNAME=C"S,VSYSRES=CPDSK1, VSYSADR=190,SYSCYL=100,SYSVOL=CPDSK2, SYSSTRT=(400,1),SYSPGCT=35, SYSPGNM=(O-34) ,SYSHRSG=(l) Using the SAVESYS Command
x X X The system to be saved must first be loaded by device address in the
traditional manner. Before its page-format image can be saved, the
system to be saved must have its execution stopped. The point at which
the operatinq system is stopped should be determined by the installation
system programmer. The SAVESYS command must then be issued; its format
is:
r---- , SAVESYS systemname L-- systemname corresponds to the identification of the saved system. This
is identical to the SYSNAME entry in the NA"ESYS macro. The user must have a CP privilege class of E to issue the SAVESYS command. Next, he should 1Pt the saved system. The virtual machine
will attempt to resume execution and immediately encounter a paqe fault.
The required page is brought into storage and execution continues. As
execution continues, subsequent page faults will bring the required
paqes into storage. A system should be saved as soon after 1PL as possible. All pages to
be saved must be resident at the time the SAVESYS command is issued.
Also, before issuing the SAVESYS command, be sure that the system is
stopped. CMS was designed to run under CP and it was.also designed so that it
could easily be saved by CP. See "Savinq the CMS System" in "Part 3.
Conversational Monitor System (CMS)" of this publication. ]Qig: The system being saved should not exceed X'79COOO' bytes. Unpredictable results may occur if you save a larger system.
134 VM/370 System Programmer's Guide

Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 Shared Segments
If one -or more segments of a saved system are designated as being "shared," a sing Ie copy of these segments in real storage can be used by
any virtual machine that loads the saved system by name. (In attached
processor mode, there are two sets of pages, page tables, and swap
tables maintained for each shared segment.) A shared segment must be
reenterable and the segment number must be included in the SYSHRSG operand of the NAMESYS macro for the saved system.
In the previous example of a DMKSNT module to create a named CMS system, the NAMESYS macro labeled FSTNAME contains the operand: SYSHRSG= (1)
This indicates that segment 1 of CMS is to be shared. When eMS is
saved, via the SAVESYS command, the pages in segment 1 are set up so
that any user loading CMS by name will share the same set of these pages
in real storage. This results in a saving of both real and external
page storage. Also, the more virtual machines using the shared segment,
the more likely it is that these pages will be frequently referenced
and, thereby, kept in real storage. As a result, the number of page
faults and the corresponding time and resources expended in page
swappinq will be reduced. SPECIAL CONSIDERATIONS FOR SHARED SEGMENTS When a saved system containing one or more shared segments is again
saved, a problem can occur if the previous system has been loaded by
name and is still in use. If users of the "old" 'system continue to
reference pages that have already been brought into paging storage, no
problems will occur. However, if after the new system has been saved,
users of the old system reference pages that had not previously been
referenced, they receive the version of the referenced page.
Any users who IPt the newly saved system share only the new copy of
the shared segment.
Also, the entire segment is saved by the SAVESYS command,
that portion occupied by the program (for example, eMS), unwanted data may also be contained in the segment.
not just
so that
The use of shared segments is not allowed in a virtual=real machine.
The maximum number of shared segments that may be defined is 78.
Discontiguous Saved Segments
with discontiguous saved segment support, you can attach and detach
segments of storage to and from your virtual machine. These segments
contain reenterable code that can be shared by many users. Thus,
programs that are required sometimes: but not all the time, can be
shared and only loaded when they are needed. Part 2. Control Proqram (CP) 135