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
April 1, 1981 Segments that are to be shared in this manner must be loaded at an
address the normal end of your virtual machine and then must be
saved. The procedure for loading and saving discontiguous segments is
similar to the procedure that already exists for loading and saving
systems. Also, discontiguous saved segments can be attached to your
virtual machine in nonshared mode for testing and debugging. In
summary, a discontiquous saved segment is a segment that: Has a name associated with it Contains only reenterable code Was previously loaded and saved Can be shared by multiple virtual machines Can be loaded bV a particular virtual machine in nonshared mode for
testing and debugging
Note: A discontiguous saved segment must not be attached by a virtual machine executing in the virtual=real area.
An example of a discontiguous saved segment is the segment of CMS that supports DOS program development and testing under eKS. This
seqment is reenterable and is named CKSOOS. The VK/370 starter system
includes an EXEC procedure that helps you load and then save this
segment. CMS contains all the necessary linkage to load the CKSOOS segment when it is needed. USER REQUIREMENTS In order to use discontiguous saved segments, you must: Allocate permanent space on a CP-owned volume to contain the saved
segment. Assign a name to the segment and specify where it is to be stored on
disk by defining an entry in the system name table (DKKSNTBL) with
the NAMESYS macro. Load and save the segment. The V"/370 starter system has EXEC procedures to help you load and save the discontiguous saved segments
for CMS (one EXEC procedure to load and save CMS/DOS, one for CMS/VSAM and AMSERV, and one for the CMS Editor, EXEC processor, and OS simulation routines). Be sure that the proper linkage for attaching and detaching saved segments is in the operating system that needs
the segment. CMS contains the linkage necessary to attach and detach
the discontiguous saved segments it supports. Usually, the direct access storage space is allocated and the system
name table entries are created during system generation. You allocate OASD space as permanent (PERM) by executing the Format/Allocate program.
This program is executed during system generation, but it is a
standalone proqram that can be executed at any time. During system
generation, you designate the CP-owned volumes by coding the SYSOWN macro the DMKSYS file. The system name table (OMKSNT) is also
created durinq system generation. If, at some time after system
generation, you wish to change the DMKSYS or OKKSNT files, you can do a
partial system generation and reassemble those files using the GENERATE EXEC GENERATE is described in the gnd You can also load and save a discontiguous saved
segment any time after system generation.
136 IBM VM/370 System Proqrammer's Guide
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