Shared Segment Modifications
The following mcdifications have beenmade to shared segments: ELIMINATION OF AUTO!ATIC UNSBARE A shared segment is defined as reentrant, read-only cede. Prior to
Release 6, a userwho modified a shared segment was unshared from the
shared copy of that segment and given a privatecOFY of the segment with
his change intact.'M/370 gave other users of that segment access to a
fresh copy of thatsegaent.- Under Release 6, if a user modifies a shared segment, except by means of the CP ADSTOP, TRICE, or STORE commands, VM/370 places the user in console function mode and returns the .odified page(s) to free storage.
The next reference to that page causes a fresh copy to bepaged in from backing storage.
a user of a shared segment causes a page of thatsegment to be
by issuing anADSTOP, TRICE, or STORE command, the user is from that shared seg.ent and given his own copy of that
Other users of thesegment continue to operate with an
copy cf thatsegment. When altered
unsharedsegment. unchanged UNPROTECTED SHARED SEGMENTS Installations that use shared segments have the option, under Release 6, of running with cut protection against changes to these segments. If a
sharedsegment is specified to be a change made by the user
of that sharedsegment is seen by all other users of that shared
segment.
This option, when selected, eliminates the need forV!/370 to scan
for changed shared pages in bothIP and UP aodes. In IP mode, a gain in pageable storage is acheived because VM/370 need not switch page table
pointers and need not maintain duplicate copies of shared segments. TheNAftESYS system generation macro instruction has been modified to support
this option. Note that shared segment protection is the default
specification. Installations that choose toemploy the unprotected
shared segments option should do so with caution and discretion.1-14 IBM VM/370 Release 6 Guide
The following mcdifications have been
Release 6, a user
shared copy of that segment and given a private
his change intact.
fresh copy of that
The next reference to that page causes a fresh copy to be
a user of a shared segment causes a page of that
by issuing an
Other users of the
copy cf that
unshared
shared
of that shared
segment.
This option, when selected, eliminates the need for
for changed shared pages in both
pointers and need not maintain duplicate copies of shared segments. The
this option. Note that shared segment protection is the default
specification. Installations that choose to
shared segments option should do so with caution and discretion.