Shared Segment Modifications
The following mcdifications have been made to shared segments: ELIMINATION OF AUTO!ATIC UNSBARE A shared segment is defined as reentrant, read-only cede. Prior to
Release 6, a user who modified a shared segment was unshared from the
shared copy of that segment and given a private cOFY of the segment with
his change intact. 'M/370 gave other users of that segment access to a
fresh copy of that segaent.- 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 be paged in from backing storage.
a user of a shared segment causes a page of that segment to be
by issuing an ADSTOP, TRICE, or STORE command, the user is from that shared seg.ent and given his own copy of that
Other users of the segment continue to operate with an
copy cf that segment. When altered
unshared segment. 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
shared segment is specified to be a change made by the user
of that shared segment is seen by all other users of that shared
segment.
This option, when selected, eliminates the need for V!/370 to scan
for changed shared pages in both IP 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. The NAftESYS system generation macro instruction has been modified to support
this option. Note that shared segment protection is the default
specification. Installations that choose to employ the unprotected
shared segments option should do so with caution and discretion. 1-14 IBM VM/370 Release 6 Guide
SRL AND PROGRAM LOGIC PUBLICATIONS The following publications contain information on modifications to
shared segments: GC20-1800 Introduction GC20-1801 Planning and System Generation Guide GC20-1806 Operator's Guide GC20-1807 System Programmer's Guide GC20-1808 System Messages GC20-i820 CP Command Reference for General Users 5Y20-0884 Data Areas and Control Block Logic 5Y20-0886 System Logic and Problem Determination Guide Section 1. Summary of Changes to the V!j370 Syste. 1-15
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