Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 Shared segments modified by the CP commands TRACE, ADSTOP, or STORE are handled differently by CP. In this case, CP gives exclusive use of
the modified segment to the virtual machine that modified it. CP provides an unmodified copy of the segment for other virtual machines.
The VM/370 default is to protect shared segments. To turn off
segment protection, use the NAMESYS macro instruction. This macro
instruction can also turn on segment protection. Instructions for using
the NAMESYS macro instruction are in the section "The NAMESYS Macro for Saved System." When segment protection is on, CP protects segments in the following
way. Before dispatching a virtual machine, CP determines if the current
virtual machine altered any pages within the shared segments. If a page
was altered, CP sends a message to the current virtual machine to
identify the altered page, makes the altered page inaccessible, and
stops the current virtual machine by placing it into console fUnction mode. CP then dispatches another virtual machine. To resume execution
on the virtual machine that CP stopped, the operator of that machine
must issue the class G BEGIN command.
To make an altered page inaccessible, CP frees the storage the page
occupied. Later, when a virtual machine references the page, CP brings
a fresh copy of the page into storage. Shared segment protection supports: • The virtual machine assist feature and Extended Control-Program
support for named shared systems. • The execution of all options of the CP STORE command in shared
segments, including branch and instruction tracing. • The execution of the CP STORE and ADSTOP commands in shared segments. • The execution of the STORE and BREAK subcommands of the eMS DEBUG command.
CP's handling of storage keys includes the following: • No distinction is made between shared and non$hared systems for
storage key fetch instruction simulation, DISPLAY command execution,
and page key handling. • A mask in control register 6 prevents the ISK (insert storage key)
and SSK (set storage key) instructions from being handled by the VMA feature. This is necessary because VMA updates the key on SSK instructions (including the SWPTABLE fields), but the new value is
not detected by the hardware change bft monitoring. CP does not permit a user of shared systems to set storage keys via
the Set Storage Key (SSK) instruction. Thus, one user cannot prevent
other users from accessing shared storage. I/O activity into shared segments is monitored by
translators. A channel protection error occurs if a
attempts to read data into a shared segment.
channel program
virtual machine Part 2. Control Program (CP) 1ij1

Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829
The STCP command may be used to alter shared When the STep command is used to alter shared segments, the change is reflected to all
users of the shared segments; the altered shared system is not assigned
to the user issuing the STCP command. Whenever the STCP command is
issued for a shared segment, storage is updated and the page that
changed is written to the paging volume, thus reflecting the change to
all users of the shared segment. VIRTUAL MACHINE OPERATION If you issue a STORE, ADSTOP, or TRACE command that alters a storage
location within a shared segment, you receive the following message: DMKVMA181E SHARED COpy SYSTEM name REPLACED WITH NON-SHARED COpy Execution continues in your virtual machine; however, you are now
executing your own copy of the shared system in nonshared mode. The
nonshared system you are executing includes the change you just made;
all other users of the shared system continue to execute in shared mode
and are not affected by your change.
If you alter a shared page by any means other than the TRACE, ADSTOP, or STORE command, you receive the following message: DMKVMA456W CP ENTERED; name SHARED PAGE hexloc ALTERED You must enter the BEGIN command to continue execution. The altered
page will be returned to free storage by CP, and you may continue with
an unaltered system in shared mode.
If you issue an STCP command that alters the storage of a shared
segment, storage is altered and the page altered is written to the
paging volume. All users, including you, remain in shared mode and the
change becomes part of the shared system. If operations overlap and you
issue a STCP command for a shared page that is about to be assigned to a
particular user as nonshared (because he just altered it), you receive
the following message: DMKCDS161E SHARED PAGE hexloc ALTERED BY userid You should check that you issued the STCP command correctly and then
wait until the fresh copy of the saved system is loaded before reissuing
the STCP command.
In attached processor systems it is invalid to issue the STep command
to a shared segment. The STORE function is not performed, and the user the following message: DMKCDS004E INVALID HEXLOC - xxxxxx
142 IBM VM/370 System programmer's Guide