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

Apr il 1, 1981
The Virtual Machine Communication Facility The Virtual Machine communication Facility (VMCF) is part of the CP component of VM/370. VKCF provides virtual machines with the ability to
send data to and receive data from any other virtual machine. VMCF is made up of five data transfer subfunctions, seven control
subfunctions, a special external interrupt (code X'4001') to
asynchronously alert virtual machines to pending messages, and an
external interrupt message header to pass control information (and data,
at times) to another user. VMCF is implemented by means of subfunctions invoked using the DIAGNOSE instruction with a code of X'68' and a special 40-byte parameter list called VMCPARM. A VMCF subfunction is indicated by a
particular subfunction code in the VMCPFUNC field in the parameter list. BQte: Before you can use any other VMCF subfunction, you must use the AUTHORIZE subfunction for communications. Before you can communicate
with another user, that user must also have used the AUTHORIZE su bfu nct ion.
A special external interrupt (code X'4001') is used by module DMKVKC to notify one virtual machine of a pending transfer of data. This
interrupt is also used to synchronize sending and receiving of data. Along with this interrupt, the virtual machine receives a message
header that is logged into a preassigned virtual storage area. This
message header is used to define the type of request and to provide data
transfer information, such as length of data. The message header is
also used to notify the originator of a transaction of the success or
failure of the transaction. In this case, the message header includes
such information as residual counts and data transfer return codes.
Figure 14 lists the VMCF subfunction$ and qives a brief description
of each. The subfunctions are described in detail in the section
"Descriptions of VMCF Subfunctions." Messages and data are directed to other virtual machines logically
via the userid. Data is transferred in up to 2048-byte blocks from the
sending virtual machine's storage to the receiving virtual machine's
storage. The amount of data that can be moved in a single transfer is
limited only by the sizes of virtual machine storage of the respective
virtual machines. Use of real storage is minimal. Only one real storage
paqe need be locked during the data transfer.
The special message facility uses VKCF to send messages from one
virtual machine storage area to another virtual machine storage area.
For a description of the special message facility and how it uses VMCF, see "Special Message Facility" in this section. Part 2. Control Proqram (CP) 143