When all communications with the DASD Block I/O System Service are
completed, you can terminate communications by issuing either an IUCV SEVER
or/and IUCV RETRIEVE BUFFER.
DASD Block I/O System Service 197

The Special Message Facility
The Special Message Facility enables a virtual machine to send messages to another
virtual machine by issuing the CP SMSG command. The. Special Message Facility
may be used. with the Virtual Machine Communication Facility (VMCF) or with
the Inter-User Communication Vehicle (lUCV). However, the sending virtual
machine does not need to perform the initialization required by VMCF or IUCV.
Initialization is handled by CP and is described later in this topic.
To send a message, a virtual machine need only prepare the message text --up to
129 bytes --and issue the class G command, SMSG. Parameters on the SMSG command identify the USERID of the receiving virtual machine and specify the
message text. The format of the message text must be acceptable to the receiving
virtual machine. The SMSG command is described in the VM / SP CP Command
Reference for General Users.
Before the receiving virtual machine can receive special messages via VMCF, it
must: • Enable itself to receive external interrupts. Set bit 31 of control register 0 to a value of 1.
Authorize itself by issuing DIAGNOSE Code X'68', AUTHORIZE. The
parameter list, VMCPARM, specified with DIAGNOSE Code X'68' must con­
tain a pointer to an external-interrupt buffer, must specify a buffer length of
169 bytes, and must have the special message flag (VMCPSMSG) turned on.
Turn on this special message flag (VMCPSMSG) by setting VMCPSMSG to a
value of B' l' or by issuing the class G command, SET SMSG ON. For infor­
mation on using DIAGNOSE Code X'68', see "Description of VMCF Subfunctions" and "Invoking VMCF Subfunctions." To understand how a special message is presented to the receiving virtual machine
via VMCF, see "The SENDX Protocol" in the section "VMCF Protocol". Before the receiving virtual machine can receive special messages via IUCV, it must
do the following: • Enable itself to receive external interrupts Set bit 30 of control register 0 to a value of 1 • Issue the IUCV DECLARE BUFFER function
Issue the IUCV CONNECT function to the CP Message System Service
Turn on the special message flag by issuing the class G command SET SMSG IUCV.
When a virtual machine no longer wishes to accept special messages, it may turn
off the special message flag by issuing the command SET SMSG OFF. To resume
receiving messages, the virtual machine may issue the command SET SMSG ON or
198 VM/SP System Programmer's Guide