assist is automatically disabled. When external interrupt tracing is
completed, virtual interval timer assist is reenabled. Virtual Machine Communication Facility The Virtual Machine Communication Facility (VMCF) allows any logged-on
user of VM/370 to transfer messages, control data, data files, or
combinations of all three to another virtual machine running under the same VM/370 system. Information is transferred directly from one
virtual storage to the other virtual storage with CP buffering the
information. Only one data page frame must be locked at anyone tiae.
The amount of data that can be transferred is limited only by the
virtual storage sizes of the virtual machines involved. VMCF contains five data movement and seven centrol functions and is
invoked by a virtual machine via the DIAGNOSE interface (code X'0068'). A special external interrupt code, X'4001', notifies a virtual machine that a VMCl communication is pending. A virtual machine can have a
aaximum of 50 messages active at anyone tiae. The number of messages
is an equate in the DMKVMC module and can be changed to accommodate different VM/370 storage sizes. When a virtual machine issues a DIAGNOSE instruction with a function
code of X'0068', the rx register contains the virtual address,
doubleword-aligned, of a 40- byte para'meter list. Thi s paralleter list (VMCPARM) contains a hexadecimal code to identify the specific VMCl subfunction. It also contains the data addresses, data lengths, and
control information that are required to execute the particular
subfunction.
The DIAGNOSE instruction, a privileged operation, is processed by DMKPRV which passes control to DMKHVC, the DIAGNOSE interface Ilodule. DMKHVC, in turn, validates the function code and, if the code is X=0068=, turns control over to D5KV5C, the aodule. DaKvac validates the VMCPARM address and length, the subfunction code, and
passes control to the aFpropriate subroutine. The VMCF subfunctions and
their codes are as follows:
Code X'OOOO' X'0001' X'0002' X'0003' X'0004' X'0005' X'0006' X'0007' X'0008' X'0009' X'OOOA' X'OOOB' S ubfunction Allow-virtual aachine communication
Disallow virtual machine communication
Initiate a SEND request
Initiate a SEND/RECV request
Initiate a SENDX request
Accept data from a SEND or SEND/RECV request
Cancel specific request you initiated
Reply to a SEND/RECV request
Reject further incoming coamunications
Resuae accepting coamunications Notify a user that you are ready for comaunications
Reject a specific incoaing coamunication
CP Introduction 1-39

Special VMCl !nterrup! Whenever a source virtual machine uses VMCl to correspond with another
virtual machine (sink), the sink is notified of the pending communication via a special external interrupt (code X'4001'). When
this interrupt is unstacked and processed, a copy of the information in
the source's parameter list is passed to the sink in an external
interrupt buffer. The buffer is defined when a user allows virtual machine communication. The contents are referred to as the external
interrupt message header. When certain transactions (SEND, SEBD/RECV, SEBDX) have been completed, a final response external interrupt is passed back to the
source. The message header associated with this interrupt contains
residual counts pertaining to the transferred data and data transfer
return codes.
Figure 13 shows the relationship between the various VMCl control blocks
and data areas. When a virtual machine allows virtual machine communication, VMCl generates a master VMCBLOK and places it at the head
of a queue pointed to by the VMCPBT field of the user's VMBLOK. Tvo fields in this master VMCBLOK define the address (VMCVADA) and length (VMCLENA) of the user's external interrupt buffer. The length must include the maximum size of any potential SEBDX data in addition to the 40 bytes for the external interrupt message header. When a source virtual machine executes a VMCl subfunction, a VMCBLOK is built, initialized with data from the parameter list (VMCPARM), and
stacked on the VMCBLOK queue pointed to by the VMCPBT field in the
sink's VMBLOK. If an XIBTBLOK for a X'4001' external interrupt has not
already been stacked for the sink machine, DMKVMC builds one and stacks
it on the IINTBLOK queue pointed to by the VMPXIBT field in the sink's VMBLOK. VMCl external interrupts are assigned a sort code of X'71FllllF', giving them the lowest priority in the external interrupt
queue. Each virtual machine clears its own VMCl control blocks. Special Messages Facility The Special Message lacility allows users to send special messages to a
virtual machine via the SMSG command. In the Special Message environment, CP acts as a source machine with the receiver of special messages being the sink. This relieves the burden from the issuer of SMSG of having to perform authorization and other setup necessary for
sending messages to the receiving virtual machine. This is performed by CP. The issuer of SMSG is responsible for sending message text that is
meaningful to the receiving virtual machine. The format and handling of
special messages is entirely up to the receiving machine, which may be
one designed by the installation or prepared by others.
Before the receiving virtual machine can accept special messages, it must be running with the Special Message flag OB, and it must have
issued AUTHORIZE (via DIAGNOSE X'68') with CP. The authorization
includes supplying the External Interrupt Buffer address and size. To
ensure receiving the entire message, the receiving virtual machine 1-40 IBM VM/370 System Logic and Problem Determination--Volume 1