Virtual interval timer assist provides hardware updating of the virtual
interval timer at virtual location 1'50'. This results in an update frequency of approximately 300 times per second, the same as for the
real interval timer. Procedures that use the virtual interval timer for
job accounting, performance measurements, and the like, will therefore
generate more accurate and repeatable time data than they would if the
virtual timer was being updated by CP routines. VM/370 Extended Control-program Support (ECPS) is controlled at two
levels: the VK/370 system and the virtual machine. At the i5i370 system level, ~~~b is automatically enabled when the system is loaded. The class A command:
set cpassist off
vi11 disable both CP assist and expanded virtual machine assist. The
class A command:
set sassist off
disables only the expanded virtual machine assist part of ECPS as ve11
as the virtual machine assist. CP assist is the only part of ECPS that
is truly independent.
At the virtual machine level, whenever ECPS is enabled on the system, both expanded virtual machine assist and virtual interval timer assist
are automatically enabled when you log on. If you issue the class G
command:
set assist off both assists as well as the existing virtual machine assist are
disabled. If you issue:
set assist notmr only the virtual interval timer assist is disabled. If CP assist is
disabled for the systea, the class A command: set sassist on
vi11 enable the virtual machine assist. You can then aachine assist and virtual interval timer assist for aachine by issuing the class A command: set assist on tar enable virtual
your virtual The restrictions on the use of ECPS are the same as those described for
the virtual machine assist feature vith one addition. Whenever a
virtual aachine traces external interrupts, the virtual interval timer 1-38 IBM VK/370 System Logic and Problem Determination--Vo1ume 1

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 ,a~r 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