Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 USING THE VIRTUAL MACHINE ASSIST FEATURE Whenever you IPL VM/370 on a processor with the virtual machine assist
feature, the feature is available for all VM/370 virtual machines.
However, the system operator's SET command can make the feature
unavailable to VM/370 and, subsequently, available again for all users.
If you do not know whether or not the virtual machine assist feature is
available to VM/370, use the class A and E QUERY command. For a
complete description of the Class A and E QUERY and SET commands, see
the !lXLJ1Q If the virtual machine assist feature is available to VM/370 when you
log on your virtual machine, it is also supported for your virtual
machine unless you are running a second-level VM/370 system in your
virtual If your VH/370 directory entry has the SVCOFF the SVC handling portion of the assist feature is not available when you
log on. The class G SET command can disable the assist feature (or only
disable SVC handling). It can also enable the assist feature, or if the
assist feature is available, enable the SVC handling. You can use the
class G QUERY SET command line to find whether you have full, partial,
or none of the assist feature available. For a complete description of
the Class G QUERY and SET commands, see the fR £2£ User.§. RESTRICTED USE OF THE VIRTUAL MACHINE ASSIST FEATURE Certain interrupts must be handled by VM/370. Consequently, the assist
feature is not available under certain circumstances. VM/370 automatically turns off the assist feature in a virtual machine that: Has set an instruction address stop Is tracing SVC and program interrupts Since an address stop is recognized by an SVC interrupt, VM/370 must
handle SVC interrupts while address stops are set. Whenever you issue
the ADSTOP command, VM/370 automatically turns off the SVC handling
portion of the assist feature for your virtual machine. The assist
feature is turned on again after the instruction is encountered and the
address stop removed. If you issue the QUERY SET command line while an
address stop is in effect, the response will indicate that the SVC handling portion of the assist feature is off. Whenever a virtual machine issues a TRACE command with the SVC, PRIV, BRANCH, INSTRUCT, or ALL operands, the virtual assist feature is
automatically turned off for that virtual machine. The assist feature
is turned on again when the tracing is completed. If the QUERY SET command line is issued while SVCs or program interrupts are being
traced, the response will indicate the assist feature is off.
The virtual machine assist feature is not available to a second-level
virtual machine, that is, a virtual machine that is running in a virtual
machine.
VM/370Extended Co-ntro-I .. P-rog-ram Support (ECPS) VM/370 Extended Control-Program Support (ECPS) extends, for specific
privileged instructions, the hardware assistance that the virtual
machine assist feature provides. ECPS also provides hardware assistance
Part 2. Control Program (CP) 99
April 1, 1981
for frequently used VM/370 functions. The use of ECPS improves VM/370 performance beyond the performance gains that the virtual machine assist
feature provides. ECPS consists of three functions: CP assist e Expanded virtual machine assist Virtual interval timer assist
CP assist provides hardware assistance for frequeutly used paths of
specific CP functions.
Expanded virtual machine assist extends the hardware assistance that
the virtual machine assist feature provides for the instructions LPSW, STNSM, STOSM, and SSM. In addition, expanded virtual machine assist
provides hardware assistance for certain other privileged instructions.
virtual interval timer assist provides hardware updating of the
virtual interval timer at virtual address X'50'. Virtual interval timer
assist updates the virtual timer at the same frequency hardware updates
the real timer, 300 times per second. Thus, virtual interval timer
assist updates the virtual timer more frequently than CP updates it.
Because the timer is updated more frequently, accounting routines may be
able to provide accounting data that is more accurate. ECPS does not support the same functions and instructions on all
processors. Figure 13.1 lists the processors on which ECPS is
available, and identifies, by processor, the functions and instructions ECPS supports. 100 IBM VM/370 System programmer's Guide
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