If the calculated working set of the highest priority virtual machine
in the eligible list is greater than the number of page frames available
for allocation, then 75 percent of the working set for that virtual machine is calculated. If the pages required for 75 percent of the
working set are available, the virtual machine is placed on Q2.
Otherwise, the virtual machine remains on the eligible list until there
are no other users on Q1 or Q2.
Executable virtual machines are sorted by "dispatching priority".
This priority is calculated each time a user is dropped from a queue and
is the ratio of processor time used while in the queue to elapsed time
in the queue. Infrequent processor users are placed at the top of the
list and are followed by more frequent processor users. When a
nonexecutable user becomes executable, he is placed on the queue based
on his dispatching When a virtual machine completes its time slice of processor usage,
it is dropped from Q2 and placed in the eligible list by user priority. When a user request in Q2 enters CP command mode, it is removed from Q2. When the request becomes executable (returns to virtual machine
execution mode), it is placed in the eligible list based on user
priority.
If a user's virtual machine is not in Q1 or Q2, it is because: • The virtual machine is on the "eligible list," waiting to be put on
Q2
-- or -- • The virtual machine execution is suspended because the user is in CP mode executing CP coamands To leave CP mode and return his virtual machine to the "eligible list" for Q2, the user can issue one of the CP commands that transfer
control to the virtual machine operating system for execution (for
example, BEGIN, IPL, EXTERNAL, and RESTART). In CP, interactive users (Q1), if any, are considered for dispatching
before noninteractive users (Q2). This means that CMS users entering
commands that do not involve disk or tape I/O operations should get fast
responses from the VM/370 system even with a large number of active
users. An installation may choose to override the CP scheduling and
dispatching scheme and force allocation of the Frocessor resource to a
specified user, regardless of its priority or operating characteristics.
The favored execution facility allows an installation to:
1. Specify that one particular virtual machine is to receive up to a
specified percentage of processor time. 2. Specify that any number of virtual machines are to remain in the
queues at all times. Assignment of the favored execution option is
discussed in the "Preferred Virtual Machines" section. Part 2. Control Program (CP) 85

Interruption Handling I/O Interrupts Input/output interrupts from completed I/O operations initiate various
completion routines and the scheduling of further I/O requests. The I/O interrupt handling routine also gathers device sense information.
Program Interrupt Program interrupts can occur in two states. If the processor is in
supervisor state, the interrupt indicates a system failure in the CP nucleus and causes the system to abnormally terminate. If the processor
is in problem state, a virtual machine is executing. CP takes control
to perform any required paging operations to satisfy the exception, or
to simulate the instruction. The fault is transparent to the virtual
machine execution. Any other program interruFt is a result of the
virtual machine processing and is reflected to the machine for handling.
Machine Check Interrupt When a machine check occurs, the CP Recovery Management Support (RMS) gains control to save data associated with the failure for the Field
Engineer. RMS analyzes the failure to determine the extent of damage.
Damage assessment results in one of the following actions being
taken: • System termination (CP disabled wait state) • Attached processor disabled (system continues in uniprocessor mode) • Selective virtual user termination • Selective virtual machine reset • Refreshing of
configuration
damaged information with no effect on system • Refreshing of damaged information with the defective storage page
removed from further system use • Error recording only for certain soft machine checks
The system operator is informed of all actions taken by the RMS routines. When a machine check occurs during VM/370 startup (before the
system is sufficiently initialized to permit RMS to operate
successfully), the processor goes into a disabled wait state and places
a completion code of X'OOB' in the leftmost bytes of the current PSi. 86 IBM 7n/37C Sy£tem Programmeris Guide