74 IBM VM/370 System Programmer's Guide
VM/370 The V5/370 Control program manages the resources of a single computer in
such a manner that multiple computing systems appear to exist. Each
"virtual" computing system, or virtual machine, is the functional
equivalent of an IBM System/370. A virtual machine is configured by recording appropriate information
in the VM/370 directory. The virtual machine configuration includes
counterparts of the components of a real IBM System/370: A virtual operator
s console Virtual storage A virtual processor Virtual I/O devices CP makes these components appear real to whichever operating system
is controlling the work flow of the virtual machine.
The virtual machines
techniques. CP overlaps
execution in another.
the idle
time of one
via multiprogramming
virtual machine with
Each virtual machine is managed at two levels. The work to be done
by the virtual machine is scheduled and controlled by so.e System/360 or System/370 operating system. The concurrent execution of multiple
virtual machines is managed by the Control Program. VM/370 performs some functions differently when running in attached
processor mode. For a description of the additional processing performed
when in attached processor mode, see 199ic Qy!de. Introduction to the VM/370 Control Program
A virtual machine is created for a user when he logs on VM/370, on the
basis of information stored in his VM/370 directory entry= The entry
for each user identification includes a list of the virtual input/output
devices associated with the particular virtual machine.
Additional information VM/370 directory entry.
class, accounting data,
dispatching priority, and
as extended control mode.
about the virtual machine is kept in the
Included are the VM/370 command normal and maximum virtual storage optional virtual machine characteristics such
The Control Program supervises the execution of virtual machines by
(1) permitting only problem state execution except in its own routines,
and (2) receiving control after all real computing system interrupts. CP intercepts each privileged instruction and simulates it if the
current program status word of the issuing virtual machine indicates a
virtual supervisor state; if the virtual machine is executing in
virtual problem state, the attemFt to execute the privileged instruction
is reflected to the virtual machine as a program interrupt. All virtual
machine interrupts (including those caused by attempting privileged
instructions) are first handled by CP, and are reflected to the virtual
machine if an analogous interrupt would have occurred on a real machine. Part 2. Control Program (CP) 75
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