1-2 IB! V!/370 System Logic and Problem Deter.ination--Volu.e 1

VM/370 The VK/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 Systea/370: • A virtual operator's console e 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 operate
techniques. CP overlaps the idle
execution in another.
concurrently
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 some 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 perforaed
when in attached processor aode, see "The Attached Processor
Environment" in this section.
Introduction to the VM/370 Control Program
A virtual machine is created for a user when he logs on VK/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 V8/370 com.and normal and maximum virtual storage s1zes, optional virtual aachine characteristics such
The Control Program supervises the execution of virtual machines by
(1) peraitting only problem state execution except in its own routines,
and (2) receiving control after all real coaputing system interrupts. CP intercepts each privileged instruction and simulates it if the
current program status word of the issuing virtual aachine indicates a
virtual supervisor state; if the virtual aachine is executing in
virtual problem state, the atteapt to execute the privileged instruction
is reflected to the virtual machine as a prograa interrupt. 111 virtual
aachine interrupts (including those caused by attempting privileged
instructions) are first handled by CP, and are reflected to the virtual
aachine if an analogous interrupt would ha've occurred on a real aachine. CP Introduction 1-3