Virtual Mach

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Virtual Machines

Conceptually, a computer system is made up of

layers. The hardware is the

lowest level in all such systems. The kernel runningat the next level uses the

hardware instructions to create a set of system calls

for use by outer layers. The

system programs above the kernel are therefore able

to use either system calls

or hardware instructions, and in some ways these

programs do not differentiate

between these two. Thus, although they are

accessed differently, they both

provide functionality that the program can use to

create even more advanced

functions. System programs, in turn, treat the

hardware and the system calls as

though they were both at the same level.

Some systems carry this scheme a step further by

allowing the system

programs to be called easily by the application

programs. As before, although

the system programs are at a level higher than that

of the other routines, theapplication programs may view everything under

them in the hierarchy as

though the latter were part of the machine itself.

This layered approach is


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taken to its logical conclusion in the concept of a

virtual machine. The VM

operating system for IBM systems is the best

example of the virtual-machineconcept, because IBM pioneered the work in this

area.

By using CPU scheduling (Chapter 6) and virtual-

memory techniques

(Chapter lo), an operating system can create the

illusion that a process has its

own processor with its own (virtual) memory. Of

course, normally, the process

has additional features, such as system calls and a

file system, that are not provided

by the bare hardware. The virtual-machine approach,

on the other hand,

does not provide any additional functionality, but

rather provides an interface

that is identical to the underlying bare hardware.

Each process is provided with

a (virtual) copy of the underlying computer (Figure

3.11).

The physical computer shares resources to create

the virtual machines. CPUscheduling can share out the CPU to create the

appearance that users have their

own processors. Spooling and a file system can

provide virtual card readers and


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virtual line printers. A normal user time-sharing

terminal provides the function

of the virtual-machine operator's console.


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A major difficulty with the virtual-machine approach

involves disk sys- I

tems. Suppose that the physical machine has three

disk drives but wants to

support seven virtual machines. Clearly, it cannot

allocate a disk drive to each 1 virtual machine.Remember that the virtual-machine software itself

will need

substantial disk space to provide virtual memory.

The solution is to provide I

I

virtual disks, which are identical in all respects

except size-termed minidisks

in IBM's VM operating system. The system

implements each minidisk by allo- I

cating as many tracks on the physical disks as the

minidisk needs. Obviously,

the sum of the sizes of all minidisks must be smaller

than the size of the physical

disk space available.

Users thus are given their own virtual machines.

They can then run any of


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the operating systems or software packages that are

available on the underlying

machine. For the IBM VM system, a user normally

runs CMS-a single-userinteractive operating system. The virtual-machine

software is concerned with

multiprogramming multiple virtual machines onto a

physical machine, but

it does not need to consider any user-support

software. This arrangement

may provide a useful partitioning into two smaller

pieces of the problem of

designing a multiuser interactive system.

Virtual Mach

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