Virtual Machines

Question 1

What is a VM?

Answer 1

A virtual machine provides multiple execution environments, creating the illusion that each env. is running on it’s own private computer.

A VM instance hosts an operating system. It executes in user mode and emulates part (or all) of the instruction set of the virtualised hardware. Privileged operation cause a trap to the VMM/hypervisor

Question 2

What are the types of virtualisation?

Answer 2

• native/bare metal/type I
• – hypervisor running directly on hardware
• – multiple OS images hosted
• hosted/embedded/type 2
• – host OS has a VM manager built in
• – e.g. can host linux under windows
• emulation
• – interpreted, emulates a different machine

Question 3

What is a hypervisor?

Answer 3

A manager for VMs. Executes in privileged mode and interfaces with the system hardware. Privileged operation cause a trap to the VMM/hypervisor.

Question 4

Pros and cons of VMs?

Answer 4

pros:

• improved hardware utilisation
• flexibility
• protection through isolation

cons:

• no direct sharing of resources (may be achieved in software)
• performance penalties