• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware.
• A virtual machine provides an interface identical to the underlying bare hardware.
• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory.
• The resources of the physical computer are shared to create the virtual machines.
  * CPU scheduling can create the appearance that users have their own processor.
  * Spooling and a file system can provide virtual card readers and virtual line printers.
  * A normal user time-sharing terminal serves as the virtual machine operator’s console.
  

IMPLEMENTAION

• Traditionally written in assembly language, operating systems can now be written in higher-level languages.
• Code written in a high-level language:
  * can be written faster.
  * is more compact.
  * is easier to understand and debug.
• An operating system is far easier to port (move to some other hardware) if it is written in a high-level language.
  
  

BENEFITS

• The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.
• A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.
• The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.
  

EXAMPLES

• Compiled Java programs are platform-neutral bytecodes executed by a Java Virtual Machine (JVM).
• JVM consists of
  - class loader
  - class verifier
  - runtime interpreter
• Just-In-Time (JIT) compilers increase performance