• Virtual Machine
  
  - Is a software implementation of a machine (computer) that executes programs like a real machine.
  
  - Provides a complete system platform which supports the execution of a complete operating system (OS).
  
  - Takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware.
  
  IMPLEMENTATION
  
  - Modes:
  
• virtual user mode and virtual monitor mode,
  
• Actual user mode and actual monitor mode
  -Time
  
• Whereas the real I/O might have taken 100 milliseconds, the virtual I/O might take less time (because it is spooled) or more time (because it is interpreted.)
  
• The CPU is being multi-programmed among many virtual machines, further slowing down the virtual machines in unpredictable ways.
  
  BENEFITS
  
  - concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines.
  
  - 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.
  
  - multiple OS environments can co-exist on the same computer, in strong isolation from each other
  
  - the virtual machine can provide an instruction set architecture (ISA) that is somewhat different from that of the real machine
  
  - application provisioning, maintenance, high availability and disaster recovery
  
  EXAMPLE
  
  - A program written in Java receives services from the Java Runtime Environment (JRE) software by issuing commands to, and receiving the expected results from, the Java software. By providing these services to the program, the Java software is acting as a "virtual machine", taking the place of the operating system or hardware for which the program would ordinarily be tailored.