Virtual Machines

• What are virtual machines? 

  • Virtual machines (VMs) are entire operating systems running inside another operating system 
  • A user running Windows 11 could run a virtual machine of MacOS 
  • This would allow them to navigate the GUI of MacOS and install software on it 
  • Running a virtual machine helps access software that is only designed to run on specific operating systems 
  • VM management software includes a Hypervisor that monitors all activity happening inside the VM 
• Structure of several virtual machines running on a single piece of hardware image

  
• Cross-platform and forwards compatibility 

  • Not all software is designed to run on all operating systems 
  • Apple commonly makes software that only runs on MacOS for performance reasons 
  • A Windows user could run a virtual machine of MacOS and install the software they need 
  • Most software needs to be updated to work on the latest versions of operating systems 
  • A user running the latest release of Windows may need to run a virtual machine of a previous release of Windows to use an application that hasn't received a forwards-compatibility update
• In Software Testing #1

  • VMs are a way to create isolated test environments, that leave the host operating system unaffected 
  • Isolated environments allow a developer to: 
  • Monitor the way their software affects system performance 
  • Test on a clean-slate system, while no other applications are running 
• • In Software Testing #2
  • VM management software can create virtual machines that act like they have older hardware 
  • This allows developers to build software that can be run on older hardware so that more users can use the software 
  • A developer can test against various operating systems, such as MacOS, Linux and Windows, for greater compatibility 
  • This is achieved through compiling into intermediate code and running against different operating systems 
• Consequences 

  • VMs share the same system hardware as the host OS 
  • Over-use of VMs can exhaust the host OS of the system of CPU, hard disk and memory 
  • VM software such as VirtualBox can set maximum limits on system resources 
  • A low-specification machine could be configured to allocate only 1GB of memory and 20% of CPU 
  • A high-specification machine could afford up to 8GB of memory and 50% of the CPU 
  • Operating systems are commonly free to download, but require an activation payment to access all features