Virtual Machines (Cambridge (CIE) A Level Computer Science): Revision Note

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

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

• 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

• 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

• In A Level Computer Science, intermediate code is generated through compilation and allows programs to run across 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