Hardware and virtual machines

The Operating System (OS) is the most important piece of software on a computer. Its main job is to act as an intermediary between the user's applications and the physical hardware. Think of it as a manager for the computer's resources. It manages the Central Processing Unit (CPU), memory (RAM), storage devices (hard drives, SSDs), and input/output (I/O) devices like keyboards and printers. By managing these resources, the OS ensures that multiple programs can run fairly and efficiently without interfering with each other. It also provides a user interface (like Windows or macOS) that hides the immense complexity of the underlying hardware, allowing us to use the computer without needing to know how the circuits work.

When you run a program, the OS loads it into memory and it becomes a 'process'. A process is a program in execution. Modern operating systems are capable of 'multitasking', which means they can handle many processes at once (e.g., a web browser, a music player, and a word processor). Since there is usually only one CPU, this is an illusion created by the OS. The OS's 'scheduler' rapidly switches the CPU's attention between different processes, giving each a small slice of time. This happens so fast that it appears they are all running simultaneously. An 'interrupt' is a signal to the processor, often from a hardware device, that requires immediate attention, causing the OS to pause the current process and handle the interrupt.

Memory management is how the OS controls the use of RAM. One common technique is 'paging'. The OS divides physical RAM into fixed-size blocks called 'frames', and it divides a program's logical address space into matching fixed-size blocks called 'pages'. When a program runs, its pages are loaded into available frames in RAM. This allows a program's memory to be non-contiguous, making memory allocation more flexible. 'Virtual memory' is an extension of paging. It allows the OS to use a portion of the hard disk as if it were extra RAM. If a required page is not in RAM (a 'page fault'), the OS swaps it in from the hard disk, potentially swapping out an unused page from RAM to make space. This lets you run programs that are larger than the available physical RAM. However, excessive swapping, known as 'disk thrashing', can severely slow down the system because hard disks are much slower than RAM.

A Virtual Machine (VM) is an emulation of a complete computer system within another computer. It's essentially a 'computer inside a computer'. A VM runs as a process on a physical 'host' machine, but to the software running inside the VM, it looks like its own dedicated piece of hardware with its own 'guest' operating system. This is made possible by a piece of software called a 'hypervisor' or Virtual Machine Monitor (VMM). The hypervisor sits between the host hardware and the VM, creating the virtual environment and managing the allocation of the host's physical resources—such as CPU time, RAM, and storage—to the various VMs running on it.

Virtual machines are incredibly useful for several reasons. A key benefit is 'server consolidation': a company can replace many under-utilised physical servers with a single, more powerful server running multiple VMs, saving on hardware costs, power, and cooling. They are also perfect for software development and testing, providing an isolated 'sandbox' environment where new code can be tested without risking the stability of the host system. You can easily run legacy applications on a modern computer by creating a VM with an older operating system. However, VMs have limitations. The main one is 'performance overhead'; because the hypervisor adds an extra layer of software, a VM will always be slightly slower than running on the native hardware. They also consume significant host resources, as each VM needs its own dedicated slice of RAM and CPU, and managing a large number of VMs can become complex.