Operating System

An operating system (OS) is the master program that sits between you and the hardware — the manager the first topic in this series named in passing — and its job is to run all the other programs, share the computer's hardware fairly among them, and give both you and those programs a simple, consistent way to use the machine instead of dealing with raw hardware directly. Windows, macOS, Linux (including the Ubuntu system the working-with-a-computer topic used for its examples), Android, and iOS are all operating systems; the device looks very different in each case, but the underlying responsibilities are the same. The first responsibility is process management: as the how-programs-run topic described, each running program is a process, and because the computer has far more programs open than it has CPU cores, the OS decides which process gets to use a core and for how long, switching between them many times a second so that everything appears to run at once. The second is memory management: the OS hands each process its own private region of the RAM introduced in the first topic, keeps one program from reading or corrupting another's memory, and uses a trick called virtual memory to let the total memory programs think they have exceed the physical RAM actually installed, by temporarily parking rarely-used data on the slower storage drive. The third is the file system: the files and directories from the working-with-a-computer topic are not physically laid out on the disk as neat folders — the OS maintains that tidy tree as a convenient illusion on top of raw numbered storage blocks, so that you and your programs can open `report.txt` by name without ever knowing where its bytes actually sit. The fourth is device management through drivers: a device driver is a small piece of OS software that knows how to talk to one specific kind of hardware — a particular printer, keyboard, or graphics card — so that ordinary programs can say "print this" or "draw this" in a general way and let the OS translate that into the exact signals the attached device needs. Tying these together is a boundary between two modes: your programs run in a restricted user mode and cannot touch hardware directly, while the core of the OS, the kernel, runs in a privileged kernel mode; whenever a program needs something only the OS may do — open a file, send data over the network, allocate memory — it makes a system call, a controlled request that crosses from user mode into the kernel and back. This design is what lets a single computer safely run many programs written by many different people at once: the OS is the referee that enforces the rules, shares the hardware, and keeps one misbehaving program from taking down the whole machine.