Operating Systems Fundamentals

An operating system is the control layer between programs and hardware. A program wants CPU time, memory, files, network access, and devices. The operating system coordinates those requests so many programs can run without directly controlling the machine.

You interact with an operating system through a graphical interface, a shell, services, and application programming interfaces. Those are user-facing layers. Under them, the operating system manages work that applications cannot safely coordinate alone: scheduling processor time, isolating memory, controlling access to files and devices, and responding to hardware events.

The central split is user mode and kernel mode. Applications normally run in user mode with limited access to the machine. Kernel mode runs the trusted core of the operating system and selected drivers. A request that needs a protected resource crosses a defined boundary, often through a system call. This split contains ordinary application failures. It also means kernel code deserves unusual care: a kernel-mode failure can affect the whole system.