An is system software that manages computer hardware/software resources and provides common services for programs.

Context: Definition and purpose of an OS

The OS provides interfaces that hide hardware access details so applications can run across different hardware without rewriting; this is via interfaces.

Context: Hardware abstraction via interfaces

Applications need to access hardware (such as memory allocation or input/output), so they use a to request OS functions.

Context: System calls as the intermediary

Limited hardware resources (CPU time and memory) cause the OS to prevent monopolization by allocating shares to each application; this is and isolation.

Context: Resource allocation and isolation

One job is blocked waiting for an I/O operation, which causes another job to use the CPU; this is enabled by (multiprogramming/time-sharing).

Context: Cause→effect: blocking I/O leads to CPU use by another job

Most OS kernels enforce two modes: user mode (restricted) and kernel mode (unrestricted); this is and protection.

Context: Kernel modes and protection

The kernel creates processes, assigns memory/resources, sets priorities, loads binaries, and starts execution; this is the execution model.

Context: Program execution model

Interrupts (also called exceptions/faults/signals/traps) let the OS efficiently react to events by changing CPU control flow away from the running program; this is -driven control flow.

Context: Interrupt-driven control flow

A technique that provides programs an illusion of nearly unlimited memory beyond physical RAM is .

Context: Virtual memory

Different OS classes target different needs, including embedded (small/simple) and real-time (timing guarantees) and hypervisors (virtual machines); these are by deployment and timing guarantees.

Context: OS types

Hard real-time requirement for exact timing causes design to often use minimal protection (for example, a library-like approach); this is a real-time design effect.

Context: Cause→effect: hard real-time leads to minimal protection

A runs virtual machines, where the VM emulates hardware like the real hardware the OS was designed to run on.

Context: Hypervisor definition

A operating system (libOS) approach provides typical OS services as libraries composed with a single application to form a unikernel.

Context: Library OS (libOS)

Context switching overhead and cache/pipeline effects cause library OS approaches to reduce overhead by avoiding protection-domain separation; this is enabled by approaches.

Context: Cause→effect: overhead leads to libOS-style reduction

A group of distinct networked computers, each potentially with its own OS and file system, possibly dispersed globally is a system.

Context: Distributed system vs multicomputer