Any four types of OS

1. Batch operating system

Jobs with similar needs are collected into a batch by an operator and run one after another without user interaction. Input is prepared on cards/tape. It gives good CPU use but long turnaround time and no interactivity. Example: payroll processing, IBM OS/360 batch mode.

2. Multiprogramming / time-sharing operating system

Several programs are kept in memory; when one waits for I/O, the CPU is given to another, so the CPU is always busy (multiprogramming). In time-sharing, the CPU is switched among users every few milliseconds so each user gets interactive response and feels that the machine is theirs. Examples: UNIX, Linux, Windows.

3. Real-time operating system (RTOS)

Used when results must be produced within a fixed time limit (deadline). In a hard real-time system missing a deadline is a failure (airbag, missile control, pacemaker); in a soft real-time system occasional misses are tolerable (video streaming). It uses priority-based scheduling and fast interrupt handling. Examples: VxWorks, QNX.

4. Distributed operating system

Manages a collection of independent computers connected by a network and makes them look like a single system. Resources and computation are shared across nodes, giving speed, reliability (failure of a node does not stop the system) and resource sharing. Examples: Amoeba, LOCUS.

Other types are multiprocessor (parallel), network and embedded/mobile operating systems.