The most important program in a computer is an operating system. A user cannot run an application program on the computer without an operating system. It’s a program designed to run other programs. Operating systems do basic works like identifying input from keyboard, sending output to monitor, managing files, and controlling peripheral devices such as printers and external drives.

There are five types of operating systems:

1. Multi-user operating system: This type of operating system let two or more users to run programs at the same time. Some operating systems allow hundreds or even thousands of users to run programs simultaneously.

2. Multitasking operating system: This type of operating system allows more than one programs to run at the same time.

3. Multiprocessing operating system: This type of operating system let a program run on more than one CPU. This is very useful in work places like schools and companies.

4. Multithreading operating system: This type of operating system can allow different parts of a single program to run simultaneously.

5. Real-time operating system: This type of operating system is made to respond the computer input instantly. DOS and UNIX are not real-time operating system because they may require seconds or minutes to respond to computer input.

After we load the operating system into the computer, it will manage all application programs in a computer. By making requests for services through a defined interface, these application programs use operating system to perform tasks.

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