1.4. Operating Systems

[Page 10]

The operating system ( OS ) is the most important program that runs on a computer to manage and control its activities. You are probably using Windows (98, NT, 2000, XP, or ME), Mac OS, or Linux. Windows is currently the most popular PC operating system. Application programs, such as a Web browser or a word processor, cannot run without an operating system. The interrelationship of hardware, operating system, application software, and the user is shown in Figure 1.6.

Figure 1.6. The operating system is the software that controls and manages the system.

The major tasks of the operating systems are:

• Controlling and monitoring system activities

• Allocating and assigning system resources

• Scheduling operations

1.4.1. Controlling and Monitoring System Activities

Operating systems are responsible for security, ensuring that unauthorized users do not access the system. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the monitor, keeping track of files and directories on the disk, and controlling peripheral devices, such as disk drives and printers. Operating systems also make sure that different programs and users running at the same time do not interfere with one other.

1.4.2. Allocating and Assigning System Resources

The OS is responsible for determining what computer resources a program needs (e.g., CPU, memory, disks, input and output devices) and for allocating and assigning them to run the program.

1.4.3. Scheduling Operations

The OS is responsible for scheduling programs to use the system resources efficiently . Many of today's operating systems support such techniques as multiprogramming , multithreading , or multiprocessing to increase system performance.

Multiprogramming allows multiple programs to run simultaneously by sharing the CPU. The CPU is much faster than the other components . As a result, it is idle most of the time; for example, while waiting for data to be transferred from the disk or from other sources. A multiprogramming OS takes advantage of this by allowing multiple programs to use the CPU when it would otherwise be idle. For example, you may use a word processor to edit a file while the Web browser is downloading a file at the same time.

---

[Page 11]

Multithreading allows concurrency within a program, so that its subunits can run at the same time. For example, a word-processing program allows users to simultaneously edit text and save it to a file. In this example, editing and saving are two tasks within the same application. These two tasks may run on separate threads concurrently.

Multiprocessing , or parallel processing, uses two or more processors together to perform a task. It is like a surgical operation where several doctors work together on one patient.