Functions of an Operating System

1One processor, more than a hundred programs

Open Task Manager on a school laptop that is only showing a browser and a word processor, and you will usually count well over a hundred running processes. The laptop may have only four processor cores and a few gigabytes of memory. Something must decide which program uses the processor next, where in memory each one lives, and who gets the printer when two people press Print at once. That is the operating system's daily work. This lesson takes its four main jobs in turn: managing the processor, the memory, the input and output devices, and the applications.

21. Processor (CPU) management

A processor core carries out one process at a time, so the OS shares it out. Every process moves between a few states:

Figure 1. Process states. A ready process is given the CPU; when its time slice ends it goes back to the queue. If it has to wait for a device, such as the disk or the keyboard, it waits without using the CPU, and another process runs.

One simple scheduling method is round robin: the ready processes stand in a queue, each gets one time slice, and any process that is not finished goes to the back of the queue.

32. Memory management

Every running process needs a share of main memory (RAM). The OS:

• allocates memory to a program when it starts and frees it when the program closes;
• protects each process's memory, so one program cannot read or damage another's data; a program that tries is stopped;
• keeps track of which parts of RAM are in use and which are free;
• uses virtual memory when RAM runs out.

43. Input and output (device) management

• Device drivers. Each printer, camera or keyboard model works differently. The OS talks to every device through a small program called a driver. Windows finds and installs most drivers by itself when you plug a device in: this is called plug and play.
• Buffering. Devices work at very different speeds. The OS holds data in a buffer, an area of memory, while it moves between them. Keys typed while the computer is busy wait in the keyboard buffer; a video loads a few seconds ahead into a buffer so it plays smoothly.
• Spooling. A printer is slow and can print only one job at a time. The OS saves each print job to a queue on the drive (the print spooler) and sends the jobs to the printer one by one, so you can keep working while it prints.
• Interrupt handling. When a device finishes a job or needs attention, for example a printer out of paper, it sends an interrupt and the OS deals with it.

54. Application management

The OS loads each program from the drive into memory and starts it, closes it when you exit, and stops it if it stops responding (End task). It also gives every application the same ready-made services through an application programming interface (API): the Save dialog, the Print dialog, copying and pasting, and access to the network all come from the OS, which is why they look alike in different programs. Finally, it controls permissions: Windows and Android ask you before a new app may use the camera, the microphone or your location.

6Other functions

The OS also manages files (folders, names and the file system on each drive, which you used in S1), security (user accounts, passwords and privileges, from S2) and the user interface (Lesson 1).

7Hands-on: watch the operating system at work

1. Press Ctrl + Shift + Esc to open Task Manager. If it opens as a small list, click More details.
2. On Processes, compare the number of Apps with the number of Background processes.
3. Click the Memory column heading to sort the processes, largest first. Note the top three.
4. Open Performance and click CPU. Note the Utilization, the number of Processes and the number of Cores.
5. Click Memory. Note In use and Available. Now open three more programs and watch In use rise; close them and watch it fall. That is memory allocation and release.
6. Close Task Manager. Then right-click Start and choose Device Manager. Expand Keyboards and Display adapters: each entry is a device the OS controls through a driver.
7. Print a one-page document, then quickly open Settings → Bluetooth & devices → Printers & scanners, choose the printer and click Open print queue. The waiting job is the spooler at work. (On Windows 10 the path starts Settings → Devices.)