Purposes of an Operating System (OS)

Computer Science A2 · System Software. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The operating system shares the processor, memory and devices so that none is left idle, and it hides the hardware behind a user interface: the user clicks Save and the OS finds the disk blocks.

A process is a program being executed. With multi-tasking, several processes seem to run at once because the processor is switched between them quickly. Each process is running, ready (waiting for the processor) or blocked (waiting for input or output). A scheduling routine chooses which ready process runs next; the kernel does the switch when an interrupt arrives.

Virtual memory uses part of the disk as extra memory, so programs larger than the free RAM can run.

Rules to remember

State changes: ready → running (chosen by the scheduler); running → ready (time slice ends); running → blocked (waits for I/O); blocked → ready (I/O done). Blocked → running is not possible.
Round robin: each process gets a fixed time slice in turn, then rejoins the end of the ready queue. Fair, with a good response time.
First come first served: processes run to completion in order of arrival; simple, but a long process holds up the others.
Shortest job first: the process with the shortest total time runs next and is not interrupted; shortest remaining time is the same idea but a new, shorter process can take the processor. Long processes may wait a very long time.
Paging: memory is divided into fixed-size blocks; pages (logical) are loaded into page frames (physical), and a page table maps one to the other.
Segmentation: memory is divided into variable-size blocks that match logical parts of the program.
Page fault: the page needed is not in RAM, so it is brought in from disk and another page may be replaced (e.g. first in first out, or least recently used). Disk thrashing: so many pages are swapped that more time goes on swapping than on running processes.

Common mistake

Students confuse ready and blocked. A ready process could run now and is only waiting for the processor; a blocked process cannot run even if the processor is free, because it is waiting for an event such as input.

Worked example

Processes P1, P2 and P3 need 5, 3 and 1 units of processor time. All arrive at time 0 in that order. Round robin is used with a time slice of 2 units. At what time does each process finish?

P1 runs 0 to 2 (3 left), P2 runs 2 to 4 (1 left), P3 runs 4 to 5 and finishes.
P1 runs 5 to 7 (1 left), P2 runs 7 to 8 and finishes.
P1 runs 8 to 9 and finishes. Check: 5 + 3 + 1 = 9 units in total.

Answer: P3 finishes at time 5, P2 at time 8 and P1 at time 9.

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