Operating systems

An operating system is a collection of programs which manages the resources of a computer system, and makes it usable. These resources include:

• Processor usage: Any sensible computer system can run several programs, and support several users, apparently at the same time, even if it only has one processor which executes its instructions sequentially. This is done by a neat trick called multiprogramming, where the processor rushes around spending a few milliseconds on each program and then moving on to the next. This is all done under the control of the operating system and should not be apparent to the user.
• Memory: Different programs running at the same time must be made to use different parts of the memory, so they don't interfere with each other. The amount of memory used by a program typically varies over time - the operating system sorts all this out.
• Permanent storage: many operations on files must be supported, as we will see shortly.
• Peripherals: As an example, for each printer the operating system must maintain a queue of things to be printed so that documents are printed one at a time, in order.
• Networks: Nowadays, computer systems consist of many separate machines connected by networks. This gives the operating system lots of extra things to do, for instance making all the above resources on one machine available on others.

Operating systems are typically very large and complex, because computer systems are large and complex. For instance, our departmental network contains around 150 PCs, 250 Sun workstations and various other computers, 10 printers, 1000 users, and network connections to the rest of the world. The workstation on which this was originally written had about 35 programs running (although as I update it, I have over 80 programs running!), the majority of which are the operating system keeping track of what's going on. If the machines sometimes seem slow, this is why!

Note: An operating system is not to be confused with a system manager, who is the overworked, underpaid, human who does the many tasks which the operating system can't do unaided, such as telling it who is allowed to use which parts of the system, setting up accounts for new users, installing new software, etc.