An operating system is, simply speaking, the software that operates the hardware on behalf of the user(s). It is that software which makes the hardware usable to the humans. Among the important functions of an operating system are that it:

• defines the user interface;
• allows sharing of resources among users, while preserving privacy;
• schedules resources;
• facilitates I/O functions;
• provides routines for applications to use peripherals.

One of the oldest views of the operating system is to consider it as a resource manager that doles out rations of scarce resources to competing processes. Although many resources, such as memory sizes and CPU speeds, have grown exponentially since this view was first developed, it remains current because the demands on these resources have more than kept pace with their growth.

It is common practice to speak of an operating system kernel, which is the...