Chapter 3. InputOutput

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3. Input/Output

One of the main functions of an operating system is to control all the computer's I/O (Input/Output) devices. It must issue commands to the devices, catch interrupts, and handle errors. It should also provide an interface between the devices and the rest of the system that is simple and easy to use. To the extent possible, the interface should be the same for all devices (device independence). The I/O code represents a significant fraction of the total operating system. Thus to really understand what an operating system does, you have to understand how I/O works. How the operating system manages I/O is the main subject of this chapter.

This chapter is organized as follows. First we will look at some of the principles of how I/O hardware is organized. Then we will look at I/O software in general. I/O software can be structured in layers, with each layer having a well-defined task to perform. We will look at these layers to see what they do and how they fit together.

After that comes a section on deadlocks. We will define deadlocks precisely, show how they are caused, give two models for analyzing them, and discuss some algorithms for preventing their occurrence.

Then we will move on to look at MINIX 3 We will start with a bird's-eye view of I/O in MINIX 3, including interrupts, device drivers, device-dependent I/O and device-independent I/O. Following that introduction, we will look at several I/O devices in detail: disks, keyboards, and displays. For each device we will look at its hardware and software.