Operating System Design

15

An Example Device Driver

It’s hard to find a good driver these days, one with

character and style.

— Unknown

15.1 Introduction

Chapters in this section of the text explore the general structure of an I/O system,

including interrupt processing and real-time clock management. The previous chapter

presents the organization of the I/O subsystem, a set of abstract I/O operations, and an

efficient implementation using a device switch table.

This chapter continues the exploration of I/O. The chapter explains how a driver

can define an I/O service at a high level of abstraction that is independent of the under-

lying hardware. The chapter also elaborates on the conceptual division of a device

driver into upper and lower halves by explaining how the two halves share data struc-

tures, such as buffers, and how they communicate. Finally, the chapter shows the de-

tails of a particular example: a driver for an asynchronous character-oriented serial de-

vice.

15.2 The Tty Abstraction

Xinu uses the name tty to refer to the abstraction of an interface used with

character-oriented serial devices such as a serial interface or a keyboard and text win-

dow.† In broad terms, a tty device supports two-way communication: a process can

send characters to the output side and/or receive characters from the input side.

Although the underlying serial hardware mechanism operates the input and output in-

†The name tty is taken from early Unix systems that used an ASCII Teletype device that consisted of a

keyboard and an associated printer mechanism.

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268 An Example Device Driver Chap. 15

dependently, the tty abstraction allows the two to be connected. For example, our tty

driver supports character echo, which means that the input side of the driver can be

configured to transmit a copy of each incoming character to the output. Echo is espe-

cially important when a user is typing on a keyboard and expects to see characters

displayed on a screen as keys are pressed.

The tty abstraction illustrates an important feature of many device drivers: multiple

modes that can be selected at run-time. In our tty driver, the three modes focus on how

the driver processes incoming characters before delivering them to an application. Fig-

ure 15.1 summarizes the three modes and gives their characteristics.

Mode Meaning

The driver delivers each incoming character as it arrives

raw without echoing the character, buffering a line of text,

performing translation, or controlling the output flow

The driver buffers input, echoes characters in a readable

cooked form, honors backspace and line kill, allows type-ahead,

handles flow control, and delivers an entire line of text

The driver handles character translation, echoing, and

cbreak flow control, but instead of buffering an entire line of text,

the driver delivers each incoming characters as it arrives

Figure 15.1 Three modes supported by the tty abstraction.

Cooked mode is intended to handle interactive keyboard input. Each time it re-

ceives a character, the driver echoes the character (i.e., transmits a copy of the character

to the output), which allows a user to see characters as they are typed. Echo is not

mandatory. Instead, the driver has a parameter to control character echoing, which

means an application can turn off echo to prompt for a password. Cooked mode sup-

ports line buffering, which means that the driver collects all characters of a line before

delivering them to a reading process. Because the tty driver performs character echo

and other functions at interrupt time, a user can type ahead, even if no application is

reading characters (e.g., a user can type the next command while the current command

is running). The chief advantage of line buffering arises from the ability to edit the

line, either by backspacing or typing a special character that erases the entire line and

allows the user to begin entering the line again.

Cooked mode provides two additional functions. First, it handles output flow con-

trol, allowing a user to temporarily stop and later restart output. When flow control is

enabled, typing control-s stops output and typing control-q restarts output. Second,

cooked mode handles input mapping. In particular, some computers or applications use

a two-character sequence of carriage return (cr) and linefeed (lf) to terminate a line of