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I/O Kit Overview

Those of you who are already familiar with writing device drivers for Mac OS 9 or for BSD will discover that writing drivers for Mac OS X requires some new ways of thinking. In creating Mac OS X, Apple has completely redesigned the Macintosh I/O architecture, providing a framework for simplified driver development that supports many categories of devices. This framework is called the I/O Kit.

From a programming perspective, the I/O Kit provides an abstract view of the system hardware to the upper layers of Mac OS X. The I/O Kit uses an object-oriented programming model, implemented in a restricted subset of C++ to promote increased code reuse.

By starting with properly designed base classes, you gain a head start in writing a new driver; with much of the driver code already written, you need only to fill in the specific code that makes your driver different. For example, all SCSI controllers deliver a fairly standard set of commands to a device, but do so via different low-level mechanisms. By properly using object-oriented programming methodology, a SCSI driver can implement those low-level transport portions without reimplementing the higher level SCSI protocol code. Similar opportunities for code reuse can be found in most types of drivers.

Part of the philosophy of the I/O Kit is to make the design completely open. Rather than hiding parts of the API in an attempt to protect developers from themselves, all of the I/O Kit source is available as part of Darwin. You can use the source code as an aid to designing (and debugging) new drivers.

Instead of hiding the interfaces, Apple’s designers have chosen to lead by example. Sample code and classes show the recommended (easy) way to write a driver. However, you are not prevented from doing things the hard way (or the wrong way). Instead, attention has been concentrated on making the “best” ways easy to follow.

Contents:

Redesigning the I/O Model
I/O Kit Architecture
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