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Redesigning the I/O Model

You might ask why Apple chose to redesign the I/O model. At first glance, it might seem that reusing the model from Mac OS 9 or FreeBSD would have been an easier choice. There are several reasons for the decision, however.

Neither the Mac OS 9 driver model nor the FreeBSD model offered a feature set rich enough to meet the needs of Mac OS X. The underlying operating-system technology of Mac OS X is very different from that of Mac OS 9. The Mac OS X kernel is significantly more advanced than the previous Mac OS system architecture; Mac OS X needs to handle memory protection, preemption, multiprocessing, and other features not present (or substantially less pervasive) in previous versions of the Mac OS.

Although FreeBSD supports these features, the BSD driver model did not offer the automatic configuration, stacking, power management, or dynamic device-loading features required in a modern, consumer-oriented operating system.

By redesigning the I/O architecture, Apple’s engineers can take best advantage of the operating-system features in Mac OS X. For example, virtual memory (VM) is not a fundamental part of the operating system in Mac OS 9. Thus, every driver writer must know about (and write for) VM. This has presented certain complications for developers. In contrast, Mac OS X has simplified driver interaction with VM. VM capability is inherent in the Mac OS X operating system and cannot be turned off by the user. Thus, VM capabilities can be abstracted into the I/O Kit, and the code for handling VM need not be written for every driver.

Mac OS X offers an unprecedented opportunity to reuse code. In Mac OS 9, for example, all software development kits (SDKs) were independent of each other, duplicating functionality between them. In Mac OS X, the I/O Kit is delivered as part of the basic developer tools, and code is shared among its various parts.

In contrast with traditional I/O models, the reusable code model provided by the I/O Kit can decrease your development work substantially. In porting drivers from Mac OS 9, for example, the Mac OS X counterparts have been up to 75% smaller.

In general, all hardware support is provided directly by I/O Kit entities. One exception to this rule is imaging devices such as printers, scanners, and digital cameras (although these do make some use of I/O Kit functionality). Specifically, although communication with these devices is handled by the I/O Kit (for instance, under the FireWire or USB families), support for particular device characteristics is handled by user-space code (see “For More Information” for further discussion). If you need to support imaging devices, you should employ the appropriate imaging software development kit (SDK).

The I/O Kit attempts to represent, in software, the same hierarchy that exists in hardware. Some things are difficult to abstract, however. When the hardware hierarchy is difficult to represent (for example, if layering violations occur), then the I/O Kit abstractions provide less help for writing drivers.

In addition, all drivers exist to drive hardware; all hardware is different. Even with the reusable model provided by the I/O Kit, you still need to be aware of any hardware quirks that may impact a higher-level view of the device. The code to support those quirks still needs to be unique from driver to driver.

Although most developers should be able to take full advantage of I/O Kit device families (see “Families”), there will occasionally be some who cannot. Even those developers should be able to make use of parts of the I/O Kit, however. In any case, the source code is always available. You can replace functionality and modify the classes yourself if you need to do so.

In designing the I/O Kit, one goal has been to make developers’ lives easier. Unfortunately, it is not possible to make all developers’ lives uniformly easy. Therefore, a second goal of the I/O Kit design is to meet the needs of the majority of developers, without getting in the way of the minority who need lower level access to the hardware.

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