1.6.9. Device Drivers

The kernel interacts with I/O devices by means of device drivers . Device drivers are included in the kernel and consist of data structures and functions that control one or more devices, such as hard disks, keyboards, mouses, monitors, network interfaces, and devices connected to an SCSI bus. Each driver interacts with the remaining part of the kernel (even with other drivers) through a specific interface. This approach has the following advantages:

• Device-specific code can be encapsulated in a specific module.
• Vendors can add new devices without knowing the kernel source code; only the interface specifications must be known.
• The kernel deals with all devices in a uniform way and accesses them through the same interface.
• It is possible to write a device driver as a module that can be dynamically loaded in the kernel without requiring the system to be rebooted. It is also possible to dynamically unload a module that is no longer needed, therefore minimizing the size of the kernel image stored in RAM.

Figure 1-4 illustrates how device drivers interface with the rest of the kernel and with the processes.

Figure 1-4. Device driver interface

Some user programs (P) wish to operate on hardware devices. They make requests to the kernel using the usual file-related system calls and the device files normally found in the /dev directory. Actually, the device files are the user-visible portion of the device driver interface. Each device file refers to a specific device driver, which is invoked by the kernel to perform the requested operation on the hardware component.

At the time Unix was introduced, graphical terminals were uncommon and expensive, so only alphanumeric terminals were handled directly by Unix kernels. When graphical terminals became widespread, ad hoc applications such as the X Window System were introduced that ran as standard processes and accessed the I/O ports of the graphics interface and the RAM video area directly. Some recent Unix kernels, such as Linux 2.6, provide an abstraction for the frame buffer of the graphic card and allow application software to access them without needing to know anything about the I/O ports of the graphics interface (see the section "Levels of Kernel Support" in Chapter 13.)