Understanding the Linux Kernel, 3rd Edition

I/O Architecture

To make a computer work properly, data paths must be provided that let information flow between CPU(s), RAM, and the score of I/O devices that can be connected to a personal computer. These data paths, which are denoted as the buses , act as the primary communication channels inside the computer.

Any computer has a system bus that connects most of the internal hardware devices. A typical system bus is the PCI (Peripheral Component Interconnect) bus. Several other types of buses, such as ISA, EISA, MCA, SCSI, and USB, are currently in use. Typically, the same computer includes several buses of different types, linked together by hardware devices called bridges . Two high-speed buses are dedicated to the data transfers to and from the memory chips: the frontside bus connects the CPUs to the RAM controller, while the backside bus connects the CPUs directly to the external hardware cache. The host bridge links together the system bus and the frontside bus.

Any I/O device is hosted by one, and only one, bus. The bus type affects the internal design of the I/O device, as well as how the device has to be handled by the kernel. In this section, we discuss the functional characteristics common to all PC architectures, without giving details about a specific bus type.

The data path that connects a CPU to an I/O device is generically called an I/O bus. The 80 × 86 microprocessors use 16 of their address pins to address I/O devices and 8, 16, or 32 of their data pins to transfer ...