Understanding the Linux Kernel

13.1. I/O Architecture

In order 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 nowadays to a personal computer. These data paths, which are denoted collectively as the bus, act as the primary communication channel inside the computer.

Several types of buses, such as the ISA, EISA, PCI, and MCA, are currently in use. In this section we'll discuss the functional characteristics common to all PC architectures, without giving details about a specific bus type.

In fact, what is commonly denoted as bus consists of three specialized buses:

Data bus

A group of lines that transfers data in parallel. The Pentium has a 64-bit-wide data bus.

Address bus

A group of lines that transmits an address in parallel. The Pentium has a 32-bit-wide address bus.

Control bus

A group of lines that transmits control information to the connected circuits. The Pentium makes use of control lines to specify, for instance, whether the bus is used to allow data transfers between a processor and the RAM or alternatively between a processor and an I/O device. Control lines also determine whether a read or a write transfer must be performed.

When the bus connects the CPU to an I/O device, it is called an I/O bus. In this case, Intel 80x86 microprocessors use 16 out of the 32 address lines to address I/O devices and 8, 16, or 32 out of the 64 data lines to transfer data. The I/O bus, in turn, is ...