PCI

PCI. The Peripheral Component Interconnect is a local computer bus standard for attaching hardware devices inside a computer. Developed by Intel and introduced in 1992, it became the dominant expansion bus for personal computers, supplanting older standards like ISA and VESA Local Bus. It provided a high-speed, reliable pathway for connecting components such as network cards, sound cards, and graphics adapters to the motherboard.

Overview

The creation of PCI was driven by the need for a faster, more flexible bus to keep pace with advancing CPU performance, notably from companies like Intel and AMD. It was designed as a general-purpose interconnect, independent of any specific processor architecture, which allowed for widespread adoption beyond just IBM-compatible systems, including workstations from Sun Microsystems and Apple Power Macintosh computers. The standard was managed by the PCI Special Interest Group, a consortium of industry leaders. A key innovation was its support for Plug and Play configuration, which automated resource assignment and greatly simplified the installation of new hardware compared to earlier buses.

Technical Specifications

The original PCI specification defined a 32-bit parallel bus clocked at 33 MHz, yielding a theoretical peak bandwidth of 133 MB/s. A 64-bit extension running at 66 MHz was later standardized, increasing bandwidth to 533 MB/s. The bus uses a shared parallel architecture where multiple devices connect to a common set of signal lines. Communication is handled through a transaction-based protocol, with a central bus arbiter on the motherboard chipset, such as those from Intel or VIA Technologies, managing access. Key features included burst mode data transfer and a sophisticated electrical specification that allowed for both 5-volt and 3.3-volt signaling, accommodating different device technologies.

Hardware Implementation

On a motherboard, PCI slots are typically white and positioned in parallel to the rear of the case. The physical connector is standardized, with notches to prevent insertion of incompatible cards. The bus requires a controller, which was integrated into the northbridge or memory controller hub of the platform's core logic chipset, like the Intel 430LX or later chipsets for the Pentium processor. This controller managed all transactions between the CPU, system memory, and PCI devices. Termination resistors were required on the bus to prevent signal reflections. Implementation also involved precise timing constraints and load limits to ensure signal integrity across multiple expansion slots.

Software and Drivers

System software interacts with PCI hardware through a standardized configuration space, a 256-byte region defined for each device that holds information like vendor ID, device ID, and Base Address Registers. This space is enumerated by the system BIOS or operating system during the boot process, a fundamental part of the Plug and Play system. Operating systems like Microsoft Windows, Linux, and FreeBSD include core subsystems to manage this enumeration and resource allocation. Device functionality is then provided by kernel-mode drivers, which are specific to each piece of hardware, such as a NVIDIA graphics driver or a Broadcom network driver, and interface with the OS's hardware abstraction layer.

Variants and Successors

Several specialized variants of PCI were developed for different form factors and markets. PCI-X, developed by IBM, Hewlett-Packard, and Compaq, doubled the bus clock and added enhancements for the server and workstation market. For mobile and embedded systems, Mini PCI and its successor PCI Express Mini Card provided a compact form factor. The most significant evolution was the development of PCI Express, a serial, point-to-point interconnect that began to replace conventional PCI in 2004. While AGP was a dedicated high-speed port for graphics that coexisted with PCI, both were ultimately superseded by PCI Express, which became the universal standard for internal expansion in modern systems from vendors like Dell, HP Inc., and Lenovo. Category:Computer buses Category:Computer hardware standards Category:Intel