PCI

(1) (Payment Card Industry) See PCI DSS.

(2) (Peripheral Component Interconnect) The most widely used I/O bus (peripheral bus). Used in computers of all sizes, it provides a shared data path between the CPU and peripheral controllers, such as network, display, SCSI and RAID cards. However, with so many controller circuits built into the motherboard, the need for vacant PCI slots in a PC has diminished considerably.

Designed by Intel, Compaq and Digital, the PCI bus first appeared in PCs in 1993 and co-existed with the ISA bus for many years. Today, most computers have only PCI slots along with one AGP or one PCI Express slot for the display adapter.

PCI Slots
PCI runs at 33 MHz or 66 MHz and supports 32 and 64-bit data paths and bus mastering. There are generally three or four slots on the motherboard, and the quantity is based on 10 electrical loads that deal with inductance and capacitance. The PCI chipset uses three, leaving seven for peripheral controllers. A controller on the motherboard uses one load; a plug-in card uses 1.5 loads. A "PCI bridge" connects PCI buses together for more slots.

PCI Shares Interrupts; ISA Did Not
On a PC, there is a limited number of hardware interrupts (IRQs), and the PCI bus is designed to share them. Thus, on a PCI-only PC, there is never an IRQ conflict as there was on earlier machines that used the ISA bus. ISA cards required an assigned IRQ that was fixed to that peripheral device.

PCs with both ISA and PCI buses were made for several years. If there was only one IRQ remaining after the rest were reserved for ISA cards, all PCI devices could share it. In such a dual bus PC, all reserved IRQs are registered in the PC's BIOS setup. On startup, PCI reads the setup memory and configures all PCI cards automatically. For a comparison of PCI technologies, see PCI-SIG. See PCI Express, PCI-X, Concurrent PCI, CompactPCI, PXI, Mini PCI, PC data buses, PICMG and Sebring ring.

How PCI Is Connected

This illustration shows how the CPU, memory and peripherals are interconnected in a PC.

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PCI - Peripheral Component Interconnect