KarbosGuide.com. Module 2c.3

About the PCI bus

The contents:

  • Introducing the PCI bus
  • The internal and external face.
  • The future design
  • NGIO
  • Next page
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  • Introducing the PCI bus

    The PCI is the high speed bus of the 1990s. PCI stands for Peripheral Component Interconnect. This bus is made by Intel. It is used today in all PCs and other computers for connecting adapters, such as network-controllers, graphics cards, sound cards etc.

    Some graphics cards however use the AGP-bus, which is a separate bus only intended for graphics.

    The PCI bus is the central I/O bus, which you find in all PCs!

    A 32 bit bus

    The PCI is actually 32 bit wide, but in practice it functions like a 64 bit bus. Running at 33 MHz, it has a maximum transmission capacity of 132 MBps.

    According to the specifications - not in practice, it can have up to 8 units with a speed up to 200 MHz. The bus is processor independent. Therefore, it can be used with all 32 or 64 bit processors, and it is also found in other computers than PCs.

    The PCI bus is compatible with the ISA bus in that it can react on ISA bus signals, create the same IRQs, etc.

    Buffering and PnP

    The PCI bus is buffered in relation to the CPU and the peripheral components. This means, that the CPU can deliver its data to the buffer, and then proceed with other tasks. The bus handles the further transmission in its own tempo. Conversely, the PCI adapters can also transmit data to the buffer, regardless of whether the CPU is free to process them. They are placed in a queue, until the system bus can forward them to the CPU. Under optimal conditions, the PCI bus transmits 32 bits per clock tick. Sometimes, it requires two clock ticks.

    Because of this, the peripheral PCI units operate asynchronous . Therefore, the PCI (contrary to the VL bus) is not a local bus in a strict sense. Finally, the PCI bus is intelligent relative to the peripheral components, in that Plug and Play is included in the PCI specifications. All adapter cards for the PCI configure themselves. Plug and Play is abbreviated PnP.

    PCI with two faces

    On modern system boards, the PCI bus (like ISA) has two "faces:"

  • Internal PCI bus, which connects to EIDE channels on the motherboard.
  • The PCI expansion bus, which typically has 3-4 slots for PCI adapters.

    The PCI bus is continuously being developed further. There is a PCI Special Interest Group, consisting of the most significant companies (Intel, IBM, Apple, and others), which coordinate and standardize the development.

    Soon we shall see PCI with a higher bus speed (66 MHz) and greater width (64 bit). However alternative buses are also marketed. An example is the high speed AGP video bus (Accelerated Graphics Port) and the FireWire Bus. AGP is fundamentally a 66 MHz PCI bus (version 2.1) which has been enhanced with other technologies making it suitable for the graphics system.


    Another new initiative is the so-called PCI-X (also called "Project One" and Future I/O). Companies like IBM, Mylex, 3COM, Adaptec, HP and Compaq want to launch a special high speed server version of the PCI bus. This new bus (also mentioned as PCIX) allows a bandwidth of up to 1 GB per second (with a 64 bit bus running at 133 MHz). Intel is not cooperating on this project, and neither is Dell. It is going to be interesting to follow.

    Intel's NGIO (Next-Generation I/O)

    NGIO server architecture is another initiative by the companies Dell Computer, Hitachi, NEC, Siemens, Sun Microsystems and Intel to produce a new architecture for I/O on servers. This is clearly an answer to the Project One mentioned above.

    FIO to merge with NGIO

    On August 31, 1999 seven of the leading companies (Compaq, Dell, Hewlett-Packard Company, IBM, Intel, Microsoft, Sun) announced the intent to merge the best ideas of the Future I/O (FIO) and Next Generation I/O (NGIO). The new open input/output architecture will find use in servers. The bandwidth will be up to 6 GByte/sec.

    The new standard NGIO will hardly go into production before 2001.

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    Learn more

    Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.

    Read module 5b about AGP and module 5c about Firewire.

    Read about chip sets on the motherboard in module 2d

    Read more about RAM in module 2e

    Read Module 4b about hard disks.

    Read Module 4c about optical media (CDROM and DVD).

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.

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