How PCI Works

Adding a PCI Device

This motherboard has four PCI slots.
This motherboard has four PCI slots.

Let's say that you have just added a new PCI-based sound card to your Windows XP computer. Here's an example of how it would work.

  1. You open up your computer's case and plug the sound card into an empty PCI slot on the motherboard.
  2. You close the computer's case and power up the computer.
  3. The system BIOS initiates the PnP BIOS.
  4. The PnP BIOS scans the PCI bus for hardware. It does this by sending out a signal to any device connected to the bus, asking the device who it is.
  5. The sound card responds by identifying itself. The device ID is sent back across the bus to the BIOS.
  6. The PnP BIOS checks the ESCD to see if the configuration data for the sound card is already present. Since the sound card was just installed, there is no existing ESCD record for it.
  7. The PnP BIOS assigns IRQ, DMA, memory address and I/O settings to the sound card and saves the data in the ESCD.
  8. Windows XP boots up. It checks the ESCD and the PCI bus. The operating system detects that the sound card is a new device and displays a small window telling you that Windows has found new hardware and is determining what it is.
  9. In many cases, Windows XP will identify the device, find and load the necessary drivers, and you'll be ready to go. If not, the "Found New Hardware Wizard" will open up. This will direct you to install drivers off of the disc that came with the sound card.
  10. Once the driver is installed, the device should be ready for use. Some devices may require that you restart the computer before you can use them. In our example, the sound card is immediately ready for use.
  11. You want to capture some audio from an external tape deck that you have plugged into the sound card. You set up the recording software that came with the sound card and begin to record.
  12. The audio comes into the sound card via an external audio connector. The sound card converts the analog signal to a digital signal.
  13. The digital audio data from the sound card is carried across the PCI bus to the bus controller. The controller determines which device on the PCI device has priority to send data to the CPU. It also checks to see if data is going directly to the CPU or to system memory.
  14. Since the sound card is in record mode, the bus controller assigns a high priority to the data coming from it and sends the sound card's data over the bus bridge to the system bus.
  15. The system bus saves the data in system memory. Once the recording is complete, you can decide whether the data from the sound card is saved to a hard drive or retained in memory for additional processing.