PCI Configuration Headers

 

  
Figure: The PCI Configuration Header

Every PCI device in the system, including the PCI-PCI bridges has a configuration data structure that is somewhere in the PCI configuration address space. The PCI Configuration header allows the system to identify and control the device. Exactly where the header is in the PCI Configuration address space depends on where in the PCI topology that device is. For example, a PCI video card plugged into one PCI slot on the PC motherboard will have its configuration header at one location and if it is plugged into another PCI slot then its header will appear in another location in PCI Configuration memory. This does not matter, for wherever the PCI devices and bridges are the system will find and configure them using the status and configuration registers in their configuration headers. Typically systems are designed so that every PCI slot has it's PCI Configuration Header in an offset that is related to its slot on the board. So, for example, the first slot on the board might have its PCI Configuration at offset 0 and the second slot at offset 256 (all headers are the same length, 256 bytes) and so on. A system specific hardware mechanism is defined so that the PCI configuration can attempt to examine all possible PCI Configuration Headers for a given PCI bus and know which devices are present and which devices are absent simply by trying to read one of the fields in the header (usually the Vendor Identification field) and getting some sort of error. The [3, PCI Local Bus Specification,] describes one possible error message as returning 0xFFFFFFFF when attempting to read the Vendor Identification and Device Identification fields for an empty PCI slot.

Figure  shows the layout of the 256 byte PCI configuration header. It contains the following fields:

Vendor Identification

A unique number describing the originator of the PCI device. Digital's PCI Vendor Identification is 0x1011 and Intel's is 0x8086.

Device Identification

A unique number describing the device itself. For example, Digital's 21141 fast ethernet device has a device identification of 0x0009.

Status

This field gives the status of the device with the meaning of the bits of this field set by the standard. [3, PCI Local Bus Specification,].

Command

By writing to this field the system controls the device, for example allowing the device to access PCI I/O memory,

Class Code

This identifies the type of device that this is. There are standard classes for every sort of device; video, scsi and so on. The class code for SCSI is 0x0100.

Base Address Registers

These registers are used to determine and allocate the type, amount and location of PCI I/O and PCI memory space that the device can use.

Interrupt Pin

Four of the physical pins on the PCI card carry interrupts from the card to the PCI bus. The standard labels these as A, B, C and D. The Interrupt Pin field describes which of these pins this PCI device uses. Generally it is hardwired for a pariticular device. That is, every time the system boots the device uses the same interrupt pin. This information allows the interrupt handling subsystem to manage interrupts from this device,

Interrupt Line

  The Interrupt Line field of the device's PCI Configuration header is used to pass an interrupt handle between the PCI initialisation code, the device's driver and Linux's interrupt handling subsystem. The number written there is meaningless to the the device driver but it allows the interrupt handler to correctly route an interrupt from the PCI device to the correct device driver's interrupt handling code within the Linux operating system. See Chapter  on page  for details on how Linux handles interrupts.