Bus Device Function is a notation standard for identifying the PCI connectors on a computer. PCI stands for Peripheral Component Interconnect. The word “peripheral” in the name does not refer to external peripherals like printers and modems; it refers to internal devices like video cards and hard discs. A bus is a series of wires fused onto the motherboard and it carries bits of data in parallel. That means that all the bits of a chunk of data travel simultaneously. PCI buses come in 64-bit and 32-bit configurations. The 64-bit bus contains 64 wires and the 32-bit bus has 32 wires.
The Bus Device Function identifier for PCI buses is often referred to as BDF. The standard format of BDF contains three elements written with the following separators: <bus>:<device>.<function>. The bus code is a hexadecimal number left padded with zeros to two or four digits. The device code is also known as the slot number. This is a two digit hexadecimal number left padded with a zero. The function code is a single hexadecimal digit.
The BDF notation creates an address in a hierarchical format. The device number is not unique throughout the system, only on its specific bus. Similarly, the function number is only a unique identifier within the context of its device and bus. Thus 00:02.5 addresses function 5 on device 02 of bus 00. 01:02.5 points to a different device and function because it is on a different bus.
Some computers are divided into different zones inside the case. These zones are called domains. The existence of domains caused the BDF notation to be extended to include a domain number. The domain is a four-digit hexadecimal number left-padded with zeros and is attached to the front of the BDF address. For example, you may see a PCI device code like 0000:00:02.5. The principle of hierarchy also applies to the domain code. This means that 0000:00:02.5 and 0001:00:02.5 refer to two different devices.
The BDF notation contains shortcuts and wildcards that enable you to specify several functions at once. Rather than listing three consecutive functions on the same device, such as 00:02.0, 00:02.1 and 00:02.2, you can use range notation -- 00:02:0-2. The wildcard “” identifies all functions on a device -- 00:02..
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