are the start of an extended capability list. Extended capabilities are very much like normal capabilities except that they can refer to any byte in the extended configuration space (by using 12 bits instead of eight), have a four-bit version number and a 16-bit capability ID. Extended capability IDs overlap with normal capability IDs, but there is no chance of confusion as they are in separate lists. Standardized registers The Device ID (DID) and Vendor ID (VID) registers identify the device (such as an IC), and are commonly called the PCI ID. The 16-bit vendor ID is allocated by the PCI-SIG. The 16-bit device ID is then assigned by the vendor. There is an inactive project to collect all known Vendor and Device IDs. (See the external links below.) The Status register is used to report which features are supported and whether certain kinds of errors have occurred. The Command register contains a bitmask of features that can be individually enabled and disabled. The Header Type register values determine the different layouts of remaining 48 bytes (64-16) of the header, depending on the function of the device. That is, Type 1 headers for Root Complex, switches, and bridges. Then Type 0 for endpoints. The Cache Line Size register must be programmed before the device is told it can use the memory-write-and-invalidate transaction. This should normally match the CPU's cache line size, but the correct setting is system dependent. This register does not apply to PCI Express. The Subsystem ID (SSID) and the Subsystem Vendor ID (SVID) differentiate specific model (such as an add-in card). While the Vendor ID is that of the chipset manufacturer, the Subsystem Vendor ID is that of the card manufacturer. The Subsystem ID is assigned by the subsystem vendor from the same number space as the Device ID. As an example, in the case of wireless network cards, the chip manufacturer might be Broadcom or Atheros, and the card manufacturer might be Netgear or D-Link. Generally, the Vendor ID–Device ID combination designates which driver the host should load in order to handle the device, as all cards with the same VID:DID combination can be handled by the same driver. The Subsystem Vendor ID–Subsystem ID combination identifies the card, which is the kind of information the driver may use to apply a minor card-specific change in its operation. Bus enumeration To address a PCI device, it must