EXtensible Host Controller Interface
eXtensible Host Controller Interface (xHCI) defines a register-level description of a Host Controller for Universal Serial bus (USB), which is capable of interfacing to USB 1.x, 2.0, and 3.0 compatible devices.
Technical Details
The xHCI controller communicates with the operating system using registers that can be located by searching the PCI configuration space for a device with a specific Class ID, Subclass ID, and Interface number. All xHCI controllers will have a Class ID of 0x0C, a Sublcass ID of 0x03, and an Interface value of 0x30. The configuration space for this device will contain two Base Address Registers: BAR0 and BAR1. These two 32-bit address fields combine to create a single 64-bit address that points to the base address of the memory mapped registers for the controller.
Capability Registers
The capability registers are located at the address specified by the PCI configuration space.
| Offset (Hex) | Name | Description |
|---|---|---|
| 00 | CAPLENGTH | Capability Register Length |
| 02 | HCIVERSION | Interface Version Number |
| 04 | HCSPARAMS1 | Structural Parameters 1 |
| 08 | HCSPARAMS2 | Structural Parameters 2 |
| 0C | HCSPARAMS3 | Structural Parameters 3 |
| 10 | HCCPARAMS | Capability Parameters |
| 14 | DBOFF | Doorbell Offset |
| 18 | RTSOFF | Runtime Registers Space Offset |
Operational Registers
The operational registers are located after the capability registers in memory, and can be found by adding the CAPLENGTH field to the base address specified in the PCI configuration space.
| Offset (Hex) | Name | Description |
|---|---|---|
| 00 | USBCMD | USB Command |
| 04 | USBSTS | USB Status |
| 08 | PAGESIZE | Page Size |
| 14 | DNCTRL | Device Notification Control |
| 18 | CRCR | Command Ring Control |
| 30 | DCBAAP | Device Context Base Address Array Pointer |
| 38 | CONFIG | Configure |
Port Registers
Runtime Registers
The runtime registers are located after the operational registers in memory, and can be found by adding the RTSOFF field to the base address specified in the PCI configuration space.
| Offset (Hex) | Name | Description |
|---|---|---|
| 00 | CAPLENGTH | Capability Register Length |
| 02 | HCIVERSION | Interface Version Number |
| 04 | HCSPARAMS1 | Structural Parameters 1 |
| 08 | HCSPARAMS2 | Structural Parameters 2 |
| 0C | HCSPARAMS3 | Structural Parameters 3 |
| 10 | HCCPARAMS | Capability Parameters |
| 14 | DBOFF | Doorbell Offset |
| 18 | RTSOFF | Runtime Registers Space Offset |
Doorbell Registers
The doorbell registers are located after the runtime registers in memory, and can be found by adding the DBOFF field to the base address specified in the PCI configuration space.
Virtual Registers
The xHCI specifications support "virtual" controllers that can be used to support multiple virtual machines running on a single physical machine. These registers must be configured and managed by the VM host, and effectively duplicate the registers above for use by the guest VMs.