In the interface descriptor, bInterfaceclass = 03h to identify the interface as a HID. Other fields that contain HID-specific information in the interface descriptor are the bInterfaceSubclass and bInterfaceProtocol fields, which can specify a boot interface.
If bInterfaceSubclass = 01h, the device supports a boot interface. A HID with a boot interface can communicate with the host even when the host hasn’t loaded its HID drivers. This situation might occur when the computer boots directly to DOS or when viewing the system setup screens that you can access on bootup, or when using Windows Safe mode for system troubleshooting.
A keyboard or mouse with a boot interface can use a simplified protocol supported by the BIOS in many hosts. The BIOS loads from ROM or other non-volatile memory on bootup and is available in any operating-system mode. The HID specification defines boot-interface protocols for keyboards and mice. If a device has a boot interface, bInterfaceProtocol indicates if the HID supports a keyboard (01h) or mouse (02h) function. The HID Usage Tables document defines the report format for keyboards and mice that use the boot protocol. The BIOS understands the boot protocol and assumes that a boot device will support the protocol, so the BIOS doesn’t need to read a report descriptor from the device.
Tags: interface, loaded, Supports A Boot, The Device
