USB device manufacturers need to support the Universal Serial Bus (USB) specification. The USB interface is implemented in two levels: The lower level of the USB protocol is implemented via a Serial Interface Engine (SIE). The higher layer of the protocol is implemented via the device firmware. Firmware consists of software programs and data that define the device's configuration and are installed semi-permanently into memory using various types of programmable ROM chips, such as PROMS, EPROMs, EEPROMs, and flash chips.