Over the next couple of slides the presentation will examine the steps involved in firmware development required to handle data transfers across the USB. As an example, the control transfer process begins as the host sends what is called a setup packet. If the packet is received with errors the SIE will do nothing and discard the packet. No device firmware is required to handle this error condition. If the packet is valid, the SIE will load the data into the FIFO and an interrupt is generated. At this point it is up to the firmware to handle the received data. Since this was an endpoint 0 transaction, the firmware sets the index to 0 so that the endpoint 0 FIFO can be read. If the packet is ready, the firmware unloads the data from the FIFO and decodes the transfer request (per chapter nine of the USB specification). Once decoded the firmware will either load data to be read by the host (IN) or get the new data sent from the host (OUT).