If no response is received from the device's function #0, the bus master performs an abort and returns an all-bits-on value ( in hexadecimal), which is an invalid VID/DID value, thus a device driver can tell that the specified combination bus/device_number/function (B/D/F) is not present. So, when a read to a function ID of zero for a given bus/device causes the master (initiator) to abort, it must then be presumed that no working device exists on that bus because devices are required to implement function number zero. In this case, reads to the remaining functions numbers (1–7) are not necessary as they also will not exist.