This read-only integer reports the current operating frequency in MHz of the PCI bus for this slot, regardless of whether a card is present or the slot is powered on or off, returning -1 if unknown. Admins use it to verify the bus is running at the expected speed, since a bus can be forced to a lower frequency by other cards sharing the same bus segment. For example, seeing a frequency of 33 MHz on a slot the admin expected to run at 66 MHz would indicate another 33 MHz-only card on the same bus segment is throttling it.