A bus controller and a control method are used in a computer system. In a bus controller, a bus controller main circuit issues a first signal to the central processing unit in response to a bus configuration cycle for indicating the presence of a first-level bus that the first group of components is coupled to. A virtual bridge device issues a second signal to the central processing unit in response to the bus configuration cycle for indicating the presence of a second-level bus that the second group of components is coupled to. A path selection unit electrically connected to the first and second groups of components via the first-level and second-level buses, respectively, outputs a normal device select signal to one of the first-level and second-level buses while outputting an invalid device select signal to the other of the first-level and second-level buses according to address data of a transaction.
