Computer system and I/O bridge

A computer system reduces an overhead by using an I/O switch high in versatility when realizing the sharing of an I/O between virtual computers. The system includes a CPU module # having plural CPU cores, an AS bridge connected to the CPU cores, and a main memory that can be accessed from the CPU cores or the AS bridge , and AS switches SW and SW that connect the AS bridge of the CPU module # and an I/O blade #. The CPU module # has a hypervisor that divides the plural CPU cores and the main memory into plural logical partitions, and the AS bridge adds virtual path information set in each of the logical partitions and path information of from the AS bridge to the I/O blade # to the path information of the AS packet to switch over the AS packet between each of the logical partitions and the I/O blade # when relaying the AS packet transmitted or received between the logical partitions and the I/O blade #