The backside bus is not used in modern CPU architectures, nor is the FSB, they have both been replaced by other faster interconnects. The L2 cache of a modern CPU is no longer on the board and thus is no longer connected by a bus but instead is part of the unified cache on the chip that the CPU has direct access to. Modern chips have everything that the BSB used to connect to now integrated into the CPU and often all connected by a Ring Bus or similar.