Lock circuit for extended buffer memory

In an information processing system comprising a main memory device, a plurality of processors, a system controller between the main memory device and the processors, and an extended buffer memory for carrying out a data transfer operation to and from the main memory device, the system controller comprises a communication lock circuit for selectively indicating a locked state and an unlocked state representative of a state of the extended buffer memory. The communication lock circuit is accessed to detect the state of the extended buffer memory each time when either a lock instruction or a lock release instruction is issued from each processor before a data transfer request is delivered from each processor to the extended buffer memory.