COMPUTER SYSTEM

PROBLEM TO BE SOLVED: To enable a processor to access resources exclusively. SOLUTION: An input/output device 223 as a resource connected to processors includes a semaphore module 4 having semaphor logic and a semaphor register. A 1st value when the resource is usable or a 2nd value when not is stored in the semaphor register under the control of the semaphor logic and when some processor sends a read command (check signal) so as to access a resource being accessed, the 2nd value is sent back, so that its unusable state is known. The semaphor is provided for I/O, so exclusive access is enable without locking a bus and no semaphore is stored in a memory, so a common area for the semaphore need not be provided in the memory.