Abstract: In a computer system, which makes an error detectable in case plural PCI target devices respond in one PCI cycle and the PCI protocol has become illicit, a processor 1 is connected over a PCI bus 10 to plural PCI devices a 100 to d 130, each of which activates corresponding target operating signal a 20 to d 50 respectively when operating as a PCI target device. The PCI bus monitor circuit 200 monitors the target address of a command executed on the PCI bus 10 and the target operating signals a 20 to d 50 from the plural PCI devices a 100 to d 130. If plural PCI target devices have responded for one PCI cycle, the PCI bus monitor circuit 200 sends an error report signal 210 to the processor unit 1.

Abstract: In a computer system, which makes an error detectable in case plural PCI target devices respond in one PCI cycle and the PCI protocol has become illicit, a processor 1 is connected over a PCI bus 10 to plural PCI devices a 100 to d 130, each of which activates corresponding target operating signal a 20 to d 50 respectively when operating as a PCI target device. The PCI bus monitor circuit 200 monitors the target address of a command executed on the PCI bus 10 and the target operating signals a 20 to d 50 from the plural PCI devices a 100 to d 130. If plural PCI target devices have responded for one PCI cycle, the PCI bus monitor circuit 200 sends an error report signal 210 to the processor unit 1.