Filter-circuit for computer system bus

Abstract

A filter circuit for filtering bus accesses from a computer bus (such as e.g. a bus conforming to the PCI standard) to a group of devices operably coupled to the computer bus in a computer system. The filter circuit operates in a filter mode and a non filter mode. In the non filter mode, the filter circuit allows bus accesses to pass from the computer bus to the group. In the filter mode, the filter circuit blocks bus accesses from passing to the group. Upon determining that a bus access is an allowable bus access, the filter circuit initiates a retry request to signal the bus master that initiated the allowable bus access to reinitiate the bus access, wherein the filter circuit allows the allowable bus access to pass to the group. The filter circuit includes a switch circuit and a control circuit that places the switch circuit in the conductive and non conductive switch states to allow bus accesses to pass and to be blocked from passing to the group, respectively. In the filter mode, the filter circuit allows bus accesses to pass from a device of the group to the computer bus. The filter circuit includes a second switch circuit for controlling the bus accesses by the device of the group. The control circuit includes a programmable logic device. The filter circuit can be used in debugging a computer system which has a malfunctioning flash ROM. A second flash ROM can be inserted into a PCI slot and the filter circuit activated to filter accesses directed towards the malfunctioning flash ROM during the boot-up of the computer system.