Computer system including arbitration mechanism allowing multiple bus masters to access a graphics bus

摘要

A bridge logic unit includes a CPU interface coupled to a CPU bus, a PCI interface coupled to a PCI bus, and an AGP interface coupled to an AGP bus. The interfaces communicate with each other through a non-local memory queue manager. The non-local memory queue manager advantageously includes an AGP arbiter for arbitrating between requests requiring ownership of the AGP bus initiated by the CPU and PCI devices. The AGP bus arbiter employs a round-robin arbitration algorithm to arbitrate between contending requests received from the CPU bus and the PCI bus. If no requests are pending from either of the buses, the AGP arbiter parks on the CPU bus by asserting an acknowledge signal to the CPU interface. Thus, the bus arbiter may advantageously accommodate low latency accesses to the AGP bus by the microprocessor. Furthermore, in order to optimize multiple burst transfer requests. another control signal (referred to as LastAddrReq) may by generated by each requesting interface. The LastAddrReq signal indicates that a particular request is the last request corresponding to a burst transaction on the associated bus. Once a requesting interface wins arbitration. the AGP arbiter parks on the winning interface and does not allow another interface to win arbitration until it detects the LastAddrReq signal asserted from the current owner of the bus. In this manner, burst cycles may advantageously be effectuated uninterrupted. Furthermore, the CPU interface can gain exclusive access to either the PCI bus or AGP bus through the use of a LOCK signal. If the CPU interface asserts the LOCK signal with the assertion of CPU_AGPREQ, and the PCI or AGP bus arbiter grants an acknowledge to the CPU interface, the CPU interface will win all further arbitration cycles until it deasserts LOCK with its next assertion of CPU_AGPREQ, thus giving high priority to the CPU interface.