Providing alternate bus master with multiple cycles of bursting access to local bus in a dual bus system including a processor local bus and a device communications bus

摘要

Disclosed method and apparatus allow for balanced usage of resources in dual bus computing systems wherein: (1) principal resources of the system-including a processor, a local bus, local bus controls, and a memory subsystem-are contained in a single system unit (e.g. a card); (2) devices are coupled to the system unit and to each other through a device communications bus that is also accessible to the processor of the system unit; (3) the system processor is required to have principal access to the local bus, as a "System Bus Master", for time critical functions such as memory refresh; and (4) the devices include one or more devices that are required to have controlling access to the resources of the system unit, and for that purpose are required to have controlling access to the local bus as Alternate Bus Master entities. The disclosed arrangement allows an Alternate Bus Master operating in a burst mode, in which several cycles of data transfer may occur consecutively, to have continuous access to the local bus after a single signal handshaking exchange with the local bus controls; subject to over-riding conditions which ensure timely accomplishment of the time critical functions controlled by the processor/System Bus Master. The over-riding conditions are detected by special logic that acts, when necessary, to interrupt Alternate Bus Master access to the local bus without affecting the state of connection between the respective Alternate Bus Master and the communications bus.