Destination selection apparatus for a bus oriented computer system

Abstract

In order to carry out data transfer among the subsystems of a bus oriented data processing system, each subsystem is coupled to the bus by a local bus adapter which controls both source and destination aspects of the information transfer. Each local bus adapter has a unique 'busy' line which may be interrogated by any local bus adapter connected to the bus. When a source subsystem wishes to transfer information to a destination subsystem, the source local bus adapter determines from the destination local bus adapter's busy line whether or not the destination subsystem is available to accept the information. If the destination subsystem is available, the source local bus adapter requests access to the bus from priority resolution apparatus. When the source request is granted, the source local bus adapter issues the message to the bus and also sets the destination local bus adapter's busy line to designate the busy state which indicates to the destination bus adapter that it must prepare to receive a message and also indicates to all subsequently requesting local bus adapters that the particular subsystem to which information just placed on the bus is to be transferred is temporarily not available to receive information from any other local bus adapter. When the message has been received by the destination local bus adapter, both the source and destination local bus adapters enter a status cycle to check the integrity of the transmitted message. If the message is received intact, both the source and destination local bus adapters complete their individual housekeeping tasks during which the destination local bus adapter turns off its busy line. If an error has occurred during the message transmission, the destination subsystem, after the transfer has been retried once, disregards the message and turns off its busy line.