SYSTEM BUS PREEMT FOR 80386 WHEN RUNNING IN AN 80386/ 82385 MICROCOMPUTER SYSTEM WITH ARBITRATION

A MULTI-BUS MICROCOMPUTER SYSTEM INCLUDES A CACHE SUBSYSTEM AND AN ARBITRATION SUPERVISOR. A CPU IS PROVIDED WITH A PREEMPT SIGNAL SOURCE WHICH GENERATES A PREEMPT SIGNAL IN CPU CYCLES EXTENDING BEYOND A SPECIFIED DURATION. THE PREEMPT SIGNAL IS EFFECTIVE AT ANY DEVICE HAVING ACCESS TO THE BUS TO INITIATE AN ORDERLY TERMINATION OF THE BUS USAGE. WHEN THAT DEVICE SIGNALS ITS TERMINATION OF BUS USAGE, THE ARBITRATION SUPERVISOR CHANGES THE STATE OF AN ARB/GRANT CONDUCTOR, WHICH HAD BEED IN THE GRANT PHASE, TO THE ARBITRATION PHASE. DURING THE ARBITRATION PHASE EACH OF THE DEVICES (OTHER THAN THE CPU) COOPERATES IN AN ARBITRATION MECHANISM FOR BUS USAGE DURING THE NEXT GRANT PHASE. ON THE OTHER HAND, THE CPU, HAVING ASSERTED PREEMPT, RESPONDS TO A SIGNAL INDICATING INITIATION OF THE ARBITRATION PHASE BY IMMEDIATELY ACCESSING THE SYSTEM BUS.(FIG. 2)