METHOD AND APPARATUS FOR SELECTIVELY POSTING WRITE CYCLES USING THE 82385 CACHE CONTROLLER

摘要

BC988-003 METHOD AND APPARATUS FOR SELECTIVELY POSTING WRITE CYCLES USING THE 82385 CACHE CONTROLLER A microcomputer system employing an 80386 CPU and an 82385 cache controller has the capability of functioning with dynamic bus sizing (where the CPU interacts with devices which may or may not be 32bits wide), as well as posted write capability. Unfortunately, the two capabilities have the possibility of an incompatibility if a write cycle is posted to a device which cannot transfer 32 bits on a single cycle. The present invention provides logic to overcome this incompatibility. An address decoder is provided to decode the tag portion or an address asserted on a CPU local bus to determine if the asserted address is inside or outside a range of addresses which define cacheable devices. Any cacheable device is by definition 32 bits wide and therefore posted writes are allowed only to cacheable devices. Accordingly, the microcomputer system employing the invention posts write cycles to cacheable devices; write cycles to non-cacheable devices are inhibited from being posted.