The energy at the I/O pins is a significant part of the overall consumption of a chip. To reduce this energy, this work extends to the data bus the working zone encoding method originally applied to encoding an external address bus. This method is based on the conjecture that programs favor a few working zones of their address space at each instant and that addresses to consecutive accesses for each zone frequently differ by a small amount. When the difference is small, instead of sending the whole address, the method sends an offset with respect to the previous address to that zone, together with an identifier for the zone. In this paper the same idea is extended to the data bus. The approach has been applied to several SPEC95 streams of references to memory along with the corresponding data values in a system with multiplexed address and multiplexed instruction/data buses. Moreover the effect of instruction and data caches is evaluated. Comparisons are given with previous methods for bus encoding, showing significant improvement in all cases except for the multiplexed address bus with instruction cache, where the best scheme depends on the overhead of the implementation.

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