Sum Addressed Decoder - What Has Been Saved?

What Has Been Saved?

A simpler data cache path would have an adder followed by a traditional decoder. For our example cache subsystem, the critical path would be a 14 bit adder, producing true and complement values, followed by an 11 bit AND gate for each row of the decoder.

In the sum-addressed design, the final AND gate in the decoder remains, although 10 bits wide instead of 11. The adder has been replaced by a four input logical expression at each bit. The latency savings comes from the speed difference between the adder and that four input expression, a savings of perhaps three simple CMOS gates.

If the reader feels that this was an inordinate amount of brain-twisting work for a three gate improvement in a multi-cycle critical path, then the reader has a better appreciation for the level to which modern CPUs are optimized.