An Investigation of the Performance of Various Instruction-Issue Buffer Topologies

In out-of-order issue superscalar microprocessors, instructions must be buffered before they are issued. This buffer can be either unified (one buffer linked to all functional units) as in the P6, distributed among the units as in the PowerPC 620, or semi-unified (a few buffers each shared by several units) as in the MIPS R10000. Of course, the size of this buffer also plays a leading role in the performance of the processor. Intensive trace-driven simulations on the SPEC92 suite have been made in order to determine the best designs and relevant choices are pointed out according to the dispatch width of the processor. 1 Introduction There are many trends and ways of implementing out-of-order superscalar architectures and except for Mike Johnson's book [8] and [2], few results have been published on the most cost-effective execution configurations, especially on out-of-order issue superscalar microprocessors. In such processors, a buffer is required to hold instructions once they are ..