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In some of today's superscalar processors (e.g.the Pentium III), the result repositories are implemented as the Reorder Buffer (ROB) slots. In such designs, the ROB is a complex multi-ported structure that occupies a significant portion of the die area and dissipates a non-trivial fraction of the total chip power, as much as 27% according to some estimates. In addition, an access to such ROB typically takes more than one cycle, impacting the IPC adversely.We propose a low-complexity and low-power ROB design that exploits the fact that the bulk of the source operand values is obtained through data forwarding to the issue queue or through direct reads of the committed register values. Our ROB design uses an organization that completely eliminates the read ports needed to read out operand values for instruction issue. Any consequential performance degradation is countered by using a small number of associatively-addressed retention latches to hold the most recent set of values written into the ROB. The contents of the retention latches are used to satisfy the operand reads for issue that would otherwise have to be read from the ROB slots. Significant savings of the ROB real estate as well as power savings in the range of 20% to 30% for the ROB are achieved using the proposed technique. At the same time, the fact that results are accessible in a single cycle from the retention latches actually leads to an overall improvement in the IPC of up to 3% on the average for SPEC 2000 benchmarks.

Abstract-The out-of-order issue queue (IQ), used in modern superscalar processors is a considerable source of energy dissipation. We consider design alternatives that result in significant reductions in the power dissipation of the IQ (by as much as 75%) through the use of comparators that dissipate energy mainly on a tag match, 0-B encoding of operands to imply the presence of bytes with all zeros and, bitline segmentation. Our results are validated by the execution of SPEC 95 benchmarks on a true hardware level, cycle-by-cycle simulator for a superscalar processor and SPICE measurements for actual layouts of the IQ in a 0.18-m CMOS process.Index Terms-Bitline segmentation, low-power comparator, low-power instruction scheduling, low-power superscalar datapath.

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