1. 09/16/2002 ICCD 2002 A Circuit-Level Implementation of Fast, Energy-Efficient CMOS Comparators for High-Performance Microprocessors* *supported in part by DARPA through the PAC-C program and NSF Oguz Ergin, Kanad Ghose, Gurhan Kucuk, Dmitry Ponomarev Department of Computer Science State University of New York Binghamton, NY 13902-6000 http://www.cs.binghamton.edu/~lowpower International Conference on Computer Design (ICCD 2002), September 16 th 2002

2. 09/16/2002 ICCD 2002 A Superscalar Datapath IQ Function Units Instruction Issue F1D1 FU1 FU2 FUm ARF Result/status forwarding buses EX Instruction dispatch Architectural Register File F2 Fetch Decode/Dispatch D2 D-cache LSQ ROB

3. 09/16/2002 ICCD 2002 Motivation Comparators are in a pervasive use in modern datapaths. They are used in: Issue queues Load-Store queues TLBs Caches Associatively-addressed Reorder Buffers Dependency checking logic CAM-based rename tables

4. 09/16/2002 ICCD 2002 Motivation (continued) Traditional comparators dissipate energy on mismatches In many cases, mismatches are much more frequent than matches To save energy, we propose two dissipate-on- match comparators: A Two stage Domino-style design Pass-logic single stage design First, traditional comparator…

5. 09/16/2002 ICCD 2002 Traditional 8-bit Pull-Down Comparator precharge Evaluation

6. 09/16/2002 ICCD 2002 Two-Stage Domino-Style Comparator Precharge Discharge Propagation Evaluation (Conditional evaluation)

7. 09/16/2002 ICCD 2002 Pass Logic, Single-Stage Comparator (PLSSC) Propagation Precharge Discharge Evaluation

8. 09/16/2002 ICCD 2002 Timing Diagrams

9. 09/16/2002 ICCD 2002 Experimental Setup (AccuPower, DATE’02) Compiled SPEC benchmarks Datapath specs Performance stats VLSI layout data SPICE deck SPICE Microarchitectural Simulator Energy/Power Estimator Power/energy stats SPICE measures of Energy per transition Transition counts, Context information

10. 09/16/2002 ICCD 2002 Variation of Response Time with Vs

11. 09/16/2002 ICCD 2002 Variation of Energy Dissipation with Vs

12. 09/16/2002 ICCD 2002 Matching Statistics: the Issue Queue Number of matching 2-bit groups % of total cases 01234 SPEC2000 Average 19.738.6299.73

13. 09/16/2002 ICCD 2002 Energy Dissipation in Various Matching Cases

14. 09/16/2002 ICCD 2002 Main Results Two-stage domino-style comparator: 65% comparator energy reduction in the issue queue About 25% increase in response time PLSSC: 75% comparator energy reduction in the issue queue Slight improvement in response time PLSSC is the design of choice for 8-bit comparands. Domino-style design is a more scalable solution.