The optimal logic depth per pipeline stage is 6 to 8 FO4 inverter delays

ABSTRACT

Microprocessor clock frequency has improved by nearly 40% annually over the past decade. This improvement has been provided, in equal measure, by smaller technologies and deeper pipelines. From our study of the SPEC 2000 benchmarks, we find that for a high-performance architecture implemented in 100nm technology, the optimal clock period is approximately 8 fan-out-of-four (FO4) inverter delays for integer benchmarks, comprised of 6 FO4 of useful work and an overhead of about 2 FO4. The optimal clock period for floating-point benchmarks is 6 FO4. We find these optimal points to be insensitive to latch and clock skew overheads. Our study indicates that further pipelining can at best improve performance of integer programs by a factor of 2 over current designs. At these high clock frequencies it will be difficult to design the instruction issue window to operate in a single cycle. Consequently, we propose and evaluate a high-frequency design called a segmented instruction window.

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INDEX TERMS

Primary Classification:
  C. Computer Systems Organization
  C.3 SPECIAL-PURPOSE AND APPLICATION-BASED SYSTEMS
      Subjects: Microprocessor/microcomputer applications

Additional Classification:
  B. Hardware
  B.2 ARITHMETIC AND LOGIC STRUCTURES
      B.2.1 Design Styles
          Subjects: Pipeline

  C. Computer Systems Organization
  C.4 PERFORMANCE OF SYSTEMS
      Subjects: Performance attributes

General Terms:
Design, Performance, Theory

Keywords:
Pipelining, instruction queue clock rate