Iron law of processor performance

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In computer architecture, the iron law of processor performance (or simply iron law of performance) describes the performance trade-off between complexity and the number of primitive instructions that processors use to perform calculations.[1] This formulation of the trade-off spurred the development[citation needed] of Reduced Instruction Set Computers (RISC) whose instruction set architectures (ISAs) leverage a smaller set of core instructions to improve performance. The term was coined by Douglas Clark[2] based on research performed by Clark and Joel Emer in the 1980s.[3]


The performance of a processor is the time it takes to execute a program: . This can be further broken down into three factors:[4]

Selection of an instruction set architecture affects , whereas is largely determined by the manufacturing technology. Classic Complex Instruction Set Computer (CISC) ISAs optimized by providing a larger set of more complex CPU instructions. Generally speaking, however, complex instructions inflate the number of clock cycles per instruction because they must be decoded into simpler micro-operations actually performed by the hardware. After converting X86 binary to the micro-operations used internally, the total number of operations is close to what is produced for a comparable RISC ISA.[5] The iron law of processor performance makes this trade-off explicit and pushes for optimization of as a whole, not just a single component.

While the iron law is credited for sparking the development of RISC architectures,[citation needed] it does not imply that a simpler ISA is always faster. If that were the case, the fastest ISA would consist of simple binary logic. A single CISC instruction can be faster than the equivalent set of RISC instructions when it enables multiple micro-operations to be performed in a single clock cycle. In practice, however, the regularity of RISC instructions allowed a pipelined implementation where the total execution time of an instruction was (typically) ~5 clock cycles, but each instruction followed the previous instruction ~1 clock cycle later[citation needed]. CISC processors can also achieve higher performance using techniques such as modular extensions, predictive logic, compressed instructions, and macro-operation fusion.[6][5][7]

See also[edit]


  1. ^ Eeckhout, Lieven (2010). Computer Architecture Performance Evaluation Methods. Morgan & Claypool. pp. 5–6. ISBN 9781608454679. Retrieved 9 March 2021.
  2. ^ Joel, Emer (2021-04-13), YArch 2021 Keynote, retrieved 2021-09-02
  3. ^ A Characterization of Processor Performance in the VAX-11/780, Joel S. Emer, Douglas W. Clark, 1984, IEEE
  4. ^ Asanovic, Krste (2019). "Lecture 4 - Pipelining" (PDF). Department of Electrical Engineering and Computer Sciences at UC Berkeley (Lecture Slides). p. 2. Archived from the original on 2020-03-11. Retrieved 2020-03-11.
  5. ^ a b Celio, Christopher; Dabbelt, Palmer; Patterson, David A.; Asanović, Krste (2016-07-08). "The Renewed Case for the Reduced Instruction Set Computer: Avoiding ISA Bloat with Macro-Op Fusion for RISC-V". arXiv:1607.02318 [cs.AR].
  6. ^ Engheim, Erik (2020-12-28). "The Genius of RISC-V Microprocessors". Medium. Retrieved 2021-03-11.
  7. ^ Celio, Christopher (2016-07-26), A Comparison of RISC V, ARM, and x86, retrieved 2021-03-11