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: ${\displaystyle \mathrm {\tfrac {Time}{Program}} }$. This can be further broken down into three factors:^[4]

$${\displaystyle \mathrm {{\frac {Instructions}{Program}}\times {\frac {ClockCycles}{Instruction}}\times {\frac {Time}{ClockCycles}}} }$$

Selection of an instruction set architecture affects ${\displaystyle \mathrm {{\tfrac {Instructions}{Program}}\times {\tfrac {ClockCycles}{Instruction}}} }$, whereas ${\displaystyle \mathrm {\tfrac {Time}{ClockCycles}} }$ is largely determined by the manufacturing technology. Classic Complex Instruction Set Computer (CISC) ISAs optimized ${\displaystyle \mathrm {\tfrac {Instructions}{Program}} }$ by providing a larger set of more complex CPU instructions. Generally speaking, however, complex instructions inflate the number of clock cycles per instruction ${\displaystyle \mathrm {\tfrac {ClockCycles}{Instruction}} }$ 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 ${\displaystyle \mathrm {\tfrac {Time}{Program}} }$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]

• Reduced instruction set computer
• Classic RISC pipeline