Iron law of processor performance
Beginner
The rule that a program’s time is: how many steps it has, times the ticks each step takes, times how long one tick lasts.
Novice
Run time = instructions in the program × average cycles per instruction (CPI) × time per cycle. Every design idea changes one or more of the three, often making one better and another worse.
Expert
. Instruction count depends on the ISA and compiler, CPI on the ISA and microarchitecture, cycle time on the microarchitecture and circuits. Credited to Emer and Clark’s VAX-11/780 study.
Explained in Instructions and pipelines (Architectures).
See also: CPI (cycles per instruction), IPC (instructions per cycle).