The timing of the fetch, decode, and execute phases depends on the internal construction of the processor and the complexity of the instructions it executes. The quantum time unit for measuring operations is known as a clock cycle. The logic that directs operations within a processor is controlled by an external clock, which is simply a circuit that generates a square wave with a fixed period. The number of clock cycles required to carry out an operation determines the amount of time it will take.
One cannot simply assume that if a multiplication can be done in nanoseconds then it will take nanoseconds to perform n multiplications or that if a branch instruction takes nanoseconds the next instruction will begin execution nanoseconds following the branch. The actual timings depend on the organization of the memory system and the communication channels that connect the processor to the memory; these are the topics of the next two sections.