What is your definition of cycle?
An example will clarify. Let's assume a Pentium 4 processor is running at a (clock) speed of 3.2 gigahertz. This means this CPU uses (consumes) 3,200,000,000 clock cycles per second. If all machine instructions could execute in a single cycle each, this CPU could execute 3,200,000,000 machine instructions per second. In a 1.6 gigahertz similar processor, it would require 2 seconds to execute the same 3,200,000,000 machine instructions.
Of course, there are many machine instructions which require more than a cycle to execute like the integer division instruction or the floating point square root instruction. Therefore, the overall performance of a program can be more accurately measured in cycles because this measurement is independant of the clock speed of the processor. Regardless of the clock speed, a typical machine instruction will consume the same amount of cycles when executed on the same type and brand of computer.
So, part of the optimization process in the art of programming, the clever selection of specific machine instructions to achieve a given task is crucial for the creation of the fastest programs. The fastest programs consumes the smallest amount of cycles.
