Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 50

Answer
The reliability improvement would be
Despite an impressive 4150X improvement in reliability of one module, from
the system's perspective, the change has a measurable but small benefit.
In the examples above, we needed the fraction consumed by the new and improved version;
often it is difficult to measure these times directly. In the next section, we will see another way
of doing such comparisons based on the use of an equation that decomposes the CPU execu-
tion time into three separate components. If we know how an alternative affects these three
components, we can determine its overall performance. Furthermore, it is often possible to
build simulators that measure these components before the hardware is actually designed.
The Processor Performance Equation
Essentially all computers are constructed using a clock running at a constant rate. These dis-
crete time events are called ticks, clock ticks, clock periods, clocks, cycles , or clock cycles . Computer
designers refer to the time of a clock period by its duration (e.g., 1 ns) or by its rate (e.g., 1
GHz). CPU time for a program can then be expressed two ways:
or
In addition to the number of clock cycles needed to execute a program, we can also count
the number of instructions executed—the instruction path length or instruction count (IC). If we
know the number of clock cycles and the instruction count, we can calculate the average num-
ber of clock cycles per instruction (CPI). Because it is easier to work with, and because we will
deal with simple processors in this chapter, we use CPI. Designers sometimes also use instruc-
tions per clock (IPC), which is the inverse of CPI.