CompTIA A+ Certification All-in-One Exam Guide

Parallel Execution
Modern CPUs can process multiple commands and parts of commands in
parallel, known as parallel execution. Early processors had to do everything
in a strict, linear fashion. The CPUs accomplish this parallelism through
multiple pipelines, dedicated cache, and the capability to work with multiple
threads or programs at one time. To understand the mighty leap in efficiency
gained from parallel execution, you need insight into the processing stages.
Pipelining To get a command from the data bus, do the calculation, and
then send the answer back out onto the data bus, a CPU takes at least four
steps (each of these steps is called a stage):
1. Fetch Get the data from the EDB.
2. Decode Figure out what type of command needs to be executed.
3. Execute Perform the calculation.
4. Write Send the data back onto the EDB.
Smart, discrete circuits inside the CPU handle each of these stages. In
early CPUs, when a command was placed on the data bus, each stage did its
job and the CPU handed back the answer before starting the next command,
requiring at least four clock cycles to process a command. In every clock
cycle, three of the four circuits sat idle. Today, the circuits are organized in a
conveyer-belt fashion called a pipeline. With pipelining, each stage does its
job with each clock-cycle pulse, creating a much more efficient process. The
CPU has multiple circuits doing multiple jobs, so let’s add pipelining to the
Man in the Box analogy. Now, it’s Men in the Box (see Figure 3-24)!