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Upgrading And Repairing PCs 21st Edition: Processor Features

Superscalar Execution

The fifth-generation Pentium and newer processors feature multiple internal instruction execution pipelines, which enable them to execute multiple instructions at the same time. The 486 and all preceding chips can perform only a single instruction at a time. Intel calls the capability to execute more than one instruction at a time superscalar technology.

Superscalar architecture was initially associated with high-output reduced instruction set computer (RISC) chips. A RISC chip has a less complicated instruction set with fewer and simpler instructions. Although each instruction accomplishes less, the overall clock speed can be higher, which usually increases performance. The Pentium is one of the first complex instruction set computer (CISC) chips to be considered superscalar. A CISC chip uses a richer, fuller-featured instruction set, which has more complicated instructions. As an example, say you wanted to instruct a robot to screw in a light bulb. Using CISC instructions, you would say the following:

  1. Pick up the bulb.
  2. Insert it into the socket.
  3. Rotate clockwise until tight.

Using RISC instructions, you would say something more along the lines of the following:

  1. Lower hand.
  2. Grasp bulb.
  3. Raise hand.
  4. Insert bulb into socket.
  5. Rotate clockwise one turn.
  6. Is bulb tight? If not, repeat step 5.
  7. End.

Overall, many more RISC instructions are required to do the job because each instruction is simpler (reduced) and does less. The advantage is that there are fewer overall commands the robot (or processor) has to deal with, and it can execute the individual commands more quickly, and thus in many cases execute the complete task (or program) more quickly as well. The debate goes on whether RISC or CISC is really better, but in reality there is no such thing as a pure RISC or CISC chip—it is all just a matter of definition, and the lines are somewhat arbitrary.

Intel and compatible processors have generally been regarded as CISC chips, although the fifth- and later-generation versions have many RISC attributes and internally break down CISC instructions into RISC versions.

Note: The ARM processor used by Windows RT tablets is a RISC processor. Windows RT uses the same tile-based interface as Windows 8, but x86 software is not compatible with Windows RT.