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Instruction Parallelism

The Instruction Cycle

In order for a single instruction to be executed by the CPU, it must go through the instruction cycle (also sometimes referred to as the fetch-execute cycle). While this cycle can vary from CPU to CPU, they typically consist of the following stages:

  • Fetch
  • Decode
  • Execute
  • Memory Access
  • Registry Write-Back

Instruction Pipelining

Instruction pipelining is a hardware-based technique in which the processor attempts to improve the throughput of a group of instructions by simultaneously processing as many instructions as effectively possible.

Parallelism Costs

In instruction pipelining, an increase in the number of steps in a pipeline causes the following side effects:

  • More expensive to manufacture
  • More power is needed to run
  • An increase in the temperature of the hardware

Hazards of Parallelism

In instruction parallelism, there are three types of hazards: Structural, Data, and Control.

There is no way to remove all hazards from a pipeline - manufacturers can only reduce the risk/impact.

Structural Hazards

Structural hazards are a limitation of the hardware itself. Structural hazards occur when there are not enough resources to execute multiple instructions.

Data Hazards

Data hazards occur when an instruction is dependent on another instruction still in the pipeline.

Control Hazards

Control hazards occur when the system doesn’t know which set of instructions will need to be processed. This occurs with branches, loops, or conditional statements.

Superscalar Architecture

Processors that take advantage of superscalar methodology are designed to use a methodology of parallelism where instructions are sent to different execution units at the same time, allowing for more than one instruction to be processed in a single clock cycle. In a superscalar processor, each execution unit (such as an ALU) is within a single CPU.

Parallelism Limitations

Instruction Parallelism processing has limitations that put a restriction on the number of simultaneous processes that are possible. These include:

  • The level of parallelism in the instruction set it is working with.
  • The amount of overhead needed to find dependencies within the instruction set.
  • The cost of examining different branches in an instruction set.

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