Computer Science – 15.1 Processors, Parallel Processing and Virtual Machines | e-Consult

Pipelining is a technique used in RISC processors to increase instruction throughput by overlapping the execution of multiple instructions. It's analogous to an assembly line in a factory, where different stages of instruction execution are performed concurrently. A typical RISC pipeline consists of stages such as Instruction Fetch (IF), Decode (ID), Execute (EX), Memory Access (MEM), and Write Back (WB). Each stage performs a specific operation on the instruction.

How Pipelining Improves Performance:

• Increased Throughput: Instead of waiting for one instruction to complete all stages before starting the next, pipelining allows multiple instructions to be in different stages of execution simultaneously. This significantly increases the number of instructions completed per unit time.
• Reduced Latency (potentially): While individual instruction latency might not decrease, the overall time to execute a sequence of instructions can be reduced due to the parallel execution.

Potential Challenges:

• Hazards: Pipelining introduces hazards that can stall the pipeline. These include:

  • Data Hazards: Occur when an instruction depends on the result of a previous instruction that is still in the pipeline. Solutions include forwarding (bypassing) and stalling.
  • Control Hazards: Occur due to branch instructions. The pipeline may need to be flushed if a branch is taken, leading to performance degradation. Solutions include branch prediction and delayed branching.
  • Structural Hazards: Occur when multiple instructions require the same hardware resource at the same time. Solutions include adding hardware resources or stalling.

• Increased Complexity: Implementing a pipeline adds complexity to the processor design, requiring careful design and control logic to manage the different stages and handle hazards.