Pipelining is a technique used in computer architecture to increase the instruction throughput (number of instructions executed per unit time) of a CPU. It involves dividing the execution process of instructions into distinct stages, with each stage completing a part of the instruction. These stages are connected in a series, forming a pipeline, where each stage processes a different instruction simultaneously.

Stages of a Pipeline:

The primary motivation behind pipelining is to keep every part of the processor busy, thereby improving efficiency and increasing the number of instructions that can be completed in a given time. The pipeline is divided into stages, each corresponding to a different part of the instruction cycle (such as fetching the instruction, decoding it, executing it, and writing the result back). As soon as one stage of the pipeline completes its task on an instruction, the next instruction can enter that stage, allowing the CPU to process multiple instructions simultaneously.

For example, a simple pipeline might include the following stages:

1. Fetch: The instruction is fetched from memory.
2. Decode: The fetched instruction is decoded, and the necessary operands are fetched from registers.
3. Execute: The instruction is executed (e.g., an arithmetic operation is performed).
4. Memory Access: Any necessary memory operations are performed (e.g., loading data from memory).
5. Write-Back: The result of the instruction is written back to the appropriate register.

These stages operate in parallel, meaning that while one instruction is being decoded, another can be fetched, a third can be executed, and so on.

Pipeline Registers

Between each stage in the pipeline, there are pipeline registers that store intermediate data and control information. These registers hold the output of one stage so that it can be used as input by the next stage. The use of pipeline registers ensures that each stage operates independently of the others, allowing the stages to work in parallel without interfering with each other.

Pipeline Operation: In an ideal pipeline, each stage completes in one clock cycle, allowing a new instruction to be fetched at each cycle. This means that once the pipeline is full, one instruction completes every clock cycle. For example, a five-stage pipeline would have up to five instructions in different stages of execution at any given time.

Pipeline Performance: Pipelining increases the throughput of the CPU without increasing the clock speed. However, it does not reduce the time it takes to complete a single instruction (latency). The performance improvement due to pipelining is mainly due to overlapping the execution of multiple instructions.

Pipeline Hazards: While pipelining improves throughput, it introduces several challenges known as pipeline hazards:

1. Data Hazards: Occur when instructions that exhibit data dependencies modify data in different stages of the pipeline. Common types include:
   • RAW (Read After Write): An instruction needs to read a location before a previous instruction writes to it.
   • WAR (Write After Read): An instruction needs to write to a location before a previous instruction reads from it.
   • WAW (Write After Write): Two instructions written to the same location.
2. Control Hazards: Arise from the pipelining of branches and other instructions that change the PC (Program Counter). If the branch outcome is not determined early in the pipeline, it can lead to incorrect instructions being fetched.
3. Structural Hazards: Occur when two or more instructions require the same hardware resource at the same time. For instance, if a pipeline lacks separate hardware for instruction fetch and data memory access, a conflict will arise.

Techniques to Handle Hazards: Several techniques are used to handle pipeline hazards:

1. Stalling: Also known as pipeline bubbling, it involves delaying subsequent instructions until the hazard is resolved.
2. Forwarding (Bypassing): This technique allows the result of an instruction to be forwarded directly to a subsequent instruction that needs it, rather than waiting for it to be written back to the register file.
3. Branch Prediction: A technique to guess the outcome of a branch instruction to avoid stalling the pipeline.
4. Out-of-Order Execution: Instructions are executed as soon as their operands are available, rather than strictly in program order, to improve resource utilization and performance.

Pipeline Bubbles

A pipeline bubble is a stall in the pipeline where one or more stages are idle, waiting for a hazard to be resolved. Bubbles occur when the pipeline cannot proceed with the next instruction because it depends on the results of a previous instruction that has not yet been completed. When a bubble is introduced, the affected stages of the pipeline do no useful work for one or more cycles, reducing the overall efficiency of the CPU.

If an instruction in the pipeline depends on a result from a previous instruction that has not yet been completed, the pipeline may stall. This creates a bubble, which moves through the pipeline until the hazard is resolved, after which normal execution resumes.

Advanced Pipelining Concepts:

1. Superscalar Execution: Involves multiple pipelines within a single processor, allowing multiple instructions to be processed simultaneously.
2. Hyper-Threading: A form of simultaneous multithreading (SMT) that allows a single CPU to execute multiple threads in parallel by duplicating certain sections of the processor—such as the register state—while sharing the execution resources.
3. Speculative Execution: Instructions are executed before the certainty of their need is known, based on prediction mechanisms. This is often combined with branch prediction.

Pipelining is a fundamental technique in modern CPU design that improves instruction throughput by overlapping the execution of multiple instructions. While it introduces complexities such as hazards, various techniques and advanced concepts have been developed to mitigate these issues and further enhance CPU performance. Pipelining remains a cornerstone of high-performance computing, enabling faster and more efficient processing.

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Frequently Asked Questions on Pipelining – FAQs

 

What is the purpose of pipelining in CPU design?

Pipelining aims to increase the instruction throughput by allowing multiple instructions to be processed simultaneously at different stages of execution.

In which type of pipeline hazard do hardware resource limitations cause conflicts?

Structural Hazards occur when hardware resources are insufficient to support all possible combinations of instructions in simultaneous overlapped execution.

Related links

• In a 5-stage pipeline, the first instruction is an ADD, the second instruction is a LOAD that causes a 2-cycle data hazard stall, and the third instruction is a branch with a 1-cycle control hazard delay. How many clock cycles are needed to complete the execution of 5 instructions?
• What is a pipeline “bubble” in a pipelined processor?
• Which of the following is a data hazard where a write operation is performed before a read operation on the same data?
• What technique involves providing an instruction with the result of a previous instruction before it is written back to the register file?
• What is a “stall” in the context of a pipelined computer?