# Pipelining in CPU Design Pipelining is a technique used in CPU design to increase instruction throughput. It divides the processing of an instruction into a series of stages, each of which can operate in parallel with the others. ## Basic Concept The basic idea behind pipelining is similar to an assembly line in a factory. Just as an assembly line allows multiple cars to be manufactured simultaneously, a pipeline allows multiple instructions to be processed at once. ## Stages of a Pipeline A typical CPU pipeline consists of the following stages: 1. **Fetch**: The CPU fetches the instruction from memory. 2. **Decode**: The CPU decodes the instruction to determine what operation to perform. 3. **Execute**: The CPU performs the operation. 4. **Memory access**: The CPU accesses memory if needed (e.g., to load or store data). 5. **Write back**: The CPU writes the results back to the register file. Each of these stages can be thought of as a separate unit that operates in parallel with the others. ## Pipeline Hazards However, pipelining also introduces the concept of hazards. These are situations that prevent the next instruction from executing in the next clock cycle. There are three types of hazards: 1. **Structural hazards**: Occur when two instructions require the same resource at the same time. 2. **Data hazards**: Occur when an instruction depends on the result of a previous instruction. 3. **Control hazards**: Occur when the flow of instructions to the pipeline is disrupted by branch instructions. To mitigate these hazards, CPU designers use techniques such as forwarding (or bypassing), where the result of an operation is forwarded directly to another operation without going through the register file, and branch prediction, where the CPU predicts the outcome of a branch instruction and fetches instructions accordingly. ## Example Consider the following sequence of instructions in assembly language: ```assembly LOAD R1, 1000 ADD R2, R1, R3 STORE 1000, R2 ``` Without pipelining, each instruction would have to complete before the next one could start. But with pipelining, once the `LOAD` instruction has been fetched, the `ADD` instruction can be fetched while the `LOAD` instruction is being decoded, and so on. This allows the CPU to make better use of its resources and process instructions more quickly.