Understanding CPU Pipeline Stages and Hazards

Understanding the CPU Pipeline Focusing on Instruction Fetch and Memory Access Stages

The CPU pipeline refers to a series of stages that process instructions in a computer system. These stages include Fetch, Decode, Execute, and Memory Access. In addition to these stages, there exist various types of hazards, including structural and data hazards, which impact the efficiency and performance of the pipeline. Let us explore each stage in detail.

Instruction Fetch (IF) Stage

The Instruction Fetch stage involves retrieving the instruction from memory and placing it in the instruction register (IR). During this stage, the processor identifies which instruction should be executed, fetches it from memory, and stores it in the IR. From this stage, instructions may be sent directly to the Execution Unit to start being processed.

Structural Hazards

Structural hazards occur when there is a dependency between instructions in the pipeline. These dependencies can lead to pipeline bubbles, where instructions stall until the necessary data is available. For example, consider an instruction such as LD R1, 0(R2) followed by SB. In this case, the beginning of the SB instruction requires the value from R1, but this value is only available once the LD instruction has been completed. As a result, the processor must either introduce a delay into the pipeline or stall the SB instruction until the LD instruction has finished executing.

To resolve this issue, the processor may employ techniques such as forwarding or reservation stations. Forwarding involves moving the value of R1 from the Data Memory Access Unit into the Instruction Decode Unit before completing the EXECUTE stage of the previous instruction. Reservation stations involve temporarily holding the instruction at an earlier stage in the pipeline until the necessary data becomes available.

Data Hazards

Data hazards arise when an instruction's execution relies on the outcome of an earlier instruction's execution, leading to unnecessary halts in the pipeline. These hazards generally fall into two categories: true and false data hazards. True data hazards occur when an instruction depends on the value of a register that may not be immediately available, while false data hazards occur when an instruction appears to depend on the value of a register, but in reality, the dependence is indirect.

To mitigate data hazards, a technique called delay slot filling can be employed. In this approach, the processor recognizes that the next instruction in the pipeline has a data dependancy on the current instruction's output. It then fills this gap with a placeholder instruction, allowing the pipeline to continue uninterrupted. Additionally, the processor can use a datapath that includes data forwarding from the Write Back stage to the Execute stage, ensuring that the necessary values are available for subsequent instructions to execute correctly.

Memory Access Stage

The Memory Access stage, also referred to as the Load Store Unit (LSU), handles operations involving loading and storing data from memory. It reads or writes data based on whether the instruction is a Load or Store command. In the event of a Load instruction, the memory is read, and the data is stored in a register. Conversely, for Store instructions, the data from a register is written to memory.

Comparison with Non-Pipelined Processors

When compared to non-pipelined processors, pipelined processors offer several advantages. They reduce the time required to execute instructions and allow multiple instructions to be partially executed simultaneously. However, pipelined processors may require additional complexity to detect and manage hazards, ultimately increasing power consumption and manufacturing costs.

Non-pipelined processors may be preferred in certain scenarios, particularly when dealing with low-cost hardware or when simplified programming models are desired. They may also facilitate better predictability and ease of programming, as hazards are generally not a concern.