RISC-V Pipeline Simulator
Multicycle execution and data hazard visualizer
Processor Configuration
How does Pipelining work in RISC-V?
Instruction pipelining is a fundamental technique in modern computer architecture that allows the execution of multiple instructions to overlap. Instead of waiting for an instruction to complete its five functional phases, the processor starts fetching the next instruction immediately.
Fundamentals of Pipelined Instruction Execution
Pipelining is a hardware optimization technique that overlaps instruction execution. Instead of processing a single instruction from start to finish (which limits clock frequency to the worst-case path), the processor is divided into sequential stages. Each instruction spends exactly one clock cycle in each stage, allowing a new instruction to finish every cycle under ideal conditions.
The Five Classical MIPS/RISC-V Stages
The academic pipeline divides execution into five distinct stages: 1) Instruction Fetch (IF), which reads the instruction from memory; 2) Instruction Decode (ID), which reads registers and extracts fields; 3) Execute (EX), which performs ALU operations or calculates branch targets; 4) Memory Access (MEM), which reads/writes data from RAM; and 5) Write-Back (WB), which writes results back to the register bank.
Structural, Data, and Control Hazards
Parallelism introduces conflicts known as hazards. Data hazards occur when an instruction depends on the result of a previous instruction that has not yet written its value back to the register bank (RAW - Read After Write). Control hazards happen when conditional branches redirect execution flow, making instructions fetched during IF invalid before the branch direction is decided in EX.
Hazard Mitigation: Forwarding vs. Stalling
To resolve data hazards, modern processors use Data Forwarding (bypassing), which routes computed values from the EX/MEM or MEM/WB pipeline registers directly to the ALU inputs, bypassing the register bank. If forwarding is impossible (such as a Load-Use hazard, where data is read from memory and immediately needed), the hardware must insert a stall (bubble), pausing the dependent instructions while allowing the loader to proceed.