Implement Forwarding to Eliminate Unnecessary Stalls

Bypass Paths

Forwarding (bypassing) routes a computed result from EX, MEM, or WB directly to the EX stage input muxes, avoiding a register-file round trip. This kills most data hazards without stalling.

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When Forwarding Fails

• Load-use: LOAD data is not ready until MEM; one stall is unavoidable
• Multi-cycle ops: divider units may stall EX for many cycles

For example, after ADD R1, R2, R3 immediately followed by SUB R4, R1, R5, the SUB in ID needs R1 while ADD is still in EX. Forwarding picks the ALU output from EX/MEM instead of reading the old register value.

For this exercise, you will implement muxes that select the youngest matching producer. This task asks you to compare IPC with and without forwarding on synthetic hazard-heavy code.

Working Through the Exercise

Keep the relevant datasheet, ISA manual, or architecture textbook chapter open while you implement. When your output disagrees with the reference trace on the same program, the bug is usually a mis-decoded opcode, a stale register read, or a flag bit left unchanged after arithmetic.

• Trace first: walk one failing input by hand before changing random lines
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

For this exercise, you will use those habits while implementing the requirement in the starter code. Microarchitectural product names change across CPU generations, but the control ideas (fetch, bypass, cache lines, vector lanes) stay stable enough to debug from first principles.