RISC-V Design · All levels

Forwarding Network and Stall Control for Data Hazards

Pipeline Implementation: Data hazards occur when a consumer instruction needs a value that has not reached the register file yet. Forwarding (bypass) paths resolve most ALU-to-ALU and ALU-to-branch dependencies by routing results from EX/MEM or MEM/WB back into EX operand muxes with priority logic for youngest producer wins. Some cases still require stalls, especially load-use hazards when data becomes valid only after memory response timing; hazard detection must freeze PC and IF/ID while injecting a controlled bubble into downstream control. Correctness depends on tight coordination between forwarding select logic, stall gating, and kill handling so that wrong-path or excepting instructions never source or write stale values. Robust implementations pair dependency matrices with constrained-random instruction streams and assertion-based checks on operand freshness.

What this topic teaches

Forwarding Network and Stall Control for Data Hazards trains mechanism-first reasoning for RISC-V design closure. Data hazards occur when a consumer instruction needs a value that has not reached the register file yet. Forwarding (bypass) paths resolve most ALU-to-ALU and ALU-to-branch dependencies by routing results from EX/MEM or MEM/WB back into EX operand muxes with priority logic for youngest producer wins. Some cases still require stalls, especially load-use hazards when data becomes valid only after memory response timing; hazard detection must freeze PC and IF/ID while injecting a controlled bubble into downstream control. Correctness depends on tight coordination between forwarding select logic, stall gating, and kill handling so that wrong-path or excepting instructions never source or write stale values. Robust implementations pair dependency matrices with constrained-random instruction streams and assertion-based checks on operand freshness.

Senior-engineer framing question

When RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction. moves, can you isolate first failing mechanism, request decisive evidence, assign owner, and decide release-safe action?

diagram

RISC-V PIPELINE DIAGRAM - Forwarding Network and Stall Control for Data Hazards

PC -> IF -> ID -> EX -> MEM -> WB
      |     |      |      |      |
  i-cache decode  ALU/BR  LSU    regfile write
              \   |
               +-> branch resolve + redirect

Hot paths:
  - branch + load-use dependencies in ID/EX
  - memory latency stretching MEM stage
  - writeback arbitration for integer/vector units

Focus: map symptom to first failing stage

Architecture visuals

Draw before you tune. Use these visuals in design reviews, interview loops, and post-silicon triage.

Decode and control map

diagram

DECODE CONTROL MAP - Forwarding Network and Stall Control for Data Hazards

opcode/funct3/funct7      controls asserted
-----------------------   ---------------------------------------
LUI / AUIPC               rd_write, imm_select(U), alu_add_pc
JAL / JALR                rd_write, pc_redirect, link_write
BRANCH                    cmp_enable, branch_type, pc_redirect
LOAD                      mem_read, rd_write, wb_sel(memory)
STORE                     mem_write, store_size, addr_calc
OP-IMM                    alu_enable, imm_select(I), rd_write
OP                        alu_enable, src2_reg, rd_write
SYSTEM / CSR              csr_readwrite, trap_check, privilege_gate
VECTOR (V extension)      vdecode, lane_mask, vtype_update

Privilege stack

diagram

PRIVILEGE MODE STACK - Forwarding Network and Stall Control for Data Hazards

            +------------------------------+
            | Machine mode (M)             |
            | firmware, PMP, trap root     |
            +---------------+--------------+
                            |
                    delegated traps
                            v
            +------------------------------+
            | Supervisor mode (S)          |
            | kernel, page tables, drivers |
            +---------------+--------------+
                            |
                    ecall / syscall
                            v
            +------------------------------+
            | User mode (U)                |
            | applications, libraries      |
            +------------------------------+

Key rule: each upward transition records cause + PC in trap CSRs.

Translation path

diagram

MMU PAGE WALK DIAGRAM - Forwarding Network and Stall Control for Data Hazards

virtual address
    |
    +--> TLB lookup hit? ---- yes ---> physical address -> cache/memory
    |             |
    |             no
    v
satp root PPN + VPN indices
    |
    +--> level-2 PTE fetch (valid?)
    |         |
    |         +-- no -> page fault trap
    v
level-1 PTE fetch -> level-0 PTE fetch
    |
    +--> permissions check (R/W/X, U/S, A/D)
             |
             +-- fail -> access fault trap
             +-- pass -> install TLB entry -> continue

Vector lane lens

diagram

VECTOR LANE VIEW - Forwarding Network and Stall Control for Data Hazards

VLEN register file
   |
   +--> lane0: ALU/MUL/permute
   +--> lane1: ALU/MUL/permute
   +--> lane2: ALU/MUL/permute
   +--> lane3: ALU/MUL/permute
            ...
mask register -> per-lane predicate enable
load/store unit -> strided/segmented access queue

Throughput model:
effective ops/cycle = active_lanes * issue_rate * mask_density

Focus: balance lane utilization and memory feed

Ownership layers

diagram

RISC-V OWNERSHIP LAYERS - Forwarding Network and Stall Control for Data Hazards

layer                  owner                         closure artifact
--------------------   ----------------------------  -----------------------------
ISA compliance         architecture/spec team        unpriv + priv test evidence
decode/control         front-end RTL owner           decode matrix + assertions
pipeline timing        microarchitecture owner       hazard/perf regression trends
memory + MMU           LSU/MMU owner                 TLB/pagewalk trace checks
privilege/CSR path     firmware + kernel interface   trap/interrupt conformance
vector subsystem       vector RTL + compiler owner   lane-utilization profiles

Evidence required

Primary metric: RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction..
Primary artifact: Forwarding select truth table, hazard detector pseudocode, and stall/bubble state machine timing diagram..
Owners to include: CPU microarchitecture lead, RTL implementation owner, verification lead, formal verification owner, silicon validation owner.
One reproducible workload and one stable comparator run.
One run with locked environment metadata for causal confidence.

Root-cause tree

diagram

ROOT CAUSE TREE - Forwarding Network and Stall Control for Data Hazards

RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction. regressed
          |
   reproducible on fixed seed?
      /                 \
    no                   yes
    |                     |
env/tool drift       first failing domain?
                     /        |         \
                  decode    execute    memory/MMU
                    |         |            |
               control map  bypass/FU   TLB/walk/perm
                    |
         privilege/CSR side effects checked?

Stop at first confirmed mechanism, then assign explicit owner + fix proof.

Movement trend

diagram

BEFORE / AFTER TREND - Forwarding Network and Stall Control for Data Hazards

RAW hazard coverage percentage with zero incorrect-commit events and minimum load-use stall cycles per kilo-instruction.
  ^
  |                           o target band
  |                    o after fix + reruns
  |             o
  |      o baseline (failing)
  +--------------------------------------------------> iteration
       capture issue      isolate mechanism      close + monitor

Use this view to confirm the gain is causal and stable across seeds.

Key takeaways

Classify mechanism before proposing fixes.
Tie every claim to one proving artifact.
Close with owner accountability and rollback criteria.

Common pitfalls

Averaging away tail behavior and mode-specific failures.
Blending results from mismatched build/runtime metadata.
Declaring closure before cross-workload validation.

Practice this lesson