Hazard Detection Basics in Decode: Mechanism

Mechanism to understand

Mechanism for Hazard Detection Basics in Decode is anchored on RAW stall cycles per kilo-instruction and percentage of avoidable stalls removed by forwarding.. Convert observations into mechanism-backed decisions with explicit ownership.

Decode is the first stage that can compare source register indices against destination registers already in flight, so it becomes the anchor point for hazard classification. Basic logic catches read-after-write hazards for operations whose results are not yet available, plus structural conflicts such as a busy multiplier or a blocked memory pipeline. The hazard unit then chooses between stalling decode, inserting bubbles, or allowing issue with forwarding assumptions. Even in simple in-order cores, edge cases matter: x0 dependencies must be ignored, CSR reads and writes can serialize unexpectedly, and load-use latency usually requires a one-cycle interlock unless the data path supports same-cycle bypass. Reliable behavior depends on synchronizing hazard state with flush events so stale dependencies do not survive control redirects.

• Name first failing path and trigger condition.

• Prove path loss with one measurable artifact.

• Assign owner who can deliver smallest reversible fix.

Pipeline mechanism sketch

RISC-V PIPELINE DIAGRAM - Hazard Detection Basics in Decode

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: keep control hazards predictable

Decode intent map

DECODE CONTROL MAP - Hazard Detection Basics in Decode

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