RISC-V Design · All levels

Extension Compatibility Policy: Mechanism

Mechanism for Extension Compatibility Policy.

Mechanism to understand

Mechanism for Extension Compatibility Policy is anchored on Software portability rate across SKUs and zero-regression compliance against declared ISA profiles.. Convert observations into mechanism-backed decisions with explicit ownership.

A compatibility policy defines mandatory, optional, and forbidden extensions per product tier, then ties those contracts to toolchain defaults, ABI rules, and runtime feature discovery. Policy quality depends on deterministic multi-version dispatch, clear deprecation windows, and compatibility tests that prevent accidental reliance on unavailable extensions. Mature teams align policy with upstream profile initiatives and publish explicit migration guidance so ecosystem partners can ship one codebase across heterogeneous RISC-V deployments.

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

diagram

RISC-V PIPELINE DIAGRAM - Extension Compatibility Policy

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

diagram

DECODE CONTROL MAP - Extension Compatibility Policy

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

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