RISC-V Design · All levels

Extension Compatibility Policy: Theory Deep Dive

Theory Deep Dive for Extension Compatibility Policy.

Theory deep dive

Theory Deep Dive 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.

Theory matters when it predicts engineering outcomes. Tie abstract ISA and microarchitecture concepts to measurable behavior in trace and counter data.

Pipeline model

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

Privilege model

diagram

PRIVILEGE MODE STACK - Extension Compatibility Policy

            +------------------------------+
            | 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.

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