Fetch in Gem5

Fetch state machine

Next PC

• src/cpu/o3/fetch.cc:

  void Fetch::fetch(bool &status_change) {
    ...
    while (numInst < fetchWidth && fetchQueue[tid].size() < fetchQueueSize
      && !predictedBranch && !quiesce) {
      ...
      do {
        ...
        predictedBranch |= lookupAndUpdateNextPC(instruction, *next_pc);
        ...
      } while ((curMacroop || dec_ptr->instReady()) &&
        numInst < fetchWidth &&
        fetchQueue[tid].size() < fetchQueueSize
      );
      ...
    }
    ...
  }
  

  • src/cpu/o3/fetch.cc:

    Fetch::lookupAndUpdateNextPC(const DynInstPtr &inst, PCStateBase &next_pc) {
      ...
      if (!inst->isControl()) {
        inst->staticInst->advancePC(next_pc);
        ...
      }
      ...
    }
    

    • src/arch/x86/insts/microop.hh:

      void advancePC(PCStateBase &pcState) const override {
        auto &xpc = pcState.as<PCState>();
          if (flags[IsLastMicroop])
              xpc.uEnd();
          else
              xpc.uAdvance();
      }