en arch microarch

Microarchitecture

Overall

Please refer to the following link.

• An Open Source FPGA-Optimized Out-of-Order RISC-V Soft Processor
• 面積効率を指向するプロセッサ「雷上動」の設計と実装 (Japanese)

About the zero register

The zero register is implemented as follows.

• An instruction that writes to the zero register is cleared the register write flag
  • Determine whether it is zero register when writing to opInfo.writeReg of Decoder.sv
• Register file is initialized by reset circuit so that all zeros are written
  • This process is written in RegisterFile.sv
• In RMT (Register Map Table), the zero register is assigned to physical register's entry 0 when initialized and this entry is not released

Reading the zero register is actually performed from the physical register, but value 0 is always read due to the above conditions

Handling exceptions

Related signals

• Mainly related units

  • Privileged/CSR_Unit.sv
  • Privileged/CSR_UnitIF.sv
  • Pipeline/MemoryBackEnd/MemoryTagAccessStage.sv
    • This module judges whether an exception occurred

• csrUnitIF.triggerInterrupt

  • Interrupt: triggered by external factors, such as timers, independent of instruction execution

• csrUnitIF.triggerExcpt

  • Exception: triggered as the instruction executes
  • Exception is further divided into fault and trap
    • fault: address biolation, etc.
    • trap: system call, etc.
  • Determine the type of exception with csrUnitIF.excptCause.
    • If csrUnitIF.excptCause is EXEC_STATE_TRAP_ECALL, trigger trap
    • If csrUnitIF.excptCause is EXEC_STATE_FAULT_LOAD_MISALIGNED, trigger fault, and so on.

Flow when a fault occurs

• Address check is performed in Pipeline/MemoryBackEnd/MemoryTagAccessStage.sv

  • Fault is registered in ExecState at this time

  // If the execution is completed correctly, check whether the fault occurred
  if (nextStage[i].execState == EXEC_STATE_SUCCESS) begin
      if (isLoad[i]) begin
          if (pipeReg[i].memMapType == MMT_ILLEGAL)
              nextStage[i].execState = EXEC_STATE_FAULT_LOAD_VIOLATION;
          else if (IsMisalignedAddress(pipeReg[i].addrOut, iqData[i].memOpInfo.memAccessMode.size))
              nextStage[i].execState = EXEC_STATE_FAULT_LOAD_MISALIGNED;
      end
      else if (isStore[i]) begin
          if (pipeReg[i].memMapType == MMT_ILLEGAL)
              nextStage[i].execState = EXEC_STATE_FAULT_STORE_VIOLATION;
          else if (IsMisalignedAddress(pipeReg[i].addrOut, iqData[i].memOpInfo.memAccessMode.size))
              nextStage[i].execState = EXEC_STATE_FAULT_STORE_MISALIGNED;
      end
  end
  

• ExecState is written to the ActiveList in write back stage

• Wait until the faulting instruction is at the top of the ActiveList

  • Faults are not subject to runtime recovery

• Detecting faulted instruction in the commit stage

• Send a recovery request to the RecoveryManager

  • In recovery, the instruction caused the fault is flushed as well

• Send a trigger request of fault to the CSR_Unit from the RecoveryManager

• Update the csr and PC according to the fault that generated CSR_Unit

  @ CSR_Unit.sv
  csrNext.mepc = port.excptCauseAddr; // Write the PC where the exception occurred
  csrNext.mtval = port.excptCauseDataAddr;// PC for ECALL / EBREAK?
  csrNext.mcause.isInterrupt = FALSE;
  csrNext.mcause.code.trapCode = ToTrapCodeFromExecState(port.excptCause);