【计算机组成与设计】Chisel取指和指令译码设计

本次试验分为三个部分：

目录

设计译码电路

设计寄存器文件

实现一个32个字的指令存储器

---

设计译码电路

输入位32bit的一个机器字，按照课本MIPS 指令格式，完成add、sub、lw、sw指令译码，其他指令一律译码成nop指令。输入信号名为Instr_word，对上述四条指令义译码输出信号名为add_op、sub_op、lw_op和sw_op，其余指令一律译码为nop；

给出Chisel设计代码和仿真测试波形，观察输入Instr_word为add R1,R2,R3; sub R0,R5,R6，lw R5,100(R2), sw R5,104(R2)、JAL RA,100(R2)时，对应的输出波形

Decode.scala

import chisel3._
 
class Decoder extends Module {
  val io = IO(new Bundle {
    val Instr_word = Input(UInt(32.W))
    val add_op = Output(Bool())
    val sub_op = Output(Bool())
    val lw_op = Output(Bool())
    val sw_op = Output(Bool())
    val nop_op = Output(Bool())
  })
  // 定义操作码
  val OPCODE_ADD = "b000000".U
  val OPCODE_SUB = "b000000".U
  val OPCODE_LW = "b100011".U
  val OPCODE_SW = "b101011".U
  //定义功能码
  val FUNCT_ADD = "b100000".U
  val FUNCT_SUB = "b100010".U
  // 提取MIPS指令的操作码
  val opcode = io.Instr_word(31, 26)
  //提取MIPS指令的功能码
  val funct = io.Instr_word(5, 0)
  // 译码
  io.add_op := opcode === OPCODE_ADD && funct === FUNCT_ADD
  io.sub_op := opcode === OPCODE_SUB && funct === FUNCT_SUB
  io.lw_op := opcode === OPCODE_LW
  io.sw_op := opcode === OPCODE_SW
  io.nop_op := !(io.add_op || io.sub_op || io.lw_op || io.sw_op)
}
 
object Decoder extends App {
  (new chisel3.stage.ChiselStage).emitVerilog(new Decoder())
}

 DecoderTest.scala

import chiseltest._
import org.scalatest.flatspec.AnyFlatSpec
import chisel3._
 
class DecoderTest extends AnyFlatSpec with ChiselScalatestTester {
  behavior of "Decoder"
  it should "correctly decode instructions" in {
    test(new Decoder).withAnnotations(Seq(WriteVcdAnnotation)) { c =>
      // Test instructions
      val addInstruction = "b000000_00010_00011_00001_00000_100000".U
      val subInstruction = "b000000_00101_00110_00000_00000_100010".U
      val lwInstruction = "b100011_00010_00101_0000000001100100".U
      val swInstruction = "b101011_00010_00101_0000000001101000".U
      val jalInstruction = "b000011_00000_00000000000000000000".U
      // Set the input instruction and evaluate the decoder
      c.io.Instr_word.poke(addInstruction)
      c.clock.step()
      c.io.add_op.expect(true)
      c.io.sub_op.expect(false)
      c.io.lw_op.expect(false)
      c.io.sw_op.expect(false)
      c.io.nop_op.expect(false)
 
      c.io.Instr_word.poke(subInstruction)
      c.clock.step()
      c.io.add_op.expect(false)
      c.io.sub_op.expect(true)
      c.io.lw_op.expect(false)
      c.io.sw_op.expect(false)
      c.io.nop_op.expect(false)
 
      c.io.Instr_word.poke(lwInstruction)
      c.clock.step()
      c.io.add_op.expect(false)
      c.io.sub_op.expect(false)
      c.io.lw_op.expect(true)
      c.io.sw_op.expect(false)
      c.io.nop_op.expect(false)
 
      c.io.Instr_word.poke(swInstruction)
      c.clock.step()
      c.io.add_op.expect(false)
      c.io.sub_op.expect(false)
      c.io.lw_op.expect(false)
      c.io.sw_op.expect(true)
      c.io.nop_op.expect(false)
 
      c.io.Instr_word.poke(jalInstruction)
      c.clock.step()
      c.io.add_op.expect(false)
      c.io.sub_op.expect(false)
      c.io.lw_op.expect(false)
      c.io.sw_op.expect(false)
      c.io.nop_op.expect(true)
    }
  }
}

设计寄存器文件

共32个32bit寄存器，允许两读一写，且0号寄存器固定读出位0。四个输入信号为RS1、RS2、WB_data、Reg_WB，寄存器输出RS1_out和RS2_out；寄存器内部保存的初始数值等同于寄存器编号

给出Chisel设计代码和仿真测试波形，观察RS1=5,RS2=8，WB_data=0x1234,Reg_WB=1的输出波形和受影响寄存器的值。

Register.scala

import chisel3._
import chisel3.util._
 
class RegisterFile extends Module {
  val io = IO(new Bundle {
    val RS1 = Input(UInt(5.W)) // RS1输入信号，用于选择要读取的寄存器
    val RS2 = Input(UInt(5.W)) // RS2输入信号，用于选择要读取的寄存器
    val WB_data = Input(UInt(32.W)) // 写入数据信号，用于写入寄存器
    val Reg_WB = Input(UInt(5.W)) // 选择写入数据的寄存器
    val RS1_out = Output(UInt(32.W)) // RS1输出数据
    val RS2_out = Output(UInt(32.W)) // RS2输出数据
  })
 
  val registers = RegInit(VecInit((0 until 32).map(_.U(32.W)))) // 32个32位寄存器，初始值等于寄存器编号
  registers(io.Reg_WB) := io.WB_data // 写入数据到寄存器
  io.RS1_out := Mux(io.RS1 === 0.U, 0.U, registers(io.RS1)) // RS1输出数据，0号寄存器固定读出位0
  io.RS2_out := Mux(io.RS2 === 0.U, 0.U, registers(io.RS2)) // RS2输出数据，0号寄存器固定读出位0
}
 
object RegisterFile extends App {
  (new chisel3.stage.ChiselStage).emitVerilog(new RegisterFile())
}

RegisterTest.scala

import chisel3._
import chiseltest._
import org.scalatest.flatspec.AnyFlatSpec
import chisel3.util._
 
class RegisterFileTest extends AnyFlatSpec with ChiselScalatestTester {
  behavior of "RegisterFile"
  it should "correctly update and read registers" in {
    test(new RegisterFile).withAnnotations(Seq(WriteVcdAnnotation)) { c =>
      // 设置输入信号
      c.io.RS1.poke(5.U)
      c.io.RS2.poke(8.U)
      c.io.WB_data.poke(0x1234.U)
      c.io.Reg_WB.poke(1.U)
 
      c.clock.step()
      c.io.RS1_out.expect(5.U)
      c.io.RS2_out.expect(8.U)
    }
  }
}
 

实现一个32个字的指令存储器

从0地址分别存储4条指令add R1,R2,R3; sub R0,R5,R6，lw R5,100(R2), sw R5,104(R2)。然后组合指令存储器、寄存器文件、译码电路，并结合PC更新电路（PC初值为0）、WB_data和Reg_WB信号产生电路，最终让电路能逐条指令取出、译码（不需要完成指令执行）。

给出Chisel设计代码和仿真测试波形，观察四条指令的执行过程波形，记录并解释其含义。

InstructionMemory.scala

import chisel3._
 
class InstructionMemory extends Module {
  val io = IO(new Bundle {
    val address = Input(UInt(5.W)) // 32个字，需要5位地址
    val instruction = Output(UInt(32.W))
  })
  // 创建一个32个字的指令存储器
  val mem = Mem(32, UInt(32.W))
  // 初始化存储器，存储MIPS指令
  mem.write(0.U, "b000000_00010_00011_00001_00000_100000".U) // add R1, R2, R3
  mem.write(1.U, "b000000_00101_00110_00000_00000_100010".U) // sub R0, R5, R6
  mem.write(2.U, "b100011_00010_00101_0000000001100100".U) // lw R5, 100(R2)
  mem.write(3.U, "b101011_00010_00101_0000000001101000".U) // sw R5, 104(R2)
  // 从存储器中读取指令
  io.instruction := mem.read(io.address)
}

Circuit.scala

import chisel3._
import chisel3.util._
 
class Circuit extends Module {
  val io = IO(new Bundle {
    // 寄存器的输入输出
    val WB_data = Input(UInt(32.W)) // 写入数据信号，用于写入寄存器
    val Reg_WB = Input(UInt(5.W)) // 选择写入数据的寄存器
    val RS1_out = Output(UInt(32.W))
    val RS2_out = Output(UInt(32.W))
    // 译码
    val add_op = Output(Bool())
    val sub_op = Output(Bool())
    val lw_op = Output(Bool())
    val sw_op = Output(Bool())
    val nop_op = Output(Bool())
  })
 
  val instructionMemory = Module(new InstructionMemory)
  val registerFile = Module(new RegisterFile)
  val decoder = Module(new Decoder)
  val pc = RegInit(0.U(5.W))
  // 根据pc的值取出指令寄存器相应指令
  instructionMemory.io.address := pc
  decoder.io.Instr_word := instructionMemory.io.instruction
  registerFile.io.RS1 := instructionMemory.io.instruction(25, 21)
  registerFile.io.RS2 := instructionMemory.io.instruction(20, 16)
  registerFile.io.WB_data := (0.U(32.W))
  registerFile.io.Reg_WB := (0.U(5.W))
  // 更新输出
  io.RS1_out := registerFile.io.RS1_out
  io.RS2_out := registerFile.io.RS2_out
  io.add_op := decoder.io.add_op
  io.sub_op := decoder.io.sub_op
  io.lw_op := decoder.io.lw_op
  io.sw_op := decoder.io.sw_op
  io.nop_op := decoder.io.nop_op
  // 更新PC
  pc := pc + 1.U
}
 
object Circuit extends App {
  (new chisel3.stage.ChiselStage).emitVerilog(new Circuit())
}

CircuitTest.scala

import chiseltest._
import org.scalatest.flatspec.AnyFlatSpec
import chisel3._
 
class CircuitTest extends AnyFlatSpec with ChiselScalatestTester {
  behavior of "Circuit"
  it should "correct circuit" in {
    test(new Circuit).withAnnotations(Seq(WriteVcdAnnotation)) { c =>
      c.clock.step()
      c.clock.step()
      c.clock.step()
      c.clock.step()
    }
  }
}