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How Chisel generates Verilog

  • Chisel is a Scala DSL, so the Chisel Compiler is written in Scala.
  • Chisel Compiler generates an intermediate language called FIR (Flexible Interpretation Representation).
  • FIR has nothing to do with Scala’s syntax
  • FIR is converted to Verilog using a converter called FIRRTL
  • FIRRTL is also written in Scala (FIR is not a Scala DSL, so I don’t think it needs to be written in Scala…) [^circt].
st=>inputoutput: Chisel(Scala)
op1=>subroutine: Chisel Compiler
io=>inputoutput: FIR(IR)
sub1=>subroutine: FIRRTL Compiler
cond=>inputoutput: Verilog


[^circt]: As a replacement for FIRRTL, a Chris Lattner-led tool called “CIRCT” has been developed using LLVM and MLIR

What’s good about Chisel?

  • You can use the features of Scala, a software description language.
  • → This results in Verilog engineers sometimes having to write seemingly incomprehensible and unintelligible statements.

Example: FIR Filter


Answer 1. By beginner

// stage size is temporary 4
class My4ElementFir(b0: Int, b1: Int, b2: Int, b3: Int) extends Module {
  val io = IO(new Bundle {
    val in = Input(UInt(8.W))
    val out = Output(UInt(8.W))

  val x_n1 = RegNext(, 0.U)
  val x_n2 = RegNext(x_n1, 0.U)
  val x_n3 = RegNext(x_n2, 0.U)
  io.out := * b0.U(8.W) + x_n1 * b1.U(8.W) +
             x_n2 * b2.U(8.W) + x_n3 * b3.U(8.W)

Answer 2. Written by me (hardware enginner)

Almost Verilog!

class MyManyDynamicElementVecFir(length: Int) extends Module {
  val io = IO(new Bundle {
    val in = Input(UInt(8.W))
    val valid = Input(Bool())
    val out = Output(UInt(8.W))
    val consts = Input(Vec(length, UInt(8.W)))

  val taps = Reg(Vec(length, UInt(8.W)));
  for (i <- 0 until length) {
    if (i == 0) { when(io.valid) { taps(i) :=     } }
    else        { when(io.valid) { taps(i) := taps(i-1) } }
  val w_fir_res = Wire(Vec(length, UInt(8.W)))
  for (i <- 0 until length) {
    val w_tap_mul = Wire(UInt(8.W))
    w_tap_mul := taps(i) * io.consts(i)
    if (i == 0) { w_fir_res(i) := 0.U }
    else        { w_fir_res(i) := w_tap_mul + w_fir_res(i-1) }
  io.out := w_fir_res(length-1)

Answer 3. Written by Chisel Expert

  • Can’t understand?

class MyManyDynamicElementVecFir(length: Int) extends Module {
  val io = IO(new Bundle {
    val in = Input(UInt(8.W))
    val valid = Input(Bool())
    val out = Output(UInt(8.W))
    val consts = Input(Vec(length, UInt(8.W)))

  val taps = Seq( ++ Seq.fill(io.consts.length - 1)(RegInit(0.U(8.W))) { case (a, b) => when (io.valid) { b := a } }

  io.out := { case (a, b) => a * b }.reduce(_ + _)

BTW, Why did Chisel choose Scala?

  • Rumor: When we built the first version of Chisel at UCB, we used Ruby and it compiled so slowly that we gave up.
  • Well, I don’t think Scala compiles fast either.
  • Scala’s feature: crazy operator overrides
  • Example 1 : A := B in normal Chisel description and A := B in TileLink have different meanings
 /** Connect this $coll to that $coll mono-directionally and element-wise.
    * This uses the [[MonoConnect]] algorithm.
    * @param that the $coll to connect to
    * @group Connect
  final def := (that: Data)(implicit sourceInfo: SourceInfo, connectionCompileOptions: CompileOptions): Unit = this.connect(that)(sourceInfo, connectionCompileOptions) // scalastyle:ignore line.size.limit
  • Example 2. Crazy AXI Bus connection (src/main/scala/subsystem/Ports.scala)
 mbus.coupleTo(s"memory_controller_port_named_$portName") {
      :*= AXI4UserYanker()
      :*= AXI4IdIndexer(idBits)
      :*= TLToAXI4()
      :*= TLWidthWidget(mbus.beatBytes)
      :*= _)

Back on topic: Why use Chisel?


Rant: If you want to make regular hardware, Verilog is definitely more convenient: !!!! .

  • Easier to write tests? → Isn’t UVM more sophisticated?
  • Does it have better error detection? → The simulator is good enough to detect common writing errors.
  • Reason for using Chisel in the cluster as seen by me
  • It’s unavoidable because Rocket-Chip is using it.
  • I want to write something in a new language and have everyone say “wow”.

So what’s the use of Chisel?

  • The evolution of technology has far outpaced the cycle of hardware design.
  • Hardware engineers, are you catching up with the evolution of technology?
  • It takes two to three years to make a chip. It takes two to three years to make a chip. Can you make a cutting-edge AI chip with this?
  • In the age of DSA, we have to build in low volume, high variety.
  • (Not that it matters.) The time should come when chips can be easily woken up by the weak in small quantities and high variety.
  • How do you manage the “complexity” of hardware?


An example of how to use Chisel: Diplomacy

  • Parameter Negotiation Framework for generating parameterized protocol implementation.


Where did Diplomacy come from?

What we want to achieve with Diplomacy

  • Flexible configuration using a single design
  • Multiple CPU configurations (RV64 / RV32) from a single design
  • Multiple CPU configurations (RV64 / RV32)
  • Multiple bus and cache configurations
    • Number of cores, bus width, cache size, SoC configuration


One example: SiFive’s Core IP Generator


If you want to develop with Chisel + Diplomacy, use IntelliJ IDEA!

Auto Completion feature is mandatory.


Misunderstandings: Diplomacy can only be used with TileLink?

No, Diplomacy is a Chisel library, so you don’t need to use TileLink to use it.

Example: Adder for Multiple Inputs connected with Diplomacy


Here are the three modules that we need to decipher

  • AdderDriver : A driver to send random values to the adder.
  • AdderNode : The adder itself, to which two or more AdderDrivers are connected to add all their values together and output the result.
  • AdderMonitor : A monitor. Monitor: A monitor that checks the values sent out by theAdderDriveragainst the results of addition generated by theAdderNode`.


Driver to send random values to the adder. Key points for reading and understanding AdderDriver: 1.

  1. What is val node = new AdderDriverNode? .
  2. what is new LazyModuleImp(this)? 3.
// class ModuleName extends LazyModule If you think of it as a module for connecting via Diplomacy.
class AdderDriver(width: Int, numOutputs: Int)(implicit p: Parameters) extends LazyModule {
    // 1. what is AdderDriverNode and what is the role of "node" in the first place?
    val node = new AdderDriverNode(Seq.fill(numOutputs)(DownwardParam(width)))

    // 2. What is LazyModuleImp() and what role does it have in Diplomacy?
    lazy val module = new LazyModuleImp(this) {
        // drive signals
        node.out.foreach { case (addend, _) => addend := randomAddend }

// AdderDriverNode is created by inheriting from the SourceNode class
class AdderDriverNode(widths: Seq[DownwardParam])(implicit valName: ValName)
  extends SourceNode(AdderNodeImp)(widths)
  • SourceNode is used for the node that will be the master.
  • SinkNode is used for nodes that are slaves.
  • AdapterNode connects multiple masters and multiple slaves. The number of masters and slaves should be the same.
  • NexusNode is used to connect multiple masters and multiple slaves. The number of masters and slaves does not have to be the same.

1. node : for connecting between modules

node is a communication port for communication between modules in Diplomacy. It has the role of exchanging various parameters between nodes and mediating the parameters.

There are two important elements in node.

  • bundle : Contains the I/O ports connected by Diplomacy. In this case, it just contains a bundle of signals for sending random values.
  • edge : Contains a set of parameters for connecting nodes of Diplomacy. These parameters are communicated between the nodes to make the final inter-node parameter adjustment (Diplomacy).
object AdderNodeImp extends SimpleNodeImp[DownwardParam, UpwardParam, EdgeParam, UInt] {
  // Edge : Contains a set of parameters for connecting Diplomacy nodes.
  def edge(pd: DownwardParam, pu: UpwardParam, p: Parameters, sourceInfo: SourceInfo) = {
    if (pd.width < pu.width) EdgeParam(pd.width) else EdgeParam(pu.width)

  // Contains the I/O ports connected by Diplomacy.
  // UInt(e.width.W) : e is the EdgeParam, i.e. the edge parameter
  def bundle(e: EdgeParam) = UInt(e.width.W)

  def render(e: EdgeParam) = RenderedEdge("blue", s "width = ${e.width}")

2. LazyModuleImp : The part that implements the circuit based on the information of node.

  1. LazyModuleImp : The part that implements the circuit based on the information of node.
  val node = new AdderDriverNode(Seq.fill(numOutputs)(DownwardParam(width)))

  lazy val module = new LazyModuleImp(this) {
    // Extract bit width information from all node edges
    val negotiatedWidths =
    // Do all inter-node connection lines have the same number of bits?
    require(negotiatedWidths.forall(_ == negotiatedWidths.head), "outputs must all have agreed on same width")

    // Use the head node's information if it is available (well, it doesn't matter which node's information is used)
    val finalWidth = negotiatedWidths.head

    // generate random numbers by FibonacciLFSR module
    val randomAddend = FibonacciLFSR.maxPeriod(finalWidth)

    // send random numbers to all output nodes
    node.out.foreach { case (addend, _) => addend := randomAddend }

If you use Diplomacy, use yEd to check the node graph diagram!


Figure. Diagram of the Adder graph generated by yEd.
  • A more complex example: Node connection diagram for a 4-core configuration in Freedom-SoC.


Figure. Connection diagram of freedom SoC nodes

Home-made CPU using TileLink + various bus connections

Try to make your own CPU.

  • I made my own CPU using Chisel, and connected it to TileLink to make a simple multi-core system.

CPU core I made: RV64 with simple 5-stage pipeline.

I made a CPU core that can run riscv-tests for now, though I haven’t tested it properly.

RISC-V pipeline processor designed with Chisel without any waveforms. I designed a pipeline processor - FPGA development diary

  • The fetch bus / data bus is very simple (non-TileLink / non-AXI bus) (link to InstBus) (link to DataBus)

  1. wrap your own CPU with LazyModule. 2.
  2. define a node of TileLink. 3.
  3. connect the node of TileLink to the node of your CPU.


Figure. Interface configuration of home-made CPU
  • There are two TileLink nodes available.
  • For instruction fetching
  • For data access
  • Both of them are TileLink, but I was forced to replace them because my own CPU has its own bus protocol.
  • Now, the CoreTop module is replaced with a module that has two TileLink interfaces.
    // Pull the ClientNode of the instruction/data bus defined in TileLink into the internal
    val (inst_out, inst_edge) = inst_node.out(0)
    val (data_out, data_edge) = data_node.out(0)
    val baseEnd = 0
    val (sizeEnd, sizeOff) = (inst_edge.bundle.sizeBits + baseEnd, baseEnd)
    val (sourceEnd, sourceOff) = (inst_edge.bundle.sourceBits + sizeEnd, sizeEnd)
    val beatBytes = inst_edge.bundle.dataBits

    // Define the instance of your own CPU
    val cpu = Module(new Cpu(conf, hartid))

    // Connect TileLink's fetch port signal to the fetch port of the home-made CPU (proprietary protocol)
    inst_out.a.valid :=
    inst_out.a.bits.address :=
    inst_out.a.bits.opcode := TLMessages.Get
    ... := inst_out.a.ready := inst_out.d.valid :=

    // Connect TileLink's data access port signal to the data access port (proprietary protocol) of the home-made CPU
    data_out.a.valid :=
    data_out.a.bits.address :=
    data_out.a.bits.opcode := TLMessages.Get
    ... := data_out.d.valid :=

Create a CoreTop and a CoreComplex module with a memory module and connect them.

class core_complex[Conf <: RVConfig]
  (conf: Conf, numCores: Int, ramBeatBytes: Int, txns: Int)(implicit p: Parameters)
  extends LazyModule {
  // Data loader for data access to memory from outside.
  val loader = LazyModule(new loader("loader"))

  // CPU cores (instantiate multiple for numCore)
  val core   = Seq.tabulate(numCores) { case i => LazyModule(new CoreTop(conf, i, "core" + i.toString)) }
  // TileLink crossbar
  val xbar   = LazyModule(new TLXbar)
  // memory with TileLink interface
  val memory = LazyModule(new TLRAM(AddressSet(0x80000000L, 0x0ffff), beatBytes = ramBeatBytes))

  // Connect the instruction fetch port and data port of the CPU core to the crossbar
  xbar.node := loader.node
  core.foreach { case (core) => {
    xbar.node := TLDelayer(0.1) := core.inst_node
    xbar.node := TLDelayer(0.1) := core.data_node
  // Connect the data loader to the crossbar
  memory.node := xbar.node


Figure. Connecting a home-made CPU to memory through TileLink to configure a simple SoC.

Set numCores to >1 for easy multi-core configuration.

With the above description of Chisel, you can easily make it multi-core. Just set nCores to 2, 3, … to nCores, and multiple CPUs will be connected to the crossbar automatically.


Figure. Connecting a home-made CPU to memory through TileLink. When connection=4
  • A simple simulation example. At first, the memory access is well arbitrated even when 4 cores request at the same time.


Figure . RTL simulation in a four-core configuration.
  • Example of Diplomacy node configuration with 4-core configuration


Figure. Diplomacy connection diagram for a four-core configuration.

Let’s learn how to connect the bus using RocketTile as an example.

Rocket-Chip Generator: Do you know why it’s named “Generator”?

  • Rocket-Chip is a comprehensive package that includes not only the design, but also Diplomacy for coordination between designs, Chisel implementation, and FIRRTL implementation itself.
  • It is called “Generator” because it includes not only the Scala implementation but also all the engines to generate the hardware.

Let’s look at the Diplomacy configuration of DefaultConfig of Rocket-Chip.

The bus connection of Rocket-Chip is basically configured using TileLink and Diplomacy.


Let’s break down little more.

Let’s take a look at the bus connections.

Connect devices using Diplomacy with RocketTile

  • RocketTile : One basic CPU core element that contains all the basic components such as RocketChip and DCache.
  • In CPU vendors, it is also called “CoreComplex”… Why do they call it “Core Complex”? I’m not sure.

(Simple) memory map by Rocket-Chip.

Start Address Stop Address Attributes Explanation
0x0000_0000 0x0000_1000 ARWX debug-controller@0
0x0000_3000 0x0000_4000 ARWX error-device@3000
0x0001_0000 0x0002_0000 RX rom@10000
0x0200_0000 0x0200_1000 ARW clint@2000000
0x0c00_0000 0x1000_0000 ARW interrupt_controller
0x6000_0000 0x8000_0000 RWX mmio-port-axi4@60000000
0x8000_0000 0x9000_0000 RWXC memory@8000_0000

Verilog structure of RocketTile

module RocketTile( // @[:freechips.rocketchip.system.DefaultConfig.fir@200426.2]

  TLXbar_8 tlMasterXbar ();

  IntXbar_4 intXbar ();

  DCache dcache ();

  Frontend frontend ();

  TLBuffer_9 buffer ();

  IntSyncCrossingSink intsink ();

  IntSyncCrossingSink_1 intsink_1 ();

  IntSyncCrossingSink_2 intsink_2 ();

  IntSyncCrossingSink_2 intsink_3 ();

  FPU fpuOpt ();

  HellaCacheArbiter dcacheArb ();

  PTW ptw ();

  Rocket core ();

I made a diagram by extracting the important parts. Blue is LazyModule, Red is NormalModule.


So, how can we decipher this from Scala? Let’s take a look at some examples to see how the nodes are connected.

  • Let’s take a look at RocketTile.scala.
class RocketTile private(
      val rocketParams: RocketTileParams,
      crossing: ClockCrossingType,
      lookup: LookupByHartIdImpl,
      q: Parameters,
      logicalTreeNode: LogicalTreeNode)
    extends BaseTile(rocketParams, crossing, lookup, q)
    with SinksExternalInterrupts
    with SourcesExternalNotifications
    with HasLazyRoCC  // implies CanHaveSharedFPU with CanHavePTW with HasHellaCache
    with HasHellaCache
    with HasICacheFrontend
/* ... */
    override lazy val module = new RocketTileModuleImp(this)
/* ... */
  • What is the implementation of RocketTileModuleImp?
// "outer" corresponds to the RocketTile above
class RocketTileModuleImp(outer: RocketTile) extends BaseTileModuleImp(outer)
    with HasFpuOpt
    with HasLazyRoCCModule
    with HasICacheFrontendModule {
/* ... */
  val core = Module(new Rocket(outer)(outer.p))
/* ... */ <>
  dcachePorts += // TODO outer.dcachePorts += () => ??

  /* For FPU, depending on options, connect */
  fpuOpt foreach { fpu => <> }

  /* Connect DCache to the data cache arbiter */ <> dcachePorts <> ptwPorts

  • FPU Connection (we are not using Diplomacy here)


  • DCache Connection


Let’s read and understand the configuration of the SoC part of Rocket-Chip.

  • When Verilog is generated by Rocket-Chip Generator, a top module called TestHarness is generated, which is a module for test execution.
  • This is also all generated from Chisel without the help of Verilog.
  • src/main/scala/system/TestHarness.scala
class TestHarness()(implicit p: Parameters) extends Module {
  val io = new Bundle {
    val success = Bool(OUTPUT)

  val ldut = LazyModule(new ExampleRocketSystem) // Instantiate ExampleRocketSystem.
  val dut = Module(ldut.module) // ExmapleRocketSystem is a LazyModule, so extract the entity inside it as dut.

  SimAXIMem.connectMem(ldut) // instantiate the object SimAXIMem and connect the memory port of ldut
  SimAXIMem.connectMMIO(ldut) // object Instantiate SimAXIMem and connect ldut's MMIO port
  ldut.l2_frontend_bus_axi4.foreach(_.tioff) // ldut's frontend port should be Tie Zero
  • SimAXIMem.connectMem(ldut)の部分を除いてみる
  def connectMem(dut: CanHaveMasterAXI4MemPort)(implicit p: Parameters): Seq[SimAXIMem] = {
    // mem_axi4 in ExampleRocketSystem shows the io port
    // is the SlavePort node of the LazyModule { case (io, (_, edge)) =>
      // Newly instantiate AXI4 memory
      // The size of the memory depends on the edge information of the AXI node
      val mem = LazyModule(new SimAXIMem(edge, size = p(ExtMem).get.master.size))
      // Connect the I/O port of the memory instance to the AXI port of the ldut
      mem.io_axi4.head <> io
  • AXI4メモリポートの仕組み (src/main/scala/subsystem/Ports.scala)
/** Adds a port to the system intended to master an AXI4 DRAM controller. */
trait CanHaveMasterAXI4MemPort { this: BaseSubsystem =>
  private val memPortParamsOpt = p(ExtMem)
  private val portName = "axi4"
  private val device = new MemoryDevice
  private val idBits =
  // Create a Diplomacy slave node for AXI4.
  // The reason why it is a slave is because the internal TileLink (master) is the master and the AXI4 node is the slave.
  val memAXI4Node = AXI4SlaveNode({ case MemoryPortParams(memPortParams, nMemoryChannels) =>
    Seq.tabulate(nMemoryChannels) { channel =>
      val base = AddressSet.misaligned(memPortParams.base, memPortParams.size)
      val filter = AddressSet(channel * mbus.blockBytes, ~((nMemoryChannels-1) * mbus.blockBytes))
  // Connect the node to the mbus (memory bus).
  mbus.coupleTo(s"memory_controller_port_named_$portName") {
    (memAXI4Node                        // AXI memory node
      :*= AXI4UserYanker()              // ↑ Add a User field to the AXI4 protocol
      :*= AXI4IdIndexer(idBits)         // ↑ Extend the ID bits of AXI4
      :*= TLToAXI4()                    // ↑ Change the protocol to AXI4
      :*= TLWidthWidget(mbus.beatBytes) // ↑ Change bus width of mbus(TileLink)
      :*= _)                            //  this means mbus

  val mem_axi4 = InModuleBody { memAXI4Node.makeIOs() }
  • Note : Why is the MMIO connected to the memory? This is a test environment, so the memory is automatically connected.


How to create a configurable module using Config / Parameter.

Motivation: Why does Rocket-Chip have so many variations of Verilog that can be generated?

$ make CONFIG=DefaultConfig         # Generate RV64GC configuration.
$ make CONFIG=RV32Config            # generate RV32GC configuration
$ make CONFIG=DualCoreConfig        # Generate SoC with 2-core configuration.
$ make CONFIG=RoCCExampleConfig     # Generate a configuration with RoCC (Rocket Custom Coprocessor).

Because each function is divided while maintaining Modularity.


Figure. In the Rocket-Chip + SoC configuration, each module is designed to be highly independent.
  • src/main/scala/system/Configs.scala
class BaseConfig extends Config(
  new WithDefaultMemPort() ++
  new WithDefaultMMIOPort() ++
  new WithDefaultSlavePort() ++
  new WithTimebase(BigInt(1000000)) ++ // 1 MHz
  new WithDTS("freechips,rocketchip-unknown", Nil) ++
  new WithNExtTopInterrupts(2) ++
  new BaseSubsystemConfig()

class DefaultConfig extends Config(new WithNBigCores(1) ++ new WithCoherentBusTopology ++ new BaseConfig)
class DefaultBufferlessConfig extends Config(new WithBufferlessBroadcastHub ++ new DefaultConfig)
class DefaultSmallConfig extends Config(new WithNSmallCores(1) ++ new WithCoherentBusTopology ++ new BaseConfig)
class DefaultRV32Config extends Config(new WithRV32 ++ new DefaultConfig)
class DualBankConfig extends Config(new WithNBanks(2) ++ new DefaultConfig)
class DualCoreConfig extends Config(new WithNBigCores(2) ++ new WithCoherentBusTopology ++ new BaseConfig)
class DualChannelConfig extends Config(new WithNMemoryChannels(2) ++ new DefaultConfig)
class EightChannelConfig extends Config(new WithNMemoryChannels(8) ++ new DefaultConfig)
class DualChannelDualBankConfig extends Config(
  new WithNMemoryChannels(2) ++
  new WithNBanks(4) ++ new DefaultConfig)
class RoccExampleConfig extends Config(new WithRoccExample ++ new DefaultConfig)

What does the above class DualBankConfig extends Config(new WithNBanks(2) ++ new DefaultConfig) mean? See about Config and Parameter.

Some techniques for passing parameters: Config

Concept of Config: build a reference table (map function) of “apply this parameter to this module”.

⇔ Verilog: “Put parameters down from the top / centralized management by package”.


図. Verilogのパラメータを階層で降ろしていく(なんとなくの)イメージ

In the case of Chisel’s Config: Create a dictionary of keys and get parameters from the keys you need in the module.

$ grep -h -R Key | grep Field
case object APBDebugRegistersKey extends Field[Map[Int, Seq[RegField]]](Map())
case object DebugModuleKey extends Field[Option[DebugModuleParams]](Some(DebugModuleParams()))
case object DebugModuleHartSelKey extends Field(DebugModuleHartSelFuncs())
case object JtagDTMKey extends Field[JtagDTMConfig](new JtagDTMKeyDefault())
case object CLINTKey extends Field[Option[CLINTParams]](None)
case object SubsystemDriveAsyncClockGroupsKey extends Field[Option[ClockGroupDriverParameters]](Some(ClockGroupDriverParameters(1)))
case object AsyncClockGroupsKey extends Field[ClockGroupEphemeralNode](ClockGroupEphemeralNode()(ValName("async_clock_groups")))
case object BroadcastKey extends Field(BroadcastParams())
case object RocketTilesKey extends Field[Seq[RocketTileParams]](Nil)
case object RocketCrossingKey extends Field[Seq[RocketCrossingParams]](List(RocketCrossingParams()))
case object PeripheryMaskROMKey extends Field[Seq[MaskROMParams]](Nil)
case object PSDTestModeBroadcastKey extends Field(

You can think of it as being like making a dictionary:.

Key Type Explain
APBDebugRegistersKey Map[Int, Seq[RegField]]
RocketTilesKey Seq[RocketTileParams]] Why are they enclosed in Seq ->
RocketTiles are placed in multiple locations in multi-core configurations, so RocketTileParams are duplicated for each Tile.
PeripheryMaskROMKey Seq[MaskROMParams]

For example, RocketTileParams is defined as follows: RocketTileParams is just like package in System Verilog?

case class RocketCoreParams(
  bootFreqHz: BigInt = 0,
  useVM: Boolean = true,
  useUser: Boolean = false,
  useDebug: Boolean = true,

Now let’s go back to the definition of DualCoreConfig again…

  • DualCoreConfig inherits from the Config class. You can think of the Config class as a set of dictionary table conversion functions based on the View. These sets of dictionary table conversion functions are finally aggregated as Parameter and passed to the design.


In each module, call the parameters defined here to get the concrete values.

  • src/main/scala/rocket/RocketCore.scala
trait HasRocketCoreParameters extends HasCoreParameters {
  lazy val rocketParams: RocketCoreParams = tileParams.core.asInstanceOf[RocketCoreParams]

  val fastLoadWord = rocketParams.fastLoadWord
  val fastLoadByte = rocketParams.fastLoadByte

  val mulDivParams = rocketParams.mulDiv.getOrElse(MulDivParams()) // TODO ask andrew about this

  require(!fastLoadByte || fastLoadWord)
class Rocket(tile: RocketTile)(implicit p: Parameters) extends CoreModule()(p)
    with HasRocketCoreParameters
    with HasCoreIO {

  val clock_en_reg = RegInit(true.B)
  // multiplier and divider
  val div = Module(new MulDiv(if (pipelinedMul) mulDivParams.copy(mulUnroll = 0) else mulDivParams, width = xLen)) := ex_reg_valid && ex_ctrl.div := ex_ctrl.alu_dw := ex_ctrl.alu_fn
  • src/main/scala/rocket/Multiplier.scala
class MulDiv(cfg: MulDivParams, width: Int, nXpr: Int = 32) extends Module {
  private def minDivLatency = (cfg.divUnroll > 0).option(if (cfg.divEarlyOut) 3 else 1 + w/cfg.divUnroll)
  private def minMulLatency = (cfg.mulUnroll > 0).option(if (cfg.mulEarlyOut) 2 else w/cfg.mulUnroll)
  def minLatency: Int = (minDivLatency ++ minMulLatency).min

  val io = IO(new MultiplierIO(width, log2Up(nXpr)))
  val req = Reg(chiselTypeOf(io.req.bits))
  val count = Reg(UInt(log2Ceil(
    ((cfg.divUnroll != 0).option(w/cfg.divUnroll + 1).toSeq ++
     (cfg.mulUnroll != 0).option(mulw/cfg.mulUnroll)).reduce(_ max _)).W))

How to use here()site()up(), etc.

  • If you want to use the value of another parameter to calculate a parameter used in Config, you can do the following in Verilog.
localparam param1 = 100;
localparam param2 = param1 * 100;

If you try to achieve this in Chisel, it will look like this. The BusWidthBytes parameter is used to calculate the address size.

class Bus128BitConfig extends Config((site, here, up) => {
  case BusWidthBytes => 128 / 8
  // BusWidthBytes is a case class, so it is not an integer
  case AddrSize => 0x100 * BusWidthBytes

class Bus64BitConfig extends Config((site, here, up) => {
  case BusWidthBytes => 64 / 8
  // BusWidthBytes is a case class, so it is not an integer
  case AddrSize => 0x200 * BusWidthBytes

[error] /home/msyksphinz/work/riscv/chisel-development/minimal-diplomacy/src/main/scala/core_complex/Configs.scala:11:26: overloaded method value * with alternatives:
[error]   (x: Double)Double <and>
[error]   (x: Float)Float <and>
[error]   (x: Long)Long <and>
[error]   (x: Int)Int <and>
[error]   (x: Char)Int <and>
[error]   (x: Short)Int <and>
[error]   (x: Byte)Int
[error]  cannot be applied to (core_complex.BusWidthBytes.type)
[error]   case AddrSize => 0x100 * BusWidthBytes  // <-- BusWidthBytes is not a number!
[error]                          ^

As you can see, BusWidthBytes itself is a case class, so you can’t use it directly as a value. So, we use information such as here() to refer to a value in the same Config for dictionary search.

class Bus128BitConfig extends Config((site, here, up) => {
  case BusWidthBytes => 128 / 8
  case AddrSize => 0x100 * here(BusWidthBytes)

class Bus64BitConfig extends Config((site, here, up) => {
  case BusWidthBytes => 64 / 8
  case AddrSize => 0x200 * here(BusWidthBytes)
// Bus128BitConfigで構成したTLRAM, 12ビットのバス幅で構成される
module TLRAM(
  input         clock,
  input         reset,
  input         auto_in_a_valid,
  input  [2:0]  auto_in_a_bits_opcode,
  input  [11:0] auto_in_a_bits_address,
  input  [31:0] auto_in_a_bits_data,
  output        auto_in_d_valid,
  output [2:0]  auto_in_d_bits_opcode,
  output [31:0] auto_in_d_bits_data

On the other hand, using site() will use a higher level Config. For example, Default2Config is configured as follows.

  • Case of using sites()Bus128BitConfig refers to BaseConfig when site() is used. So the AddrSize should be 0x100 * (256/8).
class Default2Config extends Config(
  new BaseConfig ++
  new Bus128BitConfig ++ new IfuNotConnectConfig
class BaseConfig extends Config((site, here, up) => {
  case BusWidthBytes => 256 / 8

class Bus128BitConfig extends Config((site, here, up) => {
  case BusWidthBytes => 128 / 8
  case AddrSize => 0x100 * here(BusWidthBytes)
// TLRAM configured with Bus128BitConfig, with a 12-bit bus width
module TLRAM(
  input          clock,
  input          reset,
  input          auto_in_a_valid,
  input  [2:0]   auto_in_a_bits_opcode,
  input  [12:0]  auto_in_a_bits_address,
  input  [255:0] auto_in_a_bits_data,
  output         auto_in_d_valid,
  output [2:0]   auto_in_d_bits_opcode,
  output [255:0] auto_in_d_bits_data
  • The case of using up(): In the following, two modules, Adder() and Mul(), are defined. The number of input/output bits is defined by p(BitWidth). We consider the problem of what to do if we want to define completely different bit widths for Adder() and Mul().
class Adder()(implicit p: Parameters) extends Module {
  val bitwidth = p(BitWidth)
  val io = IO(new Bundle{
    val a = Input(SInt(bitwidth.W))
    val b = Input(SInt(bitwidth.W))
    val c = Output(SInt(bitwidth.W))

  io.c := io.a + io.b

class Mul()(implicit p: Parameters) extends Module {
  val bitwidth = p(BitWidth)
  val io = IO(new Bundle{
    val a = Input(SInt(bitwidth.W))
    val b = Input(SInt(bitwidth.W))
    val c = Output(SInt((bitwidth * 2).W))

  io.c := io.a * io.b

At this time, the parameters of Adder() and Mul() are defined as follows. Adder() is assumed to be instantiated with 64 bits, and Mul() with 128 bits input bit width.

case object AdderBitWidth extends Field[Int]
case object MulBitWidth extends Field[Int]

class DefaultConfig() extends Config((site, here, up) => {
  case AdderBitWidth => 64
  case MulBitWidth => 128

We override the parameters of Adder() and Mul() when we instantiate them, but we have to define and pass adder_param and mul_param anew.

  val Adder_mod = Module(new Adder()(adder_params)) := io.a := io.b
  io.Adder_c :=

  val mul_mod = Module(new Mul()(mul_params)) := io.a := io.b
  io.mul_c :=

Here, adder_params is inherited from the above DefaultConfig, while the BitWidth parameter is inherited from the parameter used in the previous DefaultConfig. Use up() at this time. This will create:

  val adder_params = p.alter((site, here, up) => {
    case BitWidth => up(AdderBitWidth)

  val mul_params = p.alter((site, here, up) => {
    case BitWidth => up(MulBitWidth)

The BitWidth of adder_params will inherit the AdderBitWidth of DefaultConfig, and the BitWidth of mul_params will inherit the MulBitWidth of DefaultConfig. This makes it possible to inherit different parameters for the same BitWidth parameter.

The results are as follows: the input bit width of Adder is 64 bits, inherited from AddrBitWidth, and the input bit width of Mul is 128 bits, inherited from MulBitWidth.

module Adder(
  input  [63:0] io_a,
  input  [63:0] io_b,
  output [63:0] io_c
  assign io_c = $signed(io_a) + $signed(io_b); // @[up_example.scala 48:8]
module Mul(
  input  [127:0] io_a,
  input  [127:0] io_b,
  output [255:0] io_c
  assign io_c = $signed(io_a) * $signed(io_b); // @[up_example.scala 59:8]


  • Chisel usage points → Network part, not in the core. The part where flexibility is required.
  • Using Diplomacy to connect to the bus network of your own CPU
  • The bus part of Rocket-Chip is built with a lot of Diplomacy bus configuration.
  • More flexible parameterization with Config/Parameter.

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