rocket-chip/src/main/scala/rocket/RocketCore.scala

// See LICENSE.Berkeley for license details.
// See LICENSE.SiFive for license details.

package freechips.rocketchip.rocket

import Chisel._
import Chisel.ImplicitConversions._
import chisel3.core.withReset
import freechips.rocketchip.config.Parameters
import freechips.rocketchip.tile._
import freechips.rocketchip.util._
import freechips.rocketchip.util.property._
import scala.collection.immutable.ListMap
import scala.collection.mutable.ArrayBuffer

case class RocketCoreParams(
  bootFreqHz: BigInt = 0,
  useVM: Boolean = true,
  useUser: Boolean = false,
  useDebug: Boolean = true,
  useAtomics: Boolean = true,
  useAtomicsOnlyForIO: Boolean = false,
  useCompressed: Boolean = true,
  nLocalInterrupts: Int = 0,
  nBreakpoints: Int = 1,
  nPMPs: Int = 8,
  nPerfCounters: Int = 0,
  haveBasicCounters: Boolean = true,
  misaWritable: Boolean = true,
  nL2TLBEntries: Int = 0,
  mtvecInit: Option[BigInt] = Some(BigInt(0)),
  mtvecWritable: Boolean = true,
  fastLoadWord: Boolean = true,
  fastLoadByte: Boolean = false,
  tileControlAddr: Option[BigInt] = None,
  mulDiv: Option[MulDivParams] = Some(MulDivParams()),
  fpu: Option[FPUParams] = Some(FPUParams())
) extends CoreParams {
  val fetchWidth: Int = if (useCompressed) 2 else 1
  //  fetchWidth doubled, but coreInstBytes halved, for RVC:
  val decodeWidth: Int = fetchWidth / (if (useCompressed) 2 else 1)
  val retireWidth: Int = 1
  val instBits: Int = if (useCompressed) 16 else 32
}

trait HasRocketCoreParameters extends HasCoreParameters {
  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(implicit p: Parameters) extends CoreModule()(p)
    with HasRocketCoreParameters
    with HasCoreIO {

  // performance counters
  def pipelineIDToWB[T <: Data](x: T): T =
    RegEnable(RegEnable(RegEnable(x, !ctrl_killd), ex_pc_valid), mem_pc_valid)
  val perfEvents = new EventSets(Seq(
    new EventSet((mask, hits) => Mux(mask(0), wb_xcpt, wb_valid && pipelineIDToWB((mask & hits).orR)), Seq(
      ("exception", () => false.B),
      ("load", () => id_ctrl.mem && id_ctrl.mem_cmd === M_XRD && !id_ctrl.fp),
      ("store", () => id_ctrl.mem && id_ctrl.mem_cmd === M_XWR && !id_ctrl.fp),
      ("amo", () => Bool(usingAtomics) && id_ctrl.mem && (isAMO(id_ctrl.mem_cmd) || id_ctrl.mem_cmd.isOneOf(M_XLR, M_XSC))),
      ("system", () => id_ctrl.csr =/= CSR.N),
      ("arith", () => id_ctrl.wxd && !(id_ctrl.jal || id_ctrl.jalr || id_ctrl.mem || id_ctrl.fp || id_ctrl.div || id_ctrl.csr =/= CSR.N)),
      ("branch", () => id_ctrl.branch),
      ("jal", () => id_ctrl.jal),
      ("jalr", () => id_ctrl.jalr))
      ++ (if (!usingMulDiv) Seq() else Seq(
        ("mul", () => id_ctrl.div && (id_ctrl.alu_fn & ALU.FN_DIV) =/= ALU.FN_DIV),
        ("div", () => id_ctrl.div && (id_ctrl.alu_fn & ALU.FN_DIV) === ALU.FN_DIV)))
      ++ (if (!usingFPU) Seq() else Seq(
        ("fp load", () => id_ctrl.fp && io.fpu.dec.ldst && io.fpu.dec.wen),
        ("fp store", () => id_ctrl.fp && io.fpu.dec.ldst && !io.fpu.dec.wen),
        ("fp add", () => id_ctrl.fp && io.fpu.dec.fma && io.fpu.dec.swap23),
        ("fp mul", () => id_ctrl.fp && io.fpu.dec.fma && !io.fpu.dec.swap23 && !io.fpu.dec.ren3),
        ("fp mul-add", () => id_ctrl.fp && io.fpu.dec.fma && io.fpu.dec.ren3),
        ("fp div/sqrt", () => id_ctrl.fp && (io.fpu.dec.div || io.fpu.dec.sqrt)),
        ("fp other", () => id_ctrl.fp && !(io.fpu.dec.ldst || io.fpu.dec.fma || io.fpu.dec.div || io.fpu.dec.sqrt))))),
    new EventSet((mask, hits) => (mask & hits).orR, Seq(
      ("load-use interlock", () => id_ex_hazard && ex_ctrl.mem || id_mem_hazard && mem_ctrl.mem || id_wb_hazard && wb_ctrl.mem),
      ("long-latency interlock", () => id_sboard_hazard),
      ("csr interlock", () => id_ex_hazard && ex_ctrl.csr =/= CSR.N || id_mem_hazard && mem_ctrl.csr =/= CSR.N || id_wb_hazard && wb_ctrl.csr =/= CSR.N),
      ("I$ blocked", () => icache_blocked),
      ("D$ blocked", () => id_ctrl.mem && dcache_blocked),
      ("branch misprediction", () => take_pc_mem && mem_direction_misprediction),
      ("control-flow target misprediction", () => take_pc_mem && mem_misprediction && mem_cfi && !mem_direction_misprediction && !icache_blocked),
      ("flush", () => wb_reg_flush_pipe),
      ("replay", () => replay_wb))
      ++ (if (!usingMulDiv) Seq() else Seq(
        ("mul/div interlock", () => id_ex_hazard && ex_ctrl.div || id_mem_hazard && mem_ctrl.div || id_wb_hazard && wb_ctrl.div)))
      ++ (if (!usingFPU) Seq() else Seq(
        ("fp interlock", () => id_ex_hazard && ex_ctrl.fp || id_mem_hazard && mem_ctrl.fp || id_wb_hazard && wb_ctrl.fp || id_ctrl.fp && id_stall_fpu)))),
    new EventSet((mask, hits) => (mask & hits).orR, Seq(
      ("I$ miss", () => io.imem.perf.acquire),
      ("D$ miss", () => io.dmem.perf.acquire),
      ("D$ release", () => io.dmem.perf.release),
      ("ITLB miss", () => io.imem.perf.tlbMiss),
      ("DTLB miss", () => io.dmem.perf.tlbMiss),
      ("L2 TLB miss", () => io.ptw.perf.l2miss)))))

  val decode_table = {
    (if (usingMulDiv) new MDecode +: (xLen > 32).option(new M64Decode).toSeq else Nil) ++:
    (if (usingAtomics) new ADecode +: (xLen > 32).option(new A64Decode).toSeq else Nil) ++:
    (if (fLen >= 32) new FDecode +: (xLen > 32).option(new F64Decode).toSeq else Nil) ++:
    (if (fLen >= 64) new DDecode +: (xLen > 32).option(new D64Decode).toSeq else Nil) ++:
    (usingRoCC.option(new RoCCDecode)) ++:
    ((xLen > 32).option(new I64Decode)) ++:
    (usingVM.option(new SDecode)) ++:
    (usingDebug.option(new DebugDecode)) ++:
    Seq(new IDecode)
  } flatMap(_.table)

  val ex_ctrl = Reg(new IntCtrlSigs)
  val mem_ctrl = Reg(new IntCtrlSigs)
  val wb_ctrl = Reg(new IntCtrlSigs)

  val ex_reg_xcpt_interrupt  = Reg(Bool())
  val ex_reg_valid           = Reg(Bool())
  val ex_reg_rvc             = Reg(Bool())
  val ex_reg_btb_resp        = Reg(new BTBResp)
  val ex_reg_xcpt            = Reg(Bool())
  val ex_reg_flush_pipe      = Reg(Bool())
  val ex_reg_load_use        = Reg(Bool())
  val ex_reg_cause           = Reg(UInt())
  val ex_reg_replay = Reg(Bool())
  val ex_reg_pc = Reg(UInt())
  val ex_reg_inst = Reg(Bits())
  val ex_reg_raw_inst = Reg(UInt())

  val mem_reg_xcpt_interrupt  = Reg(Bool())
  val mem_reg_valid           = Reg(Bool())
  val mem_reg_rvc             = Reg(Bool())
  val mem_reg_btb_resp        = Reg(new BTBResp)
  val mem_reg_xcpt            = Reg(Bool())
  val mem_reg_replay          = Reg(Bool())
  val mem_reg_flush_pipe      = Reg(Bool())
  val mem_reg_cause           = Reg(UInt())
  val mem_reg_slow_bypass     = Reg(Bool())
  val mem_reg_load            = Reg(Bool())
  val mem_reg_store           = Reg(Bool())
  val mem_reg_sfence = Reg(Bool())
  val mem_reg_pc = Reg(UInt())
  val mem_reg_inst = Reg(Bits())
  val mem_reg_raw_inst = Reg(UInt())
  val mem_reg_wdata = Reg(Bits())
  val mem_reg_rs2 = Reg(Bits())
  val mem_br_taken = Reg(Bool())
  val take_pc_mem = Wire(Bool())

  val wb_reg_valid           = Reg(Bool())
  val wb_reg_xcpt            = Reg(Bool())
  val wb_reg_replay          = Reg(Bool())
  val wb_reg_flush_pipe      = Reg(Bool())
  val wb_reg_cause           = Reg(UInt())
  val wb_reg_sfence = Reg(Bool())
  val wb_reg_pc = Reg(UInt())
  val wb_reg_inst = Reg(Bits())
  val wb_reg_raw_inst = Reg(UInt())
  val wb_reg_wdata = Reg(Bits())
  val wb_reg_rs2 = Reg(Bits())
  val take_pc_wb = Wire(Bool())

  val take_pc_mem_wb = take_pc_wb || take_pc_mem
  val take_pc = take_pc_mem_wb

  // decode stage
  val ibuf = Module(new IBuf)
  val id_expanded_inst = ibuf.io.inst.map(_.bits.inst)
  val id_raw_inst = ibuf.io.inst.map(_.bits.raw)
  val id_inst = id_expanded_inst.map(_.bits)
  ibuf.io.imem <> io.imem.resp
  ibuf.io.kill := take_pc

  require(decodeWidth == 1 /* TODO */ && retireWidth == decodeWidth)
  val id_ctrl = Wire(new IntCtrlSigs()).decode(id_inst(0), decode_table)
  val id_raddr3 = id_expanded_inst(0).rs3
  val id_raddr2 = id_expanded_inst(0).rs2
  val id_raddr1 = id_expanded_inst(0).rs1
  val id_waddr  = id_expanded_inst(0).rd
  val id_load_use = Wire(Bool())
  val id_reg_fence = Reg(init=Bool(false))
  val id_ren = IndexedSeq(id_ctrl.rxs1, id_ctrl.rxs2)
  val id_raddr = IndexedSeq(id_raddr1, id_raddr2)
  val rf = new RegFile(31, xLen)
  val id_rs = id_raddr.map(rf.read _)
  val ctrl_killd = Wire(Bool())
  val id_npc = (ibuf.io.pc.asSInt + ImmGen(IMM_UJ, id_inst(0))).asUInt

  val csr = Module(new CSRFile(perfEvents))
  val id_csr_en = id_ctrl.csr.isOneOf(CSR.S, CSR.C, CSR.W)
  val id_system_insn = id_ctrl.csr >= CSR.I
  val id_csr_ren = id_ctrl.csr.isOneOf(CSR.S, CSR.C) && id_raddr1 === UInt(0)
  val id_csr = Mux(id_csr_ren, CSR.R, id_ctrl.csr)
  val id_sfence = id_ctrl.mem && id_ctrl.mem_cmd === M_SFENCE
  val id_csr_flush = id_sfence || id_system_insn || (id_csr_en && !id_csr_ren && csr.io.decode(0).write_flush)

  val id_illegal_insn = !id_ctrl.legal ||
    id_ctrl.div && !csr.io.status.isa('m'-'a') ||
    id_ctrl.amo && !csr.io.status.isa('a'-'a') ||
    id_ctrl.fp && (csr.io.decode(0).fp_illegal || io.fpu.illegal_rm) ||
    id_ctrl.dp && !csr.io.status.isa('d'-'a') ||
    ibuf.io.inst(0).bits.rvc && !csr.io.status.isa('c'-'a') ||
    id_ctrl.rocc && csr.io.decode(0).rocc_illegal ||
    id_csr_en && (csr.io.decode(0).read_illegal || !id_csr_ren && csr.io.decode(0).write_illegal) ||
    !ibuf.io.inst(0).bits.rvc && ((id_sfence || id_system_insn) && csr.io.decode(0).system_illegal)
  // stall decode for fences (now, for AMO.aq; later, for AMO.rl and FENCE)
  val id_amo_aq = id_inst(0)(26)
  val id_amo_rl = id_inst(0)(25)
  val id_fence_next = id_ctrl.fence || id_ctrl.amo && id_amo_rl
  val id_mem_busy = !io.dmem.ordered || io.dmem.req.valid
  when (!id_mem_busy) { id_reg_fence := false }
  val id_rocc_busy = Bool(usingRoCC) &&
    (io.rocc.busy || ex_reg_valid && ex_ctrl.rocc ||
     mem_reg_valid && mem_ctrl.rocc || wb_reg_valid && wb_ctrl.rocc)
  val id_do_fence = Wire(init = id_rocc_busy && id_ctrl.fence ||
    id_mem_busy && (id_ctrl.amo && id_amo_aq || id_ctrl.fence_i || id_reg_fence && (id_ctrl.mem || id_ctrl.rocc)))

  val bpu = Module(new BreakpointUnit(nBreakpoints))
  bpu.io.status := csr.io.status
  bpu.io.bp := csr.io.bp
  bpu.io.pc := ibuf.io.pc
  bpu.io.ea := mem_reg_wdata

  val id_xcpt0 = ibuf.io.inst(0).bits.xcpt0
  val id_xcpt1 = ibuf.io.inst(0).bits.xcpt1
  val (id_xcpt, id_cause) = checkExceptions(List(
    (csr.io.interrupt, csr.io.interrupt_cause),
    (bpu.io.debug_if,  UInt(CSR.debugTriggerCause)),
    (bpu.io.xcpt_if,   UInt(Causes.breakpoint)),
    (id_xcpt0.pf.inst, UInt(Causes.fetch_page_fault)),
    (id_xcpt0.ae.inst, UInt(Causes.fetch_access)),
    (id_xcpt1.pf.inst, UInt(Causes.fetch_page_fault)),
    (id_xcpt1.ae.inst, UInt(Causes.fetch_access)),
    (id_illegal_insn,  UInt(Causes.illegal_instruction))))

  val idCoverCauses = List(
    (CSR.debugTriggerCause, "DEBUG_TRIGGER"),
    (Causes.breakpoint, "BREAKPOINT"),
    (Causes.fetch_page_fault, "FETCH_PAGE_FAULT"),
    (Causes.fetch_access, "FETCH_ACCESS"),
    (Causes.illegal_instruction, "ILLEGAL_INSTRUCTION")
  )
  coverExceptions(id_xcpt, id_cause, "DECODE", idCoverCauses)

  val dcache_bypass_data =
    if (fastLoadByte) io.dmem.resp.bits.data(xLen-1, 0)
    else if (fastLoadWord) io.dmem.resp.bits.data_word_bypass(xLen-1, 0)
    else wb_reg_wdata

  // detect bypass opportunities
  val ex_waddr = ex_reg_inst(11,7)
  val mem_waddr = mem_reg_inst(11,7)
  val wb_waddr = wb_reg_inst(11,7)
  val bypass_sources = IndexedSeq(
    (Bool(true), UInt(0), UInt(0)), // treat reading x0 as a bypass
    (ex_reg_valid && ex_ctrl.wxd, ex_waddr, mem_reg_wdata),
    (mem_reg_valid && mem_ctrl.wxd && !mem_ctrl.mem, mem_waddr, wb_reg_wdata),
    (mem_reg_valid && mem_ctrl.wxd, mem_waddr, dcache_bypass_data))
  val id_bypass_src = id_raddr.map(raddr => bypass_sources.map(s => s._1 && s._2 === raddr))

  // execute stage
  val bypass_mux = bypass_sources.map(_._3)
  val ex_reg_rs_bypass = Reg(Vec(id_raddr.size, Bool()))
  val ex_reg_rs_lsb = Reg(Vec(id_raddr.size, UInt(width = log2Ceil(bypass_sources.size))))
  val ex_reg_rs_msb = Reg(Vec(id_raddr.size, UInt()))
  val ex_rs = for (i <- 0 until id_raddr.size)
    yield Mux(ex_reg_rs_bypass(i), bypass_mux(ex_reg_rs_lsb(i)), Cat(ex_reg_rs_msb(i), ex_reg_rs_lsb(i)))
  val ex_imm = ImmGen(ex_ctrl.sel_imm, ex_reg_inst)
  val ex_op1 = MuxLookup(ex_ctrl.sel_alu1, SInt(0), Seq(
    A1_RS1 -> ex_rs(0).asSInt,
    A1_PC -> ex_reg_pc.asSInt))
  val ex_op2 = MuxLookup(ex_ctrl.sel_alu2, SInt(0), Seq(
    A2_RS2 -> ex_rs(1).asSInt,
    A2_IMM -> ex_imm,
    A2_SIZE -> Mux(ex_reg_rvc, SInt(2), SInt(4))))

  val alu = Module(new ALU)
  alu.io.dw := ex_ctrl.alu_dw
  alu.io.fn := ex_ctrl.alu_fn
  alu.io.in2 := ex_op2.asUInt
  alu.io.in1 := ex_op1.asUInt

  // multiplier and divider
  val div = Module(new MulDiv(mulDivParams, width = xLen))
  div.io.req.valid := ex_reg_valid && ex_ctrl.div
  div.io.req.bits.dw := ex_ctrl.alu_dw
  div.io.req.bits.fn := ex_ctrl.alu_fn
  div.io.req.bits.in1 := ex_rs(0)
  div.io.req.bits.in2 := ex_rs(1)
  div.io.req.bits.tag := ex_waddr

  ex_reg_valid := !ctrl_killd
  ex_reg_replay := !take_pc && ibuf.io.inst(0).valid && ibuf.io.inst(0).bits.replay
  ex_reg_xcpt := !ctrl_killd && id_xcpt
  ex_reg_xcpt_interrupt := !take_pc && ibuf.io.inst(0).valid && csr.io.interrupt

  when (!ctrl_killd) {
    ex_ctrl := id_ctrl
    ex_reg_rvc := ibuf.io.inst(0).bits.rvc
    ex_ctrl.csr := id_csr
    when (id_fence_next) { id_reg_fence := true }
    when (id_xcpt) { // pass PC down ALU writeback pipeline for badaddr
      ex_ctrl.alu_fn := ALU.FN_ADD
      ex_ctrl.alu_dw := DW_XPR
      ex_ctrl.sel_alu1 := A1_RS1 // badaddr := instruction
      ex_ctrl.sel_alu2 := A2_ZERO
      when (id_xcpt1.asUInt.orR) { // badaddr := PC+2
        ex_ctrl.sel_alu1 := A1_PC
        ex_ctrl.sel_alu2 := A2_SIZE
        ex_reg_rvc := true
      }
      when (bpu.io.xcpt_if || id_xcpt0.asUInt.orR) { // badaddr := PC
        ex_ctrl.sel_alu1 := A1_PC
        ex_ctrl.sel_alu2 := A2_ZERO
      }
    }
    ex_reg_flush_pipe := id_ctrl.fence_i || id_csr_flush
    ex_reg_load_use := id_load_use
    when (id_sfence) {
      ex_ctrl.mem_type := Cat(id_raddr2 =/= UInt(0), id_raddr1 =/= UInt(0))
    }

    for (i <- 0 until id_raddr.size) {
      val do_bypass = id_bypass_src(i).reduce(_||_)
      val bypass_src = PriorityEncoder(id_bypass_src(i))
      ex_reg_rs_bypass(i) := do_bypass
      ex_reg_rs_lsb(i) := bypass_src
      when (id_ren(i) && !do_bypass) {
        ex_reg_rs_lsb(i) := id_rs(i)(log2Ceil(bypass_sources.size)-1, 0)
        ex_reg_rs_msb(i) := id_rs(i) >> log2Ceil(bypass_sources.size)
      }
    }
    when (id_illegal_insn) {
      val inst = Mux(ibuf.io.inst(0).bits.rvc, id_raw_inst(0)(15, 0), id_raw_inst(0))
      ex_reg_rs_bypass(0) := false
      ex_reg_rs_lsb(0) := inst(log2Ceil(bypass_sources.size)-1, 0)
      ex_reg_rs_msb(0) := inst >> log2Ceil(bypass_sources.size)
    }
  }
  when (!ctrl_killd || csr.io.interrupt || ibuf.io.inst(0).bits.replay) {
    ex_reg_cause := id_cause
    ex_reg_inst := id_inst(0)
    ex_reg_raw_inst := id_raw_inst(0)
    ex_reg_pc := ibuf.io.pc
    ex_reg_btb_resp := ibuf.io.btb_resp
  }

  // replay inst in ex stage?
  val ex_pc_valid = ex_reg_valid || ex_reg_replay || ex_reg_xcpt_interrupt
  val wb_dcache_miss = wb_ctrl.mem && !io.dmem.resp.valid
  val replay_ex_structural = ex_ctrl.mem && !io.dmem.req.ready ||
                             ex_ctrl.div && !div.io.req.ready
  val replay_ex_load_use = wb_dcache_miss && ex_reg_load_use
  val replay_ex = ex_reg_replay || (ex_reg_valid && (replay_ex_structural || replay_ex_load_use))
  val ctrl_killx = take_pc_mem_wb || replay_ex || !ex_reg_valid
  // detect 2-cycle load-use delay for LB/LH/SC
  val ex_slow_bypass = ex_ctrl.mem_cmd === M_XSC || Vec(MT_B, MT_BU, MT_H, MT_HU).contains(ex_ctrl.mem_type)
  val ex_sfence = Bool(usingVM) && ex_ctrl.mem && ex_ctrl.mem_cmd === M_SFENCE

  val (ex_xcpt, ex_cause) = checkExceptions(List(
    (ex_reg_xcpt_interrupt || ex_reg_xcpt, ex_reg_cause)))

  val exCoverCauses = idCoverCauses
  coverExceptions(ex_xcpt, ex_cause, "EXECUTE", exCoverCauses)

  // memory stage
  val mem_pc_valid = mem_reg_valid || mem_reg_replay || mem_reg_xcpt_interrupt
  val mem_br_target = mem_reg_pc.asSInt +
    Mux(mem_ctrl.branch && mem_br_taken, ImmGen(IMM_SB, mem_reg_inst),
    Mux(mem_ctrl.jal, ImmGen(IMM_UJ, mem_reg_inst),
    Mux(mem_reg_rvc, SInt(2), SInt(4))))
  val mem_npc = (Mux(mem_ctrl.jalr || mem_reg_sfence, encodeVirtualAddress(mem_reg_wdata, mem_reg_wdata).asSInt, mem_br_target) & SInt(-2)).asUInt
  val mem_wrong_npc =
    Mux(ex_pc_valid, mem_npc =/= ex_reg_pc,
    Mux(ibuf.io.inst(0).valid || ibuf.io.imem.valid, mem_npc =/= ibuf.io.pc, Bool(true)))
  val mem_npc_misaligned = !csr.io.status.isa('c'-'a') && mem_npc(1) && !mem_reg_sfence
  val mem_int_wdata = Mux(!mem_reg_xcpt && (mem_ctrl.jalr ^ mem_npc_misaligned), mem_br_target, mem_reg_wdata.asSInt).asUInt
  val mem_cfi = mem_ctrl.branch || mem_ctrl.jalr || mem_ctrl.jal
  val mem_cfi_taken = (mem_ctrl.branch && mem_br_taken) || mem_ctrl.jalr || mem_ctrl.jal
  val mem_direction_misprediction = mem_ctrl.branch && mem_br_taken =/= (usingBTB && mem_reg_btb_resp.taken)
  val mem_misprediction = if (usingBTB) mem_wrong_npc else mem_cfi_taken
  take_pc_mem := mem_reg_valid && (mem_misprediction || mem_reg_sfence)

  mem_reg_valid := !ctrl_killx
  mem_reg_replay := !take_pc_mem_wb && replay_ex
  mem_reg_xcpt := !ctrl_killx && ex_xcpt
  mem_reg_xcpt_interrupt := !take_pc_mem_wb && ex_reg_xcpt_interrupt

  // on pipeline flushes, cause mem_npc to hold the sequential npc, which
  // will drive the W-stage npc mux
  when (mem_reg_valid && mem_reg_flush_pipe) {
    mem_reg_sfence := false
  }.elsewhen (ex_pc_valid) {
    mem_ctrl := ex_ctrl
    mem_reg_rvc := ex_reg_rvc
    mem_reg_load := ex_ctrl.mem && isRead(ex_ctrl.mem_cmd)
    mem_reg_store := ex_ctrl.mem && isWrite(ex_ctrl.mem_cmd)
    mem_reg_sfence := ex_sfence
    mem_reg_btb_resp := ex_reg_btb_resp
    mem_reg_flush_pipe := ex_reg_flush_pipe
    mem_reg_slow_bypass := ex_slow_bypass

    mem_reg_cause := ex_cause
    mem_reg_inst := ex_reg_inst
    mem_reg_raw_inst := ex_reg_raw_inst
    mem_reg_pc := ex_reg_pc
    mem_reg_wdata := alu.io.out
    mem_br_taken := alu.io.cmp_out

    when (ex_ctrl.rxs2 && (ex_ctrl.mem || ex_ctrl.rocc || ex_sfence)) {
      val typ = Mux(ex_ctrl.rocc, log2Ceil(xLen/8).U, ex_ctrl.mem_type)
      mem_reg_rs2 := new StoreGen(typ, 0.U, ex_rs(1), coreDataBytes).data
    }
    when (ex_ctrl.jalr && csr.io.status.debug) {
      // flush I$ on D-mode JALR to effect uncached fetch without D$ flush
      mem_ctrl.fence_i := true
      mem_reg_flush_pipe := true
    }
  }

  val mem_breakpoint = (mem_reg_load && bpu.io.xcpt_ld) || (mem_reg_store && bpu.io.xcpt_st)
  val mem_debug_breakpoint = (mem_reg_load && bpu.io.debug_ld) || (mem_reg_store && bpu.io.debug_st)
  val (mem_ldst_xcpt, mem_ldst_cause) = checkExceptions(List(
    (mem_debug_breakpoint, UInt(CSR.debugTriggerCause)),
    (mem_breakpoint,       UInt(Causes.breakpoint))))

  val (mem_xcpt, mem_cause) = checkExceptions(List(
    (mem_reg_xcpt_interrupt || mem_reg_xcpt, mem_reg_cause),
    (mem_reg_valid && mem_npc_misaligned,    UInt(Causes.misaligned_fetch)),
    (mem_reg_valid && mem_ldst_xcpt,         mem_ldst_cause)))

  val memCoverCauses = (exCoverCauses ++ List(
    (CSR.debugTriggerCause, "DEBUG_TRIGGER"),
    (Causes.breakpoint, "BREAKPOINT"),
    (Causes.misaligned_fetch, "MISALIGNED_FETCH")
  )).distinct
  coverExceptions(mem_xcpt, mem_cause, "MEMORY", memCoverCauses)

  val dcache_kill_mem = mem_reg_valid && mem_ctrl.wxd && io.dmem.replay_next // structural hazard on writeback port
  val fpu_kill_mem = mem_reg_valid && mem_ctrl.fp && io.fpu.nack_mem
  val replay_mem  = dcache_kill_mem || mem_reg_replay || fpu_kill_mem
  val killm_common = dcache_kill_mem || take_pc_wb || mem_reg_xcpt || !mem_reg_valid
  div.io.kill := killm_common && Reg(next = div.io.req.fire())
  val ctrl_killm = killm_common || mem_xcpt || fpu_kill_mem

  // writeback stage
  wb_reg_valid := !ctrl_killm
  wb_reg_replay := replay_mem && !take_pc_wb
  wb_reg_xcpt := mem_xcpt && !take_pc_wb
  wb_reg_flush_pipe := !ctrl_killm && mem_reg_flush_pipe
  when (mem_pc_valid) {
    wb_ctrl := mem_ctrl
    wb_reg_sfence := mem_reg_sfence
    wb_reg_wdata := Mux(!mem_reg_xcpt && mem_ctrl.fp && mem_ctrl.wxd, io.fpu.toint_data, mem_int_wdata)
    when (mem_ctrl.rocc || mem_reg_sfence) {
      wb_reg_rs2 := mem_reg_rs2
    }
    wb_reg_cause := mem_cause
    wb_reg_inst := mem_reg_inst
    wb_reg_raw_inst := mem_reg_raw_inst
    wb_reg_pc := mem_reg_pc
  }

  val (wb_xcpt, wb_cause) = checkExceptions(List(
    (wb_reg_xcpt,  wb_reg_cause),
    (wb_reg_valid && wb_ctrl.mem && io.dmem.s2_xcpt.ma.st, UInt(Causes.misaligned_store)),
    (wb_reg_valid && wb_ctrl.mem && io.dmem.s2_xcpt.ma.ld, UInt(Causes.misaligned_load)),
    (wb_reg_valid && wb_ctrl.mem && io.dmem.s2_xcpt.pf.st, UInt(Causes.store_page_fault)),
    (wb_reg_valid && wb_ctrl.mem && io.dmem.s2_xcpt.pf.ld, UInt(Causes.load_page_fault)),
    (wb_reg_valid && wb_ctrl.mem && io.dmem.s2_xcpt.ae.st, UInt(Causes.store_access)),
    (wb_reg_valid && wb_ctrl.mem && io.dmem.s2_xcpt.ae.ld, UInt(Causes.load_access))
  ))

  val wbCoverCauses = List(
    (Causes.misaligned_store, "MISALIGNED_STORE"),
    (Causes.misaligned_load, "MISALIGNED_LOAD"),
    (Causes.store_page_fault, "STORE_PAGE_FAULT"),
    (Causes.load_page_fault, "LOAD_PAGE_FAULT"),
    (Causes.store_access, "STORE_ACCESS"),
    (Causes.load_access, "LOAD_ACCESS")
  )
  coverExceptions(wb_xcpt, wb_cause, "WRITEBACK", wbCoverCauses)

  val wb_wxd = wb_reg_valid && wb_ctrl.wxd
  val wb_set_sboard = wb_ctrl.div || wb_dcache_miss || wb_ctrl.rocc
  val replay_wb_common = io.dmem.s2_nack || wb_reg_replay
  val replay_wb_rocc = wb_reg_valid && wb_ctrl.rocc && !io.rocc.cmd.ready
  val replay_wb = replay_wb_common || replay_wb_rocc
  take_pc_wb := replay_wb || wb_xcpt || csr.io.eret || wb_reg_flush_pipe

  // writeback arbitration
  val dmem_resp_xpu = !io.dmem.resp.bits.tag(0).toBool
  val dmem_resp_fpu =  io.dmem.resp.bits.tag(0).toBool
  val dmem_resp_waddr = io.dmem.resp.bits.tag(5, 1)
  val dmem_resp_valid = io.dmem.resp.valid && io.dmem.resp.bits.has_data
  val dmem_resp_replay = dmem_resp_valid && io.dmem.resp.bits.replay

  div.io.resp.ready := !wb_wxd
  val ll_wdata = Wire(init = div.io.resp.bits.data)
  val ll_waddr = Wire(init = div.io.resp.bits.tag)
  val ll_wen = Wire(init = div.io.resp.fire())
  if (usingRoCC) {
    io.rocc.resp.ready := !wb_wxd
    when (io.rocc.resp.fire()) {
      div.io.resp.ready := Bool(false)
      ll_wdata := io.rocc.resp.bits.data
      ll_waddr := io.rocc.resp.bits.rd
      ll_wen := Bool(true)
    }
  }
  when (dmem_resp_replay && dmem_resp_xpu) {
    div.io.resp.ready := Bool(false)
    if (usingRoCC)
      io.rocc.resp.ready := Bool(false)
    ll_waddr := dmem_resp_waddr
    ll_wen := Bool(true)
  }

  val wb_valid = wb_reg_valid && !replay_wb && !wb_xcpt
  val wb_wen = wb_valid && wb_ctrl.wxd
  val rf_wen = wb_wen || ll_wen
  val rf_waddr = Mux(ll_wen, ll_waddr, wb_waddr)
  val rf_wdata = Mux(dmem_resp_valid && dmem_resp_xpu, io.dmem.resp.bits.data(xLen-1, 0),
                 Mux(ll_wen, ll_wdata,
                 Mux(wb_ctrl.csr =/= CSR.N, csr.io.rw.rdata,
                 wb_reg_wdata)))
  when (rf_wen) { rf.write(rf_waddr, rf_wdata) }

  // hook up control/status regfile
  csr.io.decode(0).csr := id_raw_inst(0)(31,20)
  csr.io.exception := wb_xcpt
  csr.io.cause := wb_cause
  csr.io.retire := wb_valid
  csr.io.inst(0) := (if (usingCompressed) Cat(Mux(wb_reg_raw_inst(1, 0).andR, wb_reg_inst >> 16, 0.U), wb_reg_raw_inst(15, 0)) else wb_reg_inst)
  csr.io.interrupts := io.interrupts
  csr.io.hartid := io.hartid
  io.fpu.fcsr_rm := csr.io.fcsr_rm
  csr.io.fcsr_flags := io.fpu.fcsr_flags
  csr.io.rocc_interrupt := io.rocc.interrupt
  csr.io.pc := wb_reg_pc
  val tval_valid = wb_xcpt && wb_cause.isOneOf(Causes.illegal_instruction, Causes.breakpoint,
    Causes.misaligned_load, Causes.misaligned_store,
    Causes.load_access, Causes.store_access, Causes.fetch_access,
    Causes.load_page_fault, Causes.store_page_fault, Causes.fetch_page_fault)
  csr.io.tval := Mux(tval_valid, encodeVirtualAddress(wb_reg_wdata, wb_reg_wdata), 0.U)
  io.ptw.ptbr := csr.io.ptbr
  io.ptw.status := csr.io.status
  io.ptw.pmp := csr.io.pmp
  csr.io.rw.addr := wb_reg_inst(31,20)
  csr.io.rw.cmd := Mux(wb_reg_valid, wb_ctrl.csr, CSR.N)
  csr.io.rw.wdata := wb_reg_wdata
  io.trace := csr.io.trace

  val hazard_targets = Seq((id_ctrl.rxs1 && id_raddr1 =/= UInt(0), id_raddr1),
                           (id_ctrl.rxs2 && id_raddr2 =/= UInt(0), id_raddr2),
                           (id_ctrl.wxd  && id_waddr  =/= UInt(0), id_waddr))
  val fp_hazard_targets = Seq((io.fpu.dec.ren1, id_raddr1),
                              (io.fpu.dec.ren2, id_raddr2),
                              (io.fpu.dec.ren3, id_raddr3),
                              (io.fpu.dec.wen, id_waddr))

  val sboard = new Scoreboard(32, true)
  sboard.clear(ll_wen, ll_waddr)
  def id_sboard_clear_bypass(r: UInt) = {
    // ll_waddr arrives late when D$ has ECC, so reshuffle the hazard check
    if (tileParams.dcache.get.dataECC.isInstanceOf[IdentityCode]) ll_wen && ll_waddr === r
    else div.io.resp.fire() && div.io.resp.bits.tag === r || dmem_resp_replay && dmem_resp_xpu && dmem_resp_waddr === r
  }
  val id_sboard_hazard = checkHazards(hazard_targets, rd => sboard.read(rd) && !id_sboard_clear_bypass(rd))
  sboard.set(wb_set_sboard && wb_wen, wb_waddr)

  // stall for RAW/WAW hazards on CSRs, loads, AMOs, and mul/div in execute stage.
  val ex_cannot_bypass = ex_ctrl.csr =/= CSR.N || ex_ctrl.jalr || ex_ctrl.mem || ex_ctrl.div || ex_ctrl.fp || ex_ctrl.rocc
  val data_hazard_ex = ex_ctrl.wxd && checkHazards(hazard_targets, _ === ex_waddr)
  val fp_data_hazard_ex = ex_ctrl.wfd && checkHazards(fp_hazard_targets, _ === ex_waddr)
  val id_ex_hazard = ex_reg_valid && (data_hazard_ex && ex_cannot_bypass || fp_data_hazard_ex)

  // stall for RAW/WAW hazards on CSRs, LB/LH, and mul/div in memory stage.
  val mem_mem_cmd_bh =
    if (fastLoadWord) Bool(!fastLoadByte) && mem_reg_slow_bypass
    else Bool(true)
  val mem_cannot_bypass = mem_ctrl.csr =/= CSR.N || mem_ctrl.mem && mem_mem_cmd_bh || mem_ctrl.div || mem_ctrl.fp || mem_ctrl.rocc
  val data_hazard_mem = mem_ctrl.wxd && checkHazards(hazard_targets, _ === mem_waddr)
  val fp_data_hazard_mem = mem_ctrl.wfd && checkHazards(fp_hazard_targets, _ === mem_waddr)
  val id_mem_hazard = mem_reg_valid && (data_hazard_mem && mem_cannot_bypass || fp_data_hazard_mem)
  id_load_use := mem_reg_valid && data_hazard_mem && mem_ctrl.mem

  // stall for RAW/WAW hazards on load/AMO misses and mul/div in writeback.
  val data_hazard_wb = wb_ctrl.wxd && checkHazards(hazard_targets, _ === wb_waddr)
  val fp_data_hazard_wb = wb_ctrl.wfd && checkHazards(fp_hazard_targets, _ === wb_waddr)
  val id_wb_hazard = wb_reg_valid && (data_hazard_wb && wb_set_sboard || fp_data_hazard_wb)

  val id_stall_fpu = if (usingFPU) {
    val fp_sboard = new Scoreboard(32)
    fp_sboard.set((wb_dcache_miss && wb_ctrl.wfd || io.fpu.sboard_set) && wb_valid, wb_waddr)
    fp_sboard.clear(dmem_resp_replay && dmem_resp_fpu, dmem_resp_waddr)
    fp_sboard.clear(io.fpu.sboard_clr, io.fpu.sboard_clra)

    id_csr_en && !io.fpu.fcsr_rdy || checkHazards(fp_hazard_targets, fp_sboard.read _)
  } else Bool(false)

  val dcache_blocked = Reg(Bool())
  dcache_blocked := !io.dmem.req.ready && (io.dmem.req.valid || dcache_blocked)
  val rocc_blocked = Reg(Bool())
  rocc_blocked := !wb_xcpt && !io.rocc.cmd.ready && (io.rocc.cmd.valid || rocc_blocked)

  val ctrl_stalld =
    id_ex_hazard || id_mem_hazard || id_wb_hazard || id_sboard_hazard ||
    csr.io.singleStep && (ex_reg_valid || mem_reg_valid || wb_reg_valid) ||
    id_ctrl.fp && id_stall_fpu ||
    id_ctrl.mem && dcache_blocked || // reduce activity during D$ misses
    id_ctrl.rocc && rocc_blocked || // reduce activity while RoCC is busy
    id_ctrl.div && (!(div.io.req.ready || (div.io.resp.valid && !wb_wxd)) || div.io.req.valid) || // reduce odds of replay
    id_do_fence ||
    csr.io.csr_stall
  ctrl_killd := !ibuf.io.inst(0).valid || ibuf.io.inst(0).bits.replay || take_pc_mem_wb || ctrl_stalld || csr.io.interrupt

  io.imem.req.valid := take_pc
  io.imem.req.bits.speculative := !take_pc_wb
  io.imem.req.bits.pc :=
    Mux(wb_xcpt || csr.io.eret, csr.io.evec, // exception or [m|s]ret
    Mux(replay_wb,              wb_reg_pc,   // replay
                                mem_npc))    // flush or branch misprediction
  io.imem.flush_icache := wb_reg_valid && wb_ctrl.fence_i && !io.dmem.s2_nack
  io.imem.sfence.valid := wb_reg_valid && wb_reg_sfence
  io.imem.sfence.bits.rs1 := wb_ctrl.mem_type(0)
  io.imem.sfence.bits.rs2 := wb_ctrl.mem_type(1)
  io.imem.sfence.bits.addr := wb_reg_wdata
  io.imem.sfence.bits.asid := wb_reg_rs2
  io.ptw.sfence := io.imem.sfence

  ibuf.io.inst(0).ready := !ctrl_stalld

  io.imem.btb_update.valid := mem_reg_valid && !take_pc_wb && mem_wrong_npc && (!mem_cfi || mem_cfi_taken)
  io.imem.btb_update.bits.isValid := mem_cfi
  io.imem.btb_update.bits.cfiType :=
    Mux((mem_ctrl.jal || mem_ctrl.jalr) && mem_waddr(0), CFIType.call,
    Mux(mem_ctrl.jalr && mem_reg_inst(19,15) === BitPat("b00?01"), CFIType.ret,
    Mux(mem_ctrl.jal || mem_ctrl.jalr, CFIType.jump,
    CFIType.branch)))
  io.imem.btb_update.bits.target := io.imem.req.bits.pc
  io.imem.btb_update.bits.br_pc := (if (usingCompressed) mem_reg_pc + Mux(mem_reg_rvc, UInt(0), UInt(2)) else mem_reg_pc)
  io.imem.btb_update.bits.pc := ~(~io.imem.btb_update.bits.br_pc | (coreInstBytes*fetchWidth-1))
  io.imem.btb_update.bits.prediction := mem_reg_btb_resp

  io.imem.bht_update.valid := mem_reg_valid && !take_pc_wb
  io.imem.bht_update.bits.pc := io.imem.btb_update.bits.pc
  io.imem.bht_update.bits.taken := mem_br_taken
  io.imem.bht_update.bits.mispredict := mem_wrong_npc
  io.imem.bht_update.bits.branch := mem_ctrl.branch
  io.imem.bht_update.bits.prediction := mem_reg_btb_resp.bht

  io.fpu.valid := !ctrl_killd && id_ctrl.fp
  io.fpu.killx := ctrl_killx
  io.fpu.killm := killm_common
  io.fpu.inst := id_inst(0)
  io.fpu.fromint_data := ex_rs(0)
  io.fpu.dmem_resp_val := dmem_resp_valid && dmem_resp_fpu
  io.fpu.dmem_resp_data := io.dmem.resp.bits.data_word_bypass
  io.fpu.dmem_resp_type := io.dmem.resp.bits.typ
  io.fpu.dmem_resp_tag := dmem_resp_waddr

  io.dmem.req.valid     := ex_reg_valid && ex_ctrl.mem
  val ex_dcache_tag = Cat(ex_waddr, ex_ctrl.fp)
  require(coreDCacheReqTagBits >= ex_dcache_tag.getWidth)
  io.dmem.req.bits.tag  := ex_dcache_tag
  io.dmem.req.bits.cmd  := ex_ctrl.mem_cmd
  io.dmem.req.bits.typ  := ex_ctrl.mem_type
  io.dmem.req.bits.phys := Bool(false)
  io.dmem.req.bits.addr := encodeVirtualAddress(ex_rs(0), alu.io.adder_out)
  io.dmem.invalidate_lr := wb_xcpt
  io.dmem.s1_data.data := (if (fLen == 0) mem_reg_rs2 else Mux(mem_ctrl.fp, Fill((xLen max fLen) / fLen, io.fpu.store_data), mem_reg_rs2))
  io.dmem.s1_kill := killm_common || mem_ldst_xcpt

  io.rocc.cmd.valid := wb_reg_valid && wb_ctrl.rocc && !replay_wb_common
  io.rocc.exception := wb_xcpt && csr.io.status.xs.orR
  io.rocc.cmd.bits.status := csr.io.status
  io.rocc.cmd.bits.inst := new RoCCInstruction().fromBits(wb_reg_inst)
  io.rocc.cmd.bits.rs1 := wb_reg_wdata
  io.rocc.cmd.bits.rs2 := wb_reg_rs2

  // evaluate performance counters
  val icache_blocked = !(io.imem.resp.valid || RegNext(io.imem.resp.valid))
  csr.io.counters foreach { c => c.inc := RegNext(perfEvents.evaluate(c.eventSel)) }

  if (enableCommitLog) {
    val t = csr.io.trace(0)
    val rd = wb_waddr
    val wfd = wb_ctrl.wfd
    val wxd = wb_ctrl.wxd
    val has_data = wb_wen && !wb_set_sboard

    when (t.valid && !t.exception) {
      when (wfd) {
        printf ("%d 0x%x (0x%x) f%d p%d 0xXXXXXXXXXXXXXXXX\n", t.priv, t.iaddr, t.insn, rd, rd+UInt(32))
      }
      .elsewhen (wxd && rd =/= UInt(0) && has_data) {
        printf ("%d 0x%x (0x%x) x%d 0x%x\n", t.priv, t.iaddr, t.insn, rd, rf_wdata)
      }
      .elsewhen (wxd && rd =/= UInt(0) && !has_data) {
        printf ("%d 0x%x (0x%x) x%d p%d 0xXXXXXXXXXXXXXXXX\n", t.priv, t.iaddr, t.insn, rd, rd)
      }
      .otherwise {
        printf ("%d 0x%x (0x%x)\n", t.priv, t.iaddr, t.insn)
      }
    }

    when (ll_wen && rf_waddr =/= UInt(0)) {
      printf ("x%d p%d 0x%x\n", rf_waddr, rf_waddr, rf_wdata)
    }
  }
  else {
    printf("C%d: %d [%d] pc=[%x] W[r%d=%x][%d] R[r%d=%x] R[r%d=%x] inst=[%x] DASM(%x)\n",
         io.hartid, csr.io.time(31,0), csr.io.trace(0).valid && !csr.io.trace(0).exception,
         csr.io.trace(0).iaddr(vaddrBitsExtended-1, 0),
         Mux(rf_wen && !(wb_set_sboard && wb_wen), rf_waddr, UInt(0)), rf_wdata, rf_wen,
         wb_reg_inst(19,15), Reg(next=Reg(next=ex_rs(0))),
         wb_reg_inst(24,20), Reg(next=Reg(next=ex_rs(1))),
         csr.io.trace(0).insn, csr.io.trace(0).insn)
  }

  val max_core_cycles = PlusArg("max-core-cycles",
    default = 0,
    docstring = "Kill the emulation after INT rdtime cycles. Off if 0.")
  when (max_core_cycles > UInt(0)) {
    assert (csr.io.time < max_core_cycles, "Maximum Core Cycles reached.")
  }

  def checkExceptions(x: Seq[(Bool, UInt)]) =
    (x.map(_._1).reduce(_||_), PriorityMux(x))

  def coverExceptions(exceptionValid: Bool, cause: UInt, labelPrefix: String, coverCausesLabels: Seq[(Int, String)]): Unit = {
    for ((coverCause, label) <- coverCausesLabels) {
      cover(exceptionValid && (cause === UInt(coverCause)), s"${labelPrefix}_${label}")
    }
  }

  def checkHazards(targets: Seq[(Bool, UInt)], cond: UInt => Bool) =
    targets.map(h => h._1 && cond(h._2)).reduce(_||_)

  def encodeVirtualAddress(a0: UInt, ea: UInt) = if (vaddrBitsExtended == vaddrBits) ea else {
    // efficient means to compress 64-bit VA into vaddrBits+1 bits
    // (VA is bad if VA(vaddrBits) != VA(vaddrBits-1))
    val a = a0.asSInt >> vaddrBits
    val msb = Mux(a === 0.S || a === -1.S, ea(vaddrBits), !ea(vaddrBits-1))
    Cat(msb, ea(vaddrBits-1,0))
  }

  class Scoreboard(n: Int, zero: Boolean = false)
  {
    def set(en: Bool, addr: UInt): Unit = update(en, _next | mask(en, addr))
    def clear(en: Bool, addr: UInt): Unit = update(en, _next & ~mask(en, addr))
    def read(addr: UInt): Bool = r(addr)
    def readBypassed(addr: UInt): Bool = _next(addr)

    private val _r = Reg(init=Bits(0, n))
    private val r = if (zero) (_r >> 1 << 1) else _r
    private var _next = r
    private var ens = Bool(false)
    private def mask(en: Bool, addr: UInt) = Mux(en, UInt(1) << addr, UInt(0))
    private def update(en: Bool, update: UInt) = {
      _next = update
      ens = ens || en
      when (ens) { _r := _next }
    }
  }
}

class RegFile(n: Int, w: Int, zero: Boolean = false) {
  private val rf = Mem(n, UInt(width = w))
  private def access(addr: UInt) = rf(~addr(log2Up(n)-1,0))
  private val reads = ArrayBuffer[(UInt,UInt)]()
  private var canRead = true
  def read(addr: UInt) = {
    require(canRead)
    reads += addr -> Wire(UInt())
    reads.last._2 := Mux(Bool(zero) && addr === UInt(0), UInt(0), access(addr))
    reads.last._2
  }
  def write(addr: UInt, data: UInt) = {
    canRead = false
    when (addr =/= UInt(0)) {
      access(addr) := data
      for ((raddr, rdata) <- reads)
        when (addr === raddr) { rdata := data }
    }
  }
}

object ImmGen {
  def apply(sel: UInt, inst: UInt) = {
    val sign = Mux(sel === IMM_Z, SInt(0), inst(31).asSInt)
    val b30_20 = Mux(sel === IMM_U, inst(30,20).asSInt, sign)
    val b19_12 = Mux(sel =/= IMM_U && sel =/= IMM_UJ, sign, inst(19,12).asSInt)
    val b11 = Mux(sel === IMM_U || sel === IMM_Z, SInt(0),
              Mux(sel === IMM_UJ, inst(20).asSInt,
              Mux(sel === IMM_SB, inst(7).asSInt, sign)))
    val b10_5 = Mux(sel === IMM_U || sel === IMM_Z, Bits(0), inst(30,25))
    val b4_1 = Mux(sel === IMM_U, Bits(0),
               Mux(sel === IMM_S || sel === IMM_SB, inst(11,8),
               Mux(sel === IMM_Z, inst(19,16), inst(24,21))))
    val b0 = Mux(sel === IMM_S, inst(7),
             Mux(sel === IMM_I, inst(20),
             Mux(sel === IMM_Z, inst(15), Bits(0))))

    Cat(sign, b30_20, b19_12, b11, b10_5, b4_1, b0).asSInt
  }
}