ch81/z80emu/z80emu.cc

/* z80emu.c
 * Z80 processor emulator.
 *
 * Copyright (c) 2012-2017 Lin Ke-Fong
 *
 * This code is free, do whatever you want with it.
 */

#include "z80emu.h"

#include "instructions.h"
#include "macros.h"
#include "tables.h"

namespace z80emu {
namespace {

/* The main registers are stored inside Z80_STATE as an union of arrays named
 * registers. They are referenced using indexes. Words are stored in the
 * endianness of the host processor. The alternate set of word registers AF',
 * BC', DE', and HL' is stored in the alternates member of Z80_STATE, as an
 * array using the same ordering.
 */

#ifdef Z80_BIG_ENDIAN

#define Z80_B 0
#define Z80_C 1
#define Z80_D 2
#define Z80_E 3
#define Z80_H 4
#define Z80_L 5
#define Z80_A 6
#define Z80_F 7

#define Z80_IXH 8
#define Z80_IXL 9
#define Z80_IYH 10
#define Z80_IYL 11

#else

#define Z80_B 1
#define Z80_C 0
#define Z80_D 3
#define Z80_E 2
#define Z80_H 5
#define Z80_L 4
#define Z80_A 7
#define Z80_F 6

#define Z80_IXH 9
#define Z80_IXL 8
#define Z80_IYH 11
#define Z80_IYL 10

#endif

#define Z80_BC 0
#define Z80_DE 1
#define Z80_HL 2
#define Z80_AF 3

#define Z80_IX 4
#define Z80_IY 5
#define Z80_SP 6

/* Z80's flags. */

#define Z80_S_FLAG_SHIFT 7
#define Z80_Z_FLAG_SHIFT 6
#define Z80_Y_FLAG_SHIFT 5
#define Z80_H_FLAG_SHIFT 4
#define Z80_X_FLAG_SHIFT 3
#define Z80_PV_FLAG_SHIFT 2
#define Z80_N_FLAG_SHIFT 1
#define Z80_C_FLAG_SHIFT 0

#define Z80_S_FLAG (1 << Z80_S_FLAG_SHIFT)
#define Z80_Z_FLAG (1 << Z80_Z_FLAG_SHIFT)
#define Z80_Y_FLAG (1 << Z80_Y_FLAG_SHIFT)
#define Z80_H_FLAG (1 << Z80_H_FLAG_SHIFT)
#define Z80_X_FLAG (1 << Z80_X_FLAG_SHIFT)
#define Z80_PV_FLAG (1 << Z80_PV_FLAG_SHIFT)
#define Z80_N_FLAG (1 << Z80_N_FLAG_SHIFT)
#define Z80_C_FLAG (1 << Z80_C_FLAG_SHIFT)

#define Z80_P_FLAG_SHIFT Z80_PV_FLAG_SHIFT
#define Z80_V_FLAG_SHIFT Z80_PV_FLAG_SHIFT
#define Z80_P_FLAG Z80_PV_FLAG
#define Z80_V_FLAG Z80_PV_FLAG

/* Z80's three interrupt modes. */

enum {

  Z80_INTERRUPT_MODE_0,
  Z80_INTERRUPT_MODE_1,
  Z80_INTERRUPT_MODE_2

};

#define Z80_READ_BYTE(address, x) x = board_.ReadByte(address)
#define Z80_FETCH_BYTE(address, x) x = board_.FetchByte(address)
#define Z80_READ_WORD(address, x) x = board_.ReadWord(address)
#define Z80_FETCH_WORD(address, x) x = board_.FetchWord(address)
#define Z80_WRITE_BYTE(address, x) board_.WriteByte(address, x)
#define Z80_WRITE_WORD(address, x) board_.WriteWord(address, x)
#define Z80_READ_WORD_INTERRUPT(address, x) \
  x = board_.ReadWordInterrupt(address)
#define Z80_WRITE_WORD_INTERRUPT(address, x) \
  board_.WriteWordInterrupt(address, x)
#define Z80_INPUT_BYTE(port, x) x = board_.InputByte(*this, port)
#define Z80_OUTPUT_BYTE(port, x) board_.OutputByte(port, x)

#define Z80_CATCH_HALT
#define Z80_CATCH_EI

/* Indirect (HL) or prefixed indexed (IX + d) and (IY + d) memory operands are
 * encoded using the 3 bits "110" (0x06).
 */

#define INDIRECT_HL 0x06

/* Condition codes are encoded using 2 or 3 bits.  The xor table is needed for
 * negated conditions, it is used along with the and table.
 */

constexpr uint8_t XOR_CONDITION_TABLE[8] = {

    Z80_Z_FLAG, 0, Z80_C_FLAG, 0, Z80_P_FLAG, 0, Z80_S_FLAG, 0,

};

constexpr uint8_t AND_CONDITION_TABLE[8] = {

    Z80_Z_FLAG, Z80_Z_FLAG, Z80_C_FLAG, Z80_C_FLAG,
    Z80_P_FLAG, Z80_P_FLAG, Z80_S_FLAG, Z80_S_FLAG,

};

/* RST instruction restart addresses, encoded by Y() bits of the opcode. */

constexpr uint8_t RST_TABLE[8] = {

    0x00, 0x08, 0x10, 0x18, 0x20, 0x28, 0x30, 0x38,

};

/* There is an overflow if the xor of the carry out and the carry of the most
 * significant bit is not zero.
 */

constexpr uint8_t OVERFLOW_TABLE[4] = {

    0,
    Z80_V_FLAG,
    Z80_V_FLAG,
    0,

};

}  // namespace

template <typename Board>
void Z80Emu<Board>::Reset(uint16_t pc) {
  int i;

  status_ = Status::UNDEFINED;
  AF = 0xffff;
  SP = 0xffff;
  i_ = iff1_ = iff2_ = 0;
  im_ = Z80_INTERRUPT_MODE_0;
  pc_ = pc;

  /* Build register decoding tables for both 3-bit encoded 8-bit
   * registers and 2-bit encoded 16-bit registers. When an opcode is
   * prefixed by 0xdd, HL is replaced by IX. When 0xfd prefixed, HL is
   * replaced by IY.
   */

  /* 8-bit "R" registers. */

  register_table_[0] = &registers_.byte[Z80_B];
  register_table_[1] = &registers_.byte[Z80_C];
  register_table_[2] = &registers_.byte[Z80_D];
  register_table_[3] = &registers_.byte[Z80_E];
  register_table_[4] = &registers_.byte[Z80_H];
  register_table_[5] = &registers_.byte[Z80_L];

  /* Encoding 0x06 is used for indexed memory operands and direct HL or
   * IX/IY register access.
   */

  register_table_[6] = &registers_.word[Z80_HL];
  register_table_[7] = &registers_.byte[Z80_A];

  /* "Regular" 16-bit "RR" registers. */

  register_table_[8] = &registers_.word[Z80_BC];
  register_table_[9] = &registers_.word[Z80_DE];
  register_table_[10] = &registers_.word[Z80_HL];
  register_table_[11] = &registers_.word[Z80_SP];

  /* 16-bit "SS" registers for PUSH and POP instructions (note that SP is
   * replaced by AF).
   */

  register_table_[12] = &registers_.word[Z80_BC];
  register_table_[13] = &registers_.word[Z80_DE];
  register_table_[14] = &registers_.word[Z80_HL];
  register_table_[15] = &registers_.word[Z80_AF];

  /* 0xdd and 0xfd prefixed register decoding tables. */

  for (i = 0; i < 16; i++)

    dd_register_table_[i] = fd_register_table_[i] = register_table_[i];

  dd_register_table_[4] = &registers_.byte[Z80_IXH];
  dd_register_table_[5] = &registers_.byte[Z80_IXL];
  dd_register_table_[6] = &registers_.word[Z80_IX];
  dd_register_table_[10] = &registers_.word[Z80_IX];
  dd_register_table_[14] = &registers_.word[Z80_IX];

  fd_register_table_[4] = &registers_.byte[Z80_IYH];
  fd_register_table_[5] = &registers_.byte[Z80_IYL];
  fd_register_table_[6] = &registers_.word[Z80_IY];
  fd_register_table_[10] = &registers_.word[Z80_IY];
  fd_register_table_[14] = &registers_.word[Z80_IY];
}

template <typename Board>
int Z80Emu<Board>::Interrupt(int data_on_bus) {
  status_ = Status::UNDEFINED;
  if (iff1_) {
    iff1_ = iff2_ = 0;
    r_ = (r_ & 0x80) | ((r_ + 1) & 0x7f);
    switch (im_) {
      case Z80_INTERRUPT_MODE_0: {
        /* Assuming the opcode in data_on_bus is an
         * RST instruction, accepting the interrupt
         * should take 2 + 11 = 13 cycles.
         */

        return Emulate(data_on_bus, 2, 4);
      }

      case Z80_INTERRUPT_MODE_1: {
        int elapsed_cycles;

        elapsed_cycles = 0;
        SP -= 2;
        Z80_WRITE_WORD_INTERRUPT(SP, pc_);
        pc_ = 0x0038;
        return elapsed_cycles + 13;
      }

      case Z80_INTERRUPT_MODE_2:
      default: {
        int elapsed_cycles, vector;

        elapsed_cycles = 0;
        SP -= 2;
        Z80_WRITE_WORD_INTERRUPT(SP, pc_);
        vector = i_ << 8 | data_on_bus;

#ifdef Z80_MASK_IM2_VECTOR_ADDRESS

        vector &= 0xfffe;

#endif

        Z80_READ_WORD_INTERRUPT(vector, pc_);
        return elapsed_cycles + 19;
      }
    }

  } else

    return 0;
}

template <typename Board>
int Z80Emu<Board>::NonMaskableInterrupt() {
  int elapsed_cycles;

  status_ = Status::UNDEFINED;

  iff2_ = iff1_;
  iff1_ = 0;
  r_ = (r_ & 0x80) | ((r_ + 1) & 0x7f);

  elapsed_cycles = 0;
  SP -= 2;
  Z80_WRITE_WORD_INTERRUPT(SP, pc_);
  pc_ = 0x0066;

  return elapsed_cycles + 11;
}

template <typename Board>
int Z80Emu<Board>::Emulate(int number_cycles) {
  int elapsed_cycles, pc, opcode;

  status_ = Status::UNDEFINED;
  elapsed_cycles = 0;
  pc = pc_;
  Z80_FETCH_BYTE(pc, opcode);
  pc_ = pc + 1;

  return Emulate(opcode, elapsed_cycles, number_cycles);
}

/* Actual emulation function. opcode is the first opcode to emulate, this is
 * needed by Z80Interrupt() for interrupt mode 0.
 */

template <typename Board>
int Z80Emu<Board>::Emulate(int opcode, int elapsed_cycles, int number_cycles) {
  int pc, r;

  pc = pc_;
  r = r_ & 0x7f;
  goto start_emulation;

  for (;;) {
    void **registers;
    Instruction instruction;

    Z80_FETCH_BYTE(pc, opcode);
    pc++;

  start_emulation:

    registers = register_table_;

  emulate_next_opcode:

    instruction = (Instruction)INSTRUCTION_TABLE[opcode];
    //    ++counts_[instruction];

  emulate_next_instruction:

    elapsed_cycles += 4;
    r++;
    switch (instruction) {
        /* 8-bit load group. */

      case Instruction::LD_R_R: {
        R(Y(opcode)) = R(Z(opcode));
        break;
      }

      case Instruction::LD_R_N: {
        READ_N(R(Y(opcode)));
        break;
      }

      case Instruction::LD_R_INDIRECT_HL: {
        if (registers == register_table_) {
          READ_BYTE(HL, R(Y(opcode)));

        } else {
          int d;

          READ_D(d);
          d += HL_IX_IY;
          READ_BYTE(d, S(Y(opcode)));

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::LD_INDIRECT_HL_R: {
        if (registers == register_table_) {
          WRITE_BYTE(HL, R(Z(opcode)));

        } else {
          int d;

          READ_D(d);
          d += HL_IX_IY;
          WRITE_BYTE(d, S(Z(opcode)));

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::LD_INDIRECT_HL_N: {
        int n;

        if (registers == register_table_) {
          READ_N(n);
          WRITE_BYTE(HL, n);

        } else {
          int d;

          READ_D(d);
          d += HL_IX_IY;
          READ_N(n);
          WRITE_BYTE(d, n);

          elapsed_cycles += 2;
        }

        break;
      }

      case Instruction::LD_A_INDIRECT_BC: {
        READ_BYTE(BC, A);
        break;
      }

      case Instruction::LD_A_INDIRECT_DE: {
        READ_BYTE(DE, A);
        break;
      }

      case Instruction::LD_A_INDIRECT_NN: {
        int nn;

        READ_NN(nn);
        READ_BYTE(nn, A);
        break;
      }

      case Instruction::LD_INDIRECT_BC_A: {
        WRITE_BYTE(BC, A);
        break;
      }

      case Instruction::LD_INDIRECT_DE_A: {
        WRITE_BYTE(DE, A);
        break;
      }

      case Instruction::LD_INDIRECT_NN_A: {
        int nn;

        READ_NN(nn);
        WRITE_BYTE(nn, A);
        break;
      }

      case Instruction::LD_A_I_LD_A_R: {
        int a, f;

        a = opcode == OPCODE_LD_A_I ? i_ : (r_ & 0x80) | (r & 0x7f);
        f = SZYX_FLAGS_TABLE[a];

        /* Note: On a real processor, if an interrupt
         * occurs during the execution of either
         * "LD A, I" or "LD A, R", the parity flag is
         * reset. That can never happen here.
         */

        f |= iff2_ << Z80_P_FLAG_SHIFT;
        f |= F & Z80_C_FLAG;

        AF = (a << 8) | f;

        elapsed_cycles++;

        break;
      }

      case Instruction::LD_I_A_LD_R_A: {
        if (opcode == OPCODE_LD_I_A)

          i_ = A;

        else {
          r_ = A;
          r = A & 0x7f;
        }

        elapsed_cycles++;

        break;
      }

        /* 16-bit load group. */

      case Instruction::LD_RR_NN: {
        READ_NN(RR(P(opcode)));
        break;
      }

      case Instruction::LD_HL_INDIRECT_NN: {
        int nn;

        READ_NN(nn);
        READ_WORD(nn, HL_IX_IY);
        break;
      }

      case Instruction::LD_RR_INDIRECT_NN: {
        int nn;

        READ_NN(nn);
        READ_WORD(nn, RR(P(opcode)));
        break;
      }

      case Instruction::LD_INDIRECT_NN_HL: {
        int nn;

        READ_NN(nn);
        WRITE_WORD(nn, HL_IX_IY);
        break;
      }

      case Instruction::LD_INDIRECT_NN_RR: {
        int nn;

        READ_NN(nn);
        WRITE_WORD(nn, RR(P(opcode)));
        break;
      }

      case Instruction::LD_SP_HL: {
        SP = HL_IX_IY;
        elapsed_cycles += 2;
        break;
      }

      case Instruction::PUSH_SS: {
        PUSH(SS(P(opcode)));
        elapsed_cycles++;
        break;
      }

      case Instruction::POP_SS: {
        POP(SS(P(opcode)));
        break;
      }

        /* Exchange, block transfer and search group. */

      case Instruction::EX_DE_HL: {
        EXCHANGE(DE, HL);
        break;
      }

      case Instruction::EX_AF_AF_PRIME: {
        EXCHANGE(AF, alternates_[Z80_AF]);
        break;
      }

      case Instruction::EXX: {
        EXCHANGE(BC, alternates_[Z80_BC]);
        EXCHANGE(DE, alternates_[Z80_DE]);
        EXCHANGE(HL, alternates_[Z80_HL]);
        break;
      }

      case Instruction::EX_INDIRECT_SP_HL: {
        int t;

        READ_WORD(SP, t);
        WRITE_WORD(SP, HL_IX_IY);
        HL_IX_IY = t;

        elapsed_cycles += 3;

        break;
      }

      case Instruction::LDI_LDD: {
        int n, f, d;

        READ_BYTE(HL, n);
        WRITE_BYTE(DE, n);

        f = F & SZC_FLAGS;
        f |= --BC ? Z80_P_FLAG : 0;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          n += A;
          f |= n & Z80_X_FLAG;
          f |= (n << (Z80_Y_FLAG_SHIFT - 1)) & Z80_Y_FLAG;
        }

        F = f;

        d = opcode == OPCODE_LDI ? +1 : -1;
        DE += d;
        HL += d;

        elapsed_cycles += 2;

        break;
      }

      case Instruction::LDIR_LDDR: {
        int d, f, bc, de, hl, n;

#ifdef Z80_HANDLE_SELF_MODIFYING_CODE

        int p, q;

        p = (pc - 2) & 0xffff;
        q = (pc - 1) & 0xffff;

#endif

        d = opcode == OPCODE_LDIR ? +1 : -1;

        f = F & SZC_FLAGS;
        bc = BC;
        de = DE;
        hl = HL;

        r -= 2;
        elapsed_cycles -= 8;
        for (;;) {
          r += 2;

          Z80_READ_BYTE(hl, n);
          Z80_WRITE_BYTE(de, n);

          hl += d;
          de += d;

          if (--bc)

            elapsed_cycles += 21;

          else {
            elapsed_cycles += 16;
            break;
          }

#ifdef Z80_HANDLE_SELF_MODIFYING_CODE

          if (((de - d) & 0xffff) == p || ((de - d) & 0xffff) == q) {
            f |= Z80_P_FLAG;
            pc -= 2;
            break;
          }

#endif

          if (elapsed_cycles < number_cycles)

            continue;

          else {
            f |= Z80_P_FLAG;
            pc -= 2;
            break;
          }
        }

        HL = hl;
        DE = de;
        BC = bc;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          n += A;
          f |= n & Z80_X_FLAG;
          f |= (n << (Z80_Y_FLAG_SHIFT - 1)) & Z80_Y_FLAG;
        }

        F = f;

        break;
      }

      case Instruction::CPI_CPD: {
        int a, n, z, f;

        a = A;
        READ_BYTE(HL, n);
        z = a - n;

        HL += opcode == OPCODE_CPI ? +1 : -1;

        f = (a ^ n ^ z) & Z80_H_FLAG;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          n = z - (f >> Z80_H_FLAG_SHIFT);
          f |= (n << (Z80_Y_FLAG_SHIFT - 1)) & Z80_Y_FLAG;
          f |= n & Z80_X_FLAG;
        }

        f |= SZYX_FLAGS_TABLE[z & 0xff] & SZ_FLAGS;
        f |= --BC ? Z80_P_FLAG : 0;
        F = f | Z80_N_FLAG | (F & Z80_C_FLAG);

        elapsed_cycles += 5;

        break;
      }

      case Instruction::CPIR_CPDR: {
        int d, a, bc, hl, n, z, f;

        d = opcode == OPCODE_CPIR ? +1 : -1;

        a = A;
        bc = BC;
        hl = HL;

        r -= 2;
        elapsed_cycles -= 8;
        for (;;) {
          r += 2;

          Z80_READ_BYTE(hl, n);
          z = a - n;

          hl += d;
          if (--bc && z)

            elapsed_cycles += 21;

          else {
            elapsed_cycles += 16;
            break;
          }

          if (elapsed_cycles < number_cycles)

            continue;

          else {
            pc -= 2;
            break;
          }
        }

        HL = hl;
        BC = bc;

        f = (a ^ n ^ z) & Z80_H_FLAG;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          n = z - (f >> Z80_H_FLAG_SHIFT);
          f |= (n << (Z80_Y_FLAG_SHIFT - 1)) & Z80_Y_FLAG;
          f |= n & Z80_X_FLAG;
        }

        f |= SZYX_FLAGS_TABLE[z & 0xff] & SZ_FLAGS;
        f |= bc ? Z80_P_FLAG : 0;
        F = f | Z80_N_FLAG | (F & Z80_C_FLAG);

        break;
      }

        /* 8-bit arithmetic and logical group. */

      case Instruction::ADD_R: {
        ADD(R(Z(opcode)));
        break;
      }

      case Instruction::ADD_N: {
        int n;

        READ_N(n);
        ADD(n);
        break;
      }

      case Instruction::ADD_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        ADD(x);
        break;
      }

      case Instruction::ADC_R: {
        ADC(R(Z(opcode)));
        break;
      }

      case Instruction::ADC_N: {
        int n;

        READ_N(n);
        ADC(n);
        break;
      }

      case Instruction::ADC_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        ADC(x);
        break;
      }

      case Instruction::SUB_R: {
        SUB(R(Z(opcode)));
        break;
      }

      case Instruction::SUB_N: {
        int n;

        READ_N(n);
        SUB(n);
        break;
      }

      case Instruction::SUB_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        SUB(x);
        break;
      }

      case Instruction::SBC_R: {
        SBC(R(Z(opcode)));
        break;
      }

      case Instruction::SBC_N: {
        int n;

        READ_N(n);
        SBC(n);
        break;
      }

      case Instruction::SBC_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        SBC(x);
        break;
      }

      case Instruction::AND_R: {
        AND(R(Z(opcode)));
        break;
      }

      case Instruction::AND_N: {
        int n;

        READ_N(n);
        AND(n);
        break;
      }

      case Instruction::AND_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        AND(x);
        break;
      }

      case Instruction::OR_R: {
        OR(R(Z(opcode)));
        break;
      }

      case Instruction::OR_N: {
        int n;

        READ_N(n);
        OR(n);
        break;
      }

      case Instruction::OR_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        OR(x);
        break;
      }

      case Instruction::XOR_R: {
        XOR(R(Z(opcode)));
        break;
      }

      case Instruction::XOR_N: {
        int n;

        READ_N(n);
        XOR(n);
        break;
      }

      case Instruction::XOR_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        XOR(x);
        break;
      }

      case Instruction::CP_R: {
        CP(R(Z(opcode)));
        break;
      }

      case Instruction::CP_N: {
        int n;

        READ_N(n);
        CP(n);
        break;
      }

      case Instruction::CP_INDIRECT_HL: {
        int x;

        READ_INDIRECT_HL(x);
        CP(x);
        break;
      }

      case Instruction::INC_R: {
        INC(R(Y(opcode)));
        break;
      }

      case Instruction::INC_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          INC(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          READ_D(d);
          d += HL_IX_IY;
          READ_BYTE(d, x);
          INC(x);
          WRITE_BYTE(d, x);

          elapsed_cycles += 6;
        }
        break;
      }

      case Instruction::DEC_R: {
        DEC(R(Y(opcode)));
        break;
      }

      case Instruction::DEC_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          DEC(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          READ_D(d);
          d += HL_IX_IY;
          READ_BYTE(d, x);
          DEC(x);
          WRITE_BYTE(d, x);

          elapsed_cycles += 6;
        }
        break;
      }

        /* General-purpose arithmetic and CPU control group. */

      case Instruction::DAA: {
        int a, c, d;

        /* The following algorithm is from
         * comp.sys.sinclair's FAQ.
         */

        a = A;
        if (a > 0x99 || (F & Z80_C_FLAG)) {
          c = Z80_C_FLAG;
          d = 0x60;

        } else

          c = d = 0;

        if ((a & 0x0f) > 0x09 || (F & Z80_H_FLAG)) d += 0x06;

        A += F & Z80_N_FLAG ? -d : +d;
        F = SZYXP_FLAGS_TABLE[A] | ((A ^ a) & Z80_H_FLAG) | (F & Z80_N_FLAG) |
            c;

        break;
      }

      case Instruction::CPL: {
        A = ~A;
        F = (F & (SZPV_FLAGS | Z80_C_FLAG)) | Z80_H_FLAG | Z80_N_FLAG;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          F |= (A & YX_FLAGS);
        }

        break;
      }

      case Instruction::NEG: {
        int a, f, z, c;

        a = A;
        z = -a;

        c = a ^ z;
        f = Z80_N_FLAG | (c & Z80_H_FLAG);
        f |= SZYX_FLAGS_TABLE[z &= 0xff];
        c &= 0x0180;
        f |= OVERFLOW_TABLE[c >> 7];
        f |= c >> (8 - Z80_C_FLAG_SHIFT);

        A = z;
        F = f;

        break;
      }

      case Instruction::CCF: {
        int c;

        c = F & Z80_C_FLAG;
        F = (F & SZPV_FLAGS) | (c << Z80_H_FLAG_SHIFT) | (c ^ Z80_C_FLAG);

        if constexpr (!Board::kDocumentedFlagsOnly) {
          F |= (A & YX_FLAGS);
        }

        break;
      }

      case Instruction::SCF: {
        F = (F & SZPV_FLAGS) | Z80_C_FLAG;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          F |= (A & YX_FLAGS);
        }

        break;
      }

      case Instruction::NOP: {
        break;
      }

      case Instruction::HALT: {
        board_.Halt();

#ifdef Z80_CATCH_HALT

        status_ = Status::HALT;

#else
        /* If an HALT instruction is executed, the Z80
         * keeps executing NOPs until an interrupt is
         * generated. Basically nothing happens for the
         * remaining number of cycles.
         */

        if (elapsed_cycles < number_cycles) elapsed_cycles = number_cycles;

#endif

        goto stop_emulation;
      }

      case Instruction::DI: {
        iff1_ = iff2_ = 0;

#ifdef Z80_CATCH_DI

        status_ = Z80_STATUS_FLAG_DI;
        goto stop_emulation;

#else

        /* No interrupt can be accepted right after
         * a DI or EI instruction on an actual Z80
         * processor. By adding 4 cycles to
         * number_cycles, at least one more
         * instruction will be executed. However, this
         * will fail if the next instruction has
         * multiple 0xdd or 0xfd prefixes and
         * Z80_PREFIX_FAILSAFE is defined, but that
         * is an unlikely pathological case.
         */

        number_cycles += 4;
        break;

#endif
      }

      case Instruction::EI: {
        iff1_ = iff2_ = 1;

#ifdef Z80_CATCH_EI

        status_ = Status::EI;
        goto stop_emulation;

#else

        /* See comment for DI. */

        number_cycles += 4;
        break;

#endif
      }

      case Instruction::IM_N: {
        /* "IM 0/1" (0xed prefixed opcodes 0x4e and
         * 0x6e) is treated like a "IM 0".
         */

        if ((Y(opcode) & 0x03) <= 0x01)

          im_ = Z80_INTERRUPT_MODE_0;

        else if (!(Y(opcode) & 1))

          im_ = Z80_INTERRUPT_MODE_1;

        else

          im_ = Z80_INTERRUPT_MODE_2;

        break;
      }

        /* 16-bit arithmetic group. */

      case Instruction::ADD_HL_RR: {
        int x, y, z, f, c;

        x = HL_IX_IY;
        y = RR(P(opcode));
        z = x + y;

        c = x ^ y ^ z;
        f = F & SZPV_FLAGS;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          f |= (z >> 8) & YX_FLAGS;
          f |= (c >> 8) & Z80_H_FLAG;
        }

        f |= c >> (16 - Z80_C_FLAG_SHIFT);

        HL_IX_IY = z;
        F = f;

        elapsed_cycles += 7;

        break;
      }

      case Instruction::ADC_HL_RR: {
        int x, y, z, f, c;

        x = HL;
        y = RR(P(opcode));
        z = x + y + (F & Z80_C_FLAG);

        c = x ^ y ^ z;
        f = z & 0xffff ? (z >> 8) & SYX_FLAGS : Z80_Z_FLAG;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          f |= (c >> 8) & Z80_H_FLAG;
        }

        f |= OVERFLOW_TABLE[c >> 15];
        f |= z >> (16 - Z80_C_FLAG_SHIFT);

        HL = z;
        F = f;

        elapsed_cycles += 7;

        break;
      }

      case Instruction::SBC_HL_RR: {
        int x, y, z, f, c;

        x = HL;
        y = RR(P(opcode));
        z = x - y - (F & Z80_C_FLAG);

        c = x ^ y ^ z;
        f = Z80_N_FLAG;
        f |= z & 0xffff ? (z >> 8) & SYX_FLAGS : Z80_Z_FLAG;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          f |= (c >> 8) & Z80_H_FLAG;
        }

        c &= 0x018000;
        f |= OVERFLOW_TABLE[c >> 15];
        f |= c >> (16 - Z80_C_FLAG_SHIFT);

        HL = z;
        F = f;

        elapsed_cycles += 7;

        break;
      }

      case Instruction::INC_RR: {
        int x;

        x = RR(P(opcode));
        x++;
        RR(P(opcode)) = x;

        elapsed_cycles += 2;

        break;
      }

      case Instruction::DEC_RR: {
        int x;

        x = RR(P(opcode));
        x--;
        RR(P(opcode)) = x;

        elapsed_cycles += 2;

        break;
      }

        /* Rotate and shift group. */

      case Instruction::RLCA: {
        A = (A << 1) | (A >> 7);
        F = (F & SZPV_FLAGS) | (A & (YX_FLAGS | Z80_C_FLAG));
        break;
      }

      case Instruction::RLA: {
        int a, f;

        a = A << 1;
        f = (F & SZPV_FLAGS) | (A >> 7);

        if constexpr (!Board::kDocumentedFlagsOnly) {
          f |= (a & YX_FLAGS);
        }

        A = a | (F & Z80_C_FLAG);
        F = f;

        break;
      }

      case Instruction::RRCA: {
        int c;

        c = A & 0x01;
        A = (A >> 1) | (A << 7);
        F = (F & SZPV_FLAGS) | c;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          F |= (A & YX_FLAGS);
        }

        break;
      }

      case Instruction::RRA: {
        int c;

        c = A & 0x01;
        A = (A >> 1) | ((F & Z80_C_FLAG) << 7);
        F = (F & SZPV_FLAGS) | c;

        if constexpr (!Board::kDocumentedFlagsOnly) {
          F |= (A & YX_FLAGS);
        }

        break;
      }

      case Instruction::RLC_R: {
        RLC(R(Z(opcode)));
        break;
      }

      case Instruction::RLC_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          RLC(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          RLC(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }

        break;
      }

      case Instruction::RL_R: {
        RL(R(Z(opcode)));
        break;
      }

      case Instruction::RL_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          RL(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          RL(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::RRC_R: {
        RRC(R(Z(opcode)));
        break;
      }

      case Instruction::RRC_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          RRC(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          RRC(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::RR_R: {
        RR_INSTRUCTION(R(Z(opcode)));
        break;
      }

      case Instruction::RR_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          RR_INSTRUCTION(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          RR_INSTRUCTION(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::SLA_R: {
        SLA(R(Z(opcode)));
        break;
      }

      case Instruction::SLA_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          SLA(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          SLA(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::SLL_R: {
        SLL(R(Z(opcode)));
        break;
      }

      case Instruction::SLL_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          SLL(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          SLL(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::SRA_R: {
        SRA(R(Z(opcode)));
        break;
      }

      case Instruction::SRA_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          SRA(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          SRA(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::SRL_R: {
        SRL(R(Z(opcode)));
        break;
      }

      case Instruction::SRL_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          SRL(x);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          SRL(x);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::RLD_RRD: {
        int x, y;

        READ_BYTE(HL, x);
        y = (A & 0xf0) << 8;
        y |= opcode == OPCODE_RLD
                 ? (x << 4) | (A & 0x0f)
                 : ((x & 0x0f) << 8) | ((A & 0x0f) << 4) | (x >> 4);
        WRITE_BYTE(HL, y);
        y >>= 8;

        A = y;
        F = SZYXP_FLAGS_TABLE[y] | (F & Z80_C_FLAG);

        elapsed_cycles += 4;

        break;
      }

        /* Bit set, reset, and test group. */

      case Instruction::BIT_B_R: {
        int x;

        x = R(Z(opcode)) & (1 << Y(opcode));
        F = (x ? 0 : Z80_Z_FLAG | Z80_P_FLAG) | Z80_H_FLAG | (F & Z80_C_FLAG);

        if constexpr (!Board::kDocumentedFlagsOnly) {
          F |= (x & Z80_S_FLAG) | (R(Z(opcode)) & YX_FLAGS);
        }

        break;
      }

      case Instruction::BIT_B_INDIRECT_HL: {
        int d, x;

        if (registers == register_table_) {
          d = HL;

          elapsed_cycles++;

        } else {
          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          pc += 2;

          elapsed_cycles += 5;
        }

        READ_BYTE(d, x);
        x &= 1 << Y(opcode);
        F = (x ? 0 : Z80_Z_FLAG | Z80_P_FLAG) | Z80_H_FLAG | (F & Z80_C_FLAG);

        if constexpr (!Board::kDocumentedFlagsOnly) {
          F |= (x & Z80_S_FLAG) | (d & YX_FLAGS);
        }

        break;
      }

      case Instruction::SET_B_R: {
        R(Z(opcode)) |= 1 << Y(opcode);
        break;
      }

      case Instruction::SET_B_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          x |= 1 << Y(opcode);
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          x |= 1 << Y(opcode);
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

      case Instruction::RES_B_R: {
        R(Z(opcode)) &= ~(1 << Y(opcode));
        break;
      }

      case Instruction::RES_B_INDIRECT_HL: {
        int x;

        if (registers == register_table_) {
          READ_BYTE(HL, x);
          x &= ~(1 << Y(opcode));
          WRITE_BYTE(HL, x);

          elapsed_cycles++;

        } else {
          int d;

          Z80_FETCH_BYTE(pc, d);
          d = ((signed char)d) + HL_IX_IY;

          READ_BYTE(d, x);
          x &= ~(1 << Y(opcode));
          WRITE_BYTE(d, x);

          if (Z(opcode) != INDIRECT_HL) R(Z(opcode)) = x;

          pc += 2;

          elapsed_cycles += 5;
        }
        break;
      }

        /* Jump group. */

      case Instruction::JP_NN: {
        int nn;

        Z80_FETCH_WORD(pc, nn);
        pc = nn;

        elapsed_cycles += 6;

        break;
      }

      case Instruction::JP_CC_NN: {
        int nn;

        if (CC(Y(opcode))) {
          Z80_FETCH_WORD(pc, nn);
          pc = nn;

        } else {
#ifdef Z80_FALSE_CONDITION_FETCH

          Z80_FETCH_WORD(pc, nn);

#endif

          pc += 2;
        }

        elapsed_cycles += 6;

        break;
      }

      case Instruction::JR_E: {
        int e;

        Z80_FETCH_BYTE(pc, e);
        pc += ((signed char)e) + 1;

        elapsed_cycles += 8;

        break;
      }

      case Instruction::JR_DD_E: {
        int e;

        if (DD(Q(opcode))) {
          Z80_FETCH_BYTE(pc, e);
          pc += ((signed char)e) + 1;

          elapsed_cycles += 8;

        } else {
#ifdef Z80_FALSE_CONDITION_FETCH

          Z80_FETCH_BYTE(pc, e);

#endif

          pc++;

          elapsed_cycles += 3;
        }
        break;
      }

      case Instruction::JP_HL: {
        pc = HL_IX_IY;
        break;
      }

      case Instruction::DJNZ_E: {
        int e;

        if (--B) {
          Z80_FETCH_BYTE(pc, e);
          pc += ((signed char)e) + 1;

          elapsed_cycles += 9;

        } else {
#ifdef Z80_FALSE_CONDITION_FETCH

          Z80_FETCH_BYTE(pc, e);

#endif

          pc++;

          elapsed_cycles += 4;
        }
        break;
      }

        /* Call and return group. */

      case Instruction::CALL_NN: {
        int nn;

        READ_NN(nn);
        PUSH(pc);
        pc = nn;

        elapsed_cycles++;

        break;
      }

      case Instruction::CALL_CC_NN: {
        int nn;

        if (CC(Y(opcode))) {
          READ_NN(nn);
          PUSH(pc);
          pc = nn;

          elapsed_cycles++;

        } else {
#ifdef Z80_FALSE_CONDITION_FETCH

          Z80_FETCH_WORD(pc, nn);

#endif

          pc += 2;

          elapsed_cycles += 6;
        }
        break;
      }

      case Instruction::RET: {
        POP(pc);
        if (elapsed_cycles >= number_cycles) goto stop_emulation;
        break;
      }

      case Instruction::RET_CC: {
        if (CC(Y(opcode))) {
          POP(pc);
        }
        elapsed_cycles++;
        if (elapsed_cycles >= number_cycles) goto stop_emulation;
        break;
      }

      case Instruction::RETI_RETN: {
        iff1_ = iff2_;
        POP(pc);

#if defined(Z80_CATCH_RETI) && defined(Z80_CATCH_RETN)

        status_ =
            opcode == OPCODE_RETI ? Z80_STATUS_FLAG_RETI : Z80_STATUS_FLAG_RETN;
        goto stop_emulation;

#elif defined(Z80_CATCH_RETI)

        status_ = Z80_STATUS_FLAG_RETI;
        goto stop_emulation;

#elif defined(Z80_CATCH_RETN)

        status_ = Z80_STATUS_FLAG_RETN;
        goto stop_emulation;

#else

        break;

#endif
      }

      case Instruction::RST_P: {
        PUSH(pc);
        pc = RST_TABLE[Y(opcode)];
        elapsed_cycles++;
        break;
      }

        /* Input and output group. */

      case Instruction::IN_A_N: {
        int n;

        READ_N(n);
        Z80_INPUT_BYTE(n, A);

        elapsed_cycles += 4;

        break;
      }

      case Instruction::IN_R_C: {
        int x;
        Z80_INPUT_BYTE(BC, x);
        if (Y(opcode) != INDIRECT_HL) R(Y(opcode)) = x;
        F = SZYXP_FLAGS_TABLE[x] | (F & Z80_C_FLAG);

        elapsed_cycles += 4;

        break;
      }

        /* Some of the undocumented flags for "INI", "IND",
         * "INIR", "INDR",  "OUTI", "OUTD", "OTIR", and
         * "OTDR" are really really strange. The emulator
         * implements the specifications described in "The
         * Undocumented Z80 Documented Version 0.91".
         */

      case Instruction::INI_IND: {
        int x, f;

        Z80_INPUT_BYTE(C, x);
        WRITE_BYTE(HL, x);

        f = SZYX_FLAGS_TABLE[--B & 0xff] | (x >> (7 - Z80_N_FLAG_SHIFT));
        if (opcode == OPCODE_INI) {
          HL++;
          x += (C + 1) & 0xff;

        } else {
          HL--;
          x += (C - 1) & 0xff;
        }
        f |= x & 0x0100 ? HC_FLAGS : 0;
        f |= SZYXP_FLAGS_TABLE[(x & 0x07) ^ B] & Z80_P_FLAG;
        F = f;

        elapsed_cycles += 5;

        break;
      }

      case Instruction::INIR_INDR: {
        int d, b, hl, x, f;

#ifdef Z80_HANDLE_SELF_MODIFYING_CODE

        int p, q;

        p = (pc - 2) & 0xffff;
        q = (pc - 1) & 0xffff;

#endif

        d = opcode == OPCODE_INIR ? +1 : -1;

        b = B;
        hl = HL;

        r -= 2;
        elapsed_cycles -= 8;
        for (;;) {
          r += 2;

          Z80_INPUT_BYTE(C, x);
          Z80_WRITE_BYTE(hl, x);

          hl += d;

          if (--b)

            elapsed_cycles += 21;

          else {
            f = Z80_Z_FLAG;
            elapsed_cycles += 16;
            break;
          }

#ifdef Z80_HANDLE_SELF_MODIFYING_CODE

          if (((hl - d) & 0xffff) == p || ((hl - d) & 0xffff) == q) {
            f = SZYX_FLAGS_TABLE[b];
            pc -= 2;
            break;
          }

#endif

          if (elapsed_cycles < number_cycles)

            continue;

          else {
            f = SZYX_FLAGS_TABLE[b];
            pc -= 2;
            break;
          }
        }

        HL = hl;
        B = b;

        f |= x >> (7 - Z80_N_FLAG_SHIFT);
        x += (C + d) & 0xff;
        f |= x & 0x0100 ? HC_FLAGS : 0;
        f |= SZYXP_FLAGS_TABLE[(x & 0x07) ^ b] & Z80_P_FLAG;
        F = f;

        break;
      }

      case Instruction::OUT_N_A: {
        int n;

        elapsed_cycles += 4;
        READ_N(n);
        if (Z80_OUTPUT_BYTE(n, A)) {
          goto stop_emulation;
        }

        break;
      }

      case Instruction::OUT_C_R: {
        int x;

        elapsed_cycles += 4;
        x = Y(opcode) != INDIRECT_HL ? R(Y(opcode)) : 0;
        if (Z80_OUTPUT_BYTE(C, x)) {
          goto stop_emulation;
        }

        break;
      }

      case Instruction::OUTI_OUTD: {
        int x, f;

        READ_BYTE(HL, x);
        Z80_OUTPUT_BYTE(C, x);

        HL += opcode == OPCODE_OUTI ? +1 : -1;

        f = SZYX_FLAGS_TABLE[--B & 0xff] | (x >> (7 - Z80_N_FLAG_SHIFT));
        x += HL & 0xff;
        f |= x & 0x0100 ? HC_FLAGS : 0;
        f |= SZYXP_FLAGS_TABLE[(x & 0x07) ^ B] & Z80_P_FLAG;
        F = f;

        break;
      }

      case Instruction::OTIR_OTDR: {
        int d, b, hl, x, f;

        d = opcode == OPCODE_OTIR ? +1 : -1;

        b = B;
        hl = HL;

        r -= 2;
        elapsed_cycles -= 8;
        for (;;) {
          r += 2;

          Z80_READ_BYTE(hl, x);
          Z80_OUTPUT_BYTE(C, x);

          hl += d;
          if (--b)

            elapsed_cycles += 21;

          else {
            f = Z80_Z_FLAG;
            elapsed_cycles += 16;
            break;
          }

          if (elapsed_cycles < number_cycles)

            continue;

          else {
            f = SZYX_FLAGS_TABLE[b];
            pc -= 2;
            break;
          }
        }

        HL = hl;
        B = b;

        f |= x >> (7 - Z80_N_FLAG_SHIFT);
        x += hl & 0xff;
        f |= x & 0x0100 ? HC_FLAGS : 0;
        f |= SZYXP_FLAGS_TABLE[(x & 0x07) ^ b] & Z80_P_FLAG;
        F = f;

        break;
      }

        /* Prefix group. */

      case Instruction::CB_PREFIX: {
        /* Special handling if the 0xcb prefix is
         * prefixed by a 0xdd or 0xfd prefix.
         */

        if (registers != register_table_) {
          r--;

          /* Indexed memory access routine will
           * correctly update pc.
           */

          Z80_FETCH_BYTE(pc + 1, opcode);

        } else {
          Z80_FETCH_BYTE(pc, opcode);
          pc++;
        }
        instruction = (Instruction)CB_INSTRUCTION_TABLE[opcode];

        goto emulate_next_instruction;
      }

      case Instruction::DD_PREFIX: {
        registers = dd_register_table_;

#ifdef Z80_PREFIX_FAILSAFE

        /* Ensure that at least number_cycles cycles
         * are executed.
         */

        if (elapsed_cycles < number_cycles) {
          Z80_FETCH_BYTE(pc, opcode);
          pc++;
          goto emulate_next_opcode;

        } else {
          status_ = Z80_STATUS_PREFIX;
          pc--;
          elapsed_cycles -= 4;
          goto stop_emulation;
        }

#else

        Z80_FETCH_BYTE(pc, opcode);
        pc++;
        goto emulate_next_opcode;

#endif
      }

      case Instruction::FD_PREFIX: {
        registers = fd_register_table_;

#ifdef Z80_PREFIX_FAILSAFE

        if (elapsed_cycles < number_cycles) {
          Z80_FETCH_BYTE(pc, opcode);
          pc++;
          goto emulate_next_opcode;

        } else {
          status_ = Z80_STATUS_PREFIX;
          pc--;
          elapsed_cycles -= 4;
          goto stop_emulation;
        }

#else

        Z80_FETCH_BYTE(pc, opcode);
        pc++;
        goto emulate_next_opcode;

#endif
      }

      case Instruction::ED_PREFIX: {
        registers = register_table_;
        Z80_FETCH_BYTE(pc, opcode);
        pc++;
        instruction = (Instruction)ED_INSTRUCTION_TABLE[opcode];

        goto emulate_next_instruction;
      }

        /* Special/pseudo instruction group. */

      case Instruction::ED_UNDEFINED: {
#ifdef Z80_CATCH_ED_UNDEFINED

        status_ = Z80_STATUS_FLAG_ED_UNDEFINED;
        pc -= 2;
        goto stop_emulation;

#else

        break;

#endif
      }
    }

    //    if (elapsed_cycles >= number_cycles) goto stop_emulation;
  }

stop_emulation:

  r_ = (r_ & 0x80) | (r & 0x7f);
  pc_ = pc & 0xffff;

  return elapsed_cycles;
}

template <typename Board>
uint8_t Z80Emu<Board>::ReadReg(int reg) {
  return registers_.byte[reg];
}

template <typename Board>
uint16_t Z80Emu<Board>::ReadReg16(int reg) {
  return registers_.word[reg];
}

}  // namespace z80emu