MIPS port initial commit

src/mips/simulator-mips.cc

Index: src/mips/simulator-mips.cc
===================================================================
--- src/mips/simulator-mips.cc	(revision 0)
+++ src/mips/simulator-mips.cc	(revision 0)
@@ -0,0 +1,1648 @@
+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <stdlib.h>
+#include <cstdarg>
+#include "v8.h"
+
+#include "disasm.h"
+#include "assembler.h"
+#include "globals.h"    // Need the bit_cast
+#include "mips/constants-mips.h"
+#include "mips/simulator-mips.h"
+
+namespace v8i = v8::internal;
+
+#if !defined(__mips)
+
+// Only build the simulator if not compiling for real MIPS hardware.
+namespace assembler {
+namespace mips {
+
+using ::v8::internal::Object;
+using ::v8::internal::PrintF;
+using ::v8::internal::OS;
+using ::v8::internal::ReadLine;
+using ::v8::internal::DeleteArray;
+
+// Utils functions
+bool HaveSameSign(int32_t a, int32_t b) {
+  return ((a ^ b) > 0);
+}
+
+
+// This macro provides a platform independent use of sscanf. The reason for
+// SScanF not being implemented in a platform independent was through
+// ::v8::internal::OS in the same way as SNPrintF is that the Windows C Run-Time
+// Library does not provide vsscanf.
+#define SScanF sscanf  // NOLINT
+
+// The Debugger class is used by the simulator while debugging simulated MIPS
+// code.
+class Debugger {
+ public:
+  explicit Debugger(Simulator* sim);
+  ~Debugger();
+
+  void Stop(Instruction* instr);
+  void Debug();
+
+ private:
+  // We set the breakpoint code to 0xfffff to easily recognize it.
+  static const Instr kBreakpointInstr = SPECIAL | BREAK | 0xfffff << 6;
+  static const Instr kNopInstr =  0x0;
+
+  Simulator* sim_;
+
+  int32_t GetRegisterValue(int regnum);
+  bool GetValue(const char* desc, int32_t* value);
+
+  // Set or delete a breakpoint. Returns true if successful.
+  bool SetBreakpoint(Instruction* breakpc);
+  bool DeleteBreakpoint(Instruction* breakpc);
+
+  // Undo and redo all breakpoints. This is needed to bracket disassembly and
+  // execution to skip past breakpoints when run from the debugger.
+  void UndoBreakpoints();
+  void RedoBreakpoints();
+
+  // Print all registers with a nice formatting.
+  void PrintAllRegs();
+};
+
+Debugger::Debugger(Simulator* sim) {
+  sim_ = sim;
+}
+
+Debugger::~Debugger() {
+}
+
+#ifdef GENERATED_CODE_COVERAGE
+static FILE* coverage_log = NULL;
+
+
+static void InitializeCoverage() {
+  char* file_name = getenv("V8_GENERATED_CODE_COVERAGE_LOG");
+  if (file_name != NULL) {
+    coverage_log = fopen(file_name, "aw+");
+  }
+}
+
+
+void Debugger::Stop(Instruction* instr) {
+  UNIMPLEMENTED_MIPS();
+  char* str = reinterpret_cast<char*>(instr->InstructionBits());
+  if (strlen(str) > 0) {
+    if (coverage_log != NULL) {
+      fprintf(coverage_log, "%s\n", str);
+      fflush(coverage_log);
+    }
+    instr->SetInstructionBits(0x0);  // Overwrite with nop.
+  }
+  sim_->set_pc(sim_->get_pc() + Instruction::kInstructionSize);
+}
+
+#else  // ndef GENERATED_CODE_COVERAGE
+
+#define UNSUPPORTED() printf("Unsupported instruction.\n");
+
+static void InitializeCoverage() {}
+
+
+void Debugger::Stop(Instruction* instr) {
+  const char* str = reinterpret_cast<char*>(instr->InstructionBits());
+  PrintF("Simulator hit %s\n", str);
+  sim_->set_pc(sim_->get_pc() + Instruction::kInstructionSize);
+  Debug();
+}
+#endif  // def GENERATED_CODE_COVERAGE
+
+
+int32_t Debugger::GetRegisterValue(int regnum) {
+  if (regnum == kNumSimuRegisters) {
+    return sim_->get_pc();
+  } else {
+    return sim_->get_register(regnum);
+  }
+}
+
+
+bool Debugger::GetValue(const char* desc, int32_t* value) {
+  int regnum = Registers::Number(desc);
+  if (regnum != kInvalidRegister) {
+    *value = GetRegisterValue(regnum);
+    return true;
+  } else {
+    return SScanF(desc, "%i", value) == 1;
+  }
+  return false;
+}
+
+
+bool Debugger::SetBreakpoint(Instruction* breakpc) {
+  // Check if a breakpoint can be set. If not return without any side-effects.
+  if (sim_->break_pc_ != NULL) {
+    return false;
+  }
+
+  // Set the breakpoint.
+  sim_->break_pc_ = breakpc;
+  sim_->break_instr_ = breakpc->InstructionBits();
+  // Not setting the breakpoint instruction in the code itself. It will be set
+  // when the debugger shell continues.
+  return true;
+}
+
+
+bool Debugger::DeleteBreakpoint(Instruction* breakpc) {
+  if (sim_->break_pc_ != NULL) {
+    sim_->break_pc_->SetInstructionBits(sim_->break_instr_);
+  }
+
+  sim_->break_pc_ = NULL;
+  sim_->break_instr_ = 0;
+  return true;
+}
+
+
+void Debugger::UndoBreakpoints() {
+  if (sim_->break_pc_ != NULL) {
+    sim_->break_pc_->SetInstructionBits(sim_->break_instr_);
+  }
+}
+
+
+void Debugger::RedoBreakpoints() {
+  if (sim_->break_pc_ != NULL) {
+    sim_->break_pc_->SetInstructionBits(kBreakpointInstr);
+  }
+}
+
+void Debugger::PrintAllRegs() {
+#define REG_INFO(n) Registers::Name(n), GetRegisterValue(n), GetRegisterValue(n)
+
+  PrintF("\n");
+  // at, v0, a0
+  PrintF("%3s: 0x%08x %10d\t%3s: 0x%08x %10d\t%3s: 0x%08x %10d\n",
+         REG_INFO(1), REG_INFO(2), REG_INFO(4));
+  // v1, a1
+  PrintF("%26s\t%3s: 0x%08x %10d\t%3s: 0x%08x %10d\n",
+         "", REG_INFO(3), REG_INFO(5));
+  // a2
+  PrintF("%26s\t%26s\t%3s: 0x%08x %10d\n", "", "", REG_INFO(6));
+  // a3
+  PrintF("%26s\t%26s\t%3s: 0x%08x %10d\n", "", "", REG_INFO(7));
+  PrintF("\n");
+  // t0-t7, s0-s7
+  for (int i = 0; i < 8; i++) {
+    PrintF("%3s: 0x%08x %10d\t%3s: 0x%08x %10d\n",
+           REG_INFO(8+i), REG_INFO(16+i));
+  }
+  PrintF("\n");
+  // t8, k0, LO
+  PrintF("%3s: 0x%08x %10d\t%3s: 0x%08x %10d\t%3s: 0x%08x %10d\n",
+         REG_INFO(24), REG_INFO(26), REG_INFO(32));
+  // t9, k1, HI
+  PrintF("%3s: 0x%08x %10d\t%3s: 0x%08x %10d\t%3s: 0x%08x %10d\n",
+         REG_INFO(25), REG_INFO(27), REG_INFO(33));
+  // sp, fp, gp
+  PrintF("%3s: 0x%08x %10d\t%3s: 0x%08x %10d\t%3s: 0x%08x %10d\n",
+         REG_INFO(29), REG_INFO(30), REG_INFO(28));
+  // pc
+  PrintF("%3s: 0x%08x %10d\t%3s: 0x%08x %10d\n",
+         REG_INFO(31), REG_INFO(34));
+#undef REG_INFO
+}
+
+void Debugger::Debug() {
+  intptr_t last_pc = -1;
+  bool done = false;
+
+#define COMMAND_SIZE 63
+#define ARG_SIZE 255
+
+#define STR(a) #a
+#define XSTR(a) STR(a)
+
+  char cmd[COMMAND_SIZE + 1];
+  char arg1[ARG_SIZE + 1];
+  char arg2[ARG_SIZE + 1];
+
+  // make sure to have a proper terminating character if reaching the limit
+  cmd[COMMAND_SIZE] = 0;
+  arg1[ARG_SIZE] = 0;
+  arg2[ARG_SIZE] = 0;
+
+  // Undo all set breakpoints while running in the debugger shell. This will
+  // make them invisible to all commands.
+  UndoBreakpoints();
+
+  while (!done && (sim_->get_pc() != Simulator::end_sim_pc)) {
+    if (last_pc != sim_->get_pc()) {
+      disasm::NameConverter converter;
+      disasm::Disassembler dasm(converter);
+      // use a reasonably large buffer
+      v8::internal::EmbeddedVector<char, 256> buffer;
+      dasm.InstructionDecode(buffer,
+                             reinterpret_cast<byte_*>(sim_->get_pc()));
+      PrintF("  0x%08x  %s\n", sim_->get_pc(), buffer.start());
+      last_pc = sim_->get_pc();
+    }
+    char* line = ReadLine("sim> ");
+    if (line == NULL) {
+      break;
+    } else {
+      // Use sscanf to parse the individual parts of the command line. At the
+      // moment no command expects more than two parameters.
+      int args = SScanF(line,
+                        "%" XSTR(COMMAND_SIZE) "s "
+                        "%" XSTR(ARG_SIZE) "s "
+                        "%" XSTR(ARG_SIZE) "s",
+                        cmd, arg1, arg2);
+      if ((strcmp(cmd, "si") == 0) || (strcmp(cmd, "stepi") == 0)) {
+        if (!(reinterpret_cast<Instruction*>(sim_->get_pc())->IsTrap())) {
+          sim_->InstructionDecode(
+                                reinterpret_cast<Instruction*>(sim_->get_pc()));
+        } else {
+          // Allow si to jump over generated breakpoints.
+          PrintF("/!\\ Jumping over generated breakpoint.\n");
+          sim_->set_pc(sim_->get_pc() + Instruction::kInstructionSize);
+        }
+      } else if ((strcmp(cmd, "c") == 0) || (strcmp(cmd, "cont") == 0)) {
+        // Execute the one instruction we broke at with breakpoints disabled.
+        sim_->InstructionDecode(reinterpret_cast<Instruction*>(sim_->get_pc()));
+        // Leave the debugger shell.
+        done = true;
+      } else if ((strcmp(cmd, "p") == 0) || (strcmp(cmd, "print") == 0)) {
+        if (args == 2) {
+          int32_t value;
+          if (strcmp(arg1, "all") == 0) {
+            PrintAllRegs();
+          } else {
+            if (GetValue(arg1, &value)) {
+              PrintF("%s: 0x%08x %d \n", arg1, value, value);
+            } else {
+              PrintF("%s unrecognized\n", arg1);
+            }
+          }
+        } else {
+          PrintF("print <register>\n");
+        }
+      } else if ((strcmp(cmd, "po") == 0)
+                 || (strcmp(cmd, "printobject") == 0)) {
+        if (args == 2) {
+          int32_t value;
+          if (GetValue(arg1, &value)) {
+            Object* obj = reinterpret_cast<Object*>(value);
+            PrintF("%s: \n", arg1);
+#ifdef DEBUG
+            obj->PrintLn();
+#else
+            obj->ShortPrint();
+            PrintF("\n");
+#endif
+          } else {
+            PrintF("%s unrecognized\n", arg1);
+          }
+        } else {
+          PrintF("printobject <value>\n");
+        }
+      } else if ((strcmp(cmd, "disasm") == 0) || (strcmp(cmd, "dpc") == 0)) {
+        disasm::NameConverter converter;
+        disasm::Disassembler dasm(converter);
+        // use a reasonably large buffer
+        v8::internal::EmbeddedVector<char, 256> buffer;
+
+        byte_* cur = NULL;
+        byte_* end = NULL;
+
+        if (args == 1) {
+          cur = reinterpret_cast<byte_*>(sim_->get_pc());
+          end = cur + (10 * Instruction::kInstructionSize);
+        } else if (args == 2) {
+          int32_t value;
+          if (GetValue(arg1, &value)) {
+            cur = reinterpret_cast<byte_*>(value);
+            // no length parameter passed, assume 10 instructions
+            end = cur + (10 * Instruction::kInstructionSize);
+          }
+        } else {
+          int32_t value1;
+          int32_t value2;
+          if (GetValue(arg1, &value1) && GetValue(arg2, &value2)) {
+            cur = reinterpret_cast<byte_*>(value1);
+            end = cur + (value2 * Instruction::kInstructionSize);
+          }
+        }
+
+        while (cur < end) {
+          dasm.InstructionDecode(buffer, cur);
+          PrintF("  0x%08x  %s\n", cur, buffer.start());
+          cur += Instruction::kInstructionSize;
+        }
+      } else if (strcmp(cmd, "gdb") == 0) {
+        PrintF("relinquishing control to gdb\n");
+        v8::internal::OS::DebugBreak();
+        PrintF("regaining control from gdb\n");
+      } else if (strcmp(cmd, "break") == 0) {
+        if (args == 2) {
+          int32_t value;
+          if (GetValue(arg1, &value)) {
+            if (!SetBreakpoint(reinterpret_cast<Instruction*>(value))) {
+              PrintF("setting breakpoint failed\n");
+            }
+          } else {
+            PrintF("%s unrecognized\n", arg1);
+          }
+        } else {
+          PrintF("break <address>\n");
+        }
+      } else if (strcmp(cmd, "del") == 0) {
+        if (!DeleteBreakpoint(NULL)) {
+          PrintF("deleting breakpoint failed\n");
+        }
+      } else if (strcmp(cmd, "flags") == 0) {
+        PrintF("No flags on MIPS !\n");
+      } else if (strcmp(cmd, "unstop") == 0) {
+          PrintF("Unstop command not implemented on MIPS.");
+      } else if ((strcmp(cmd, "stat") == 0) || (strcmp(cmd, "st") == 0)) {
+        // Print registers and disassemble
+        PrintAllRegs();
+        PrintF("\n");
+
+        disasm::NameConverter converter;
+        disasm::Disassembler dasm(converter);
+        // use a reasonably large buffer
+        v8::internal::EmbeddedVector<char, 256> buffer;
+
+        byte_* cur = NULL;
+        byte_* end = NULL;
+
+        if (args == 1) {
+          cur = reinterpret_cast<byte_*>(sim_->get_pc());
+          end = cur + (10 * Instruction::kInstructionSize);
+        } else if (args == 2) {
+          int32_t value;
+          if (GetValue(arg1, &value)) {
+            cur = reinterpret_cast<byte_*>(value);
+            // no length parameter passed, assume 10 instructions
+            end = cur + (10 * Instruction::kInstructionSize);
+          }
+        } else {
+          int32_t value1;
+          int32_t value2;
+          if (GetValue(arg1, &value1) && GetValue(arg2, &value2)) {
+            cur = reinterpret_cast<byte_*>(value1);
+            end = cur + (value2 * Instruction::kInstructionSize);
+          }
+        }
+
+        while (cur < end) {
+          dasm.InstructionDecode(buffer, cur);
+          PrintF("  0x%08x  %s\n", cur, buffer.start());
+          cur += Instruction::kInstructionSize;
+        }
+      } else if ((strcmp(cmd, "h") == 0) || (strcmp(cmd, "help") == 0)) {
+        PrintF("cont\n");
+        PrintF("  continue execution (alias 'c')\n");
+        PrintF("stepi\n");
+        PrintF("  step one instruction (alias 'si')\n");
+        PrintF("print <register>\n");
+        PrintF("  print register content (alias 'p')\n");
+        PrintF("  use register name 'all' to print all registers\n");
+        PrintF("printobject <register>\n");
+        PrintF("  print an object from a register (alias 'po')\n");
+        PrintF("flags\n");
+        PrintF("  print flags\n");
+        PrintF("disasm [<instructions>]\n");
+        PrintF("disasm [[<address>] <instructions>]\n");
+        PrintF("  disassemble code, default is 10 instructions from pc\n");
+        PrintF("gdb\n");
+        PrintF("  enter gdb\n");
+        PrintF("break <address>\n");
+        PrintF("  set a break point on the address\n");
+        PrintF("del\n");
+        PrintF("  delete the breakpoint\n");
+        PrintF("unstop\n");
+        PrintF("  ignore the stop instruction at the current location");
+        PrintF(" from now on\n");
+      } else {
+        PrintF("Unknown command: %s\n", cmd);
+      }
+    }
+    DeleteArray(line);
+  }
+
+  // Add all the breakpoints back to stop execution and enter the debugger
+  // shell when hit.
+  RedoBreakpoints();
+
+#undef COMMAND_SIZE
+#undef ARG_SIZE
+
+#undef STR
+#undef XSTR
+}
+
+
+// Create one simulator per thread and keep it in thread local storage.
+static v8::internal::Thread::LocalStorageKey simulator_key;
+
+
+bool Simulator::initialized_ = false;
+
+
+void Simulator::Initialize() {
+  if (initialized_) return;
+  simulator_key = v8::internal::Thread::CreateThreadLocalKey();
+  initialized_ = true;
+  ::v8::internal::ExternalReference::set_redirector(&RedirectExternalReference);
+}
+
+
+Simulator::Simulator() {
+  Initialize();
+  // Setup simulator support first. Some of this information is needed to
+  // setup the architecture state.
+  size_t stack_size = 1 * 1024*1024;  // allocate 1MB for stack
+  stack_ = reinterpret_cast<char*>(malloc(stack_size));
+  pc_modified_ = false;
+  icount_ = 0;
+  break_pc_ = NULL;
+  break_instr_ = 0;
+
+  // Setup architecture state.
+  // All registers are initialized to zero to start with.
+  for (int i = 0; i < kNumSimuRegisters; i++) {
+    registers_[i] = 0;
+  }
+
+  // The sp is initialized to point to the bottom (high address) of the
+  // allocated stack area. To be safe in potential stack underflows we leave
+  // some buffer below.
+  registers_[sp] = reinterpret_cast<int32_t>(stack_) + stack_size - 64;
+  // The ra and pc are initialized to a known bad value that will cause an
+  // access violation if the simulator ever tries to execute it.
+  registers_[pc] = bad_ra;
+  registers_[ra] = bad_ra;
+  InitializeCoverage();
+}
+
+
+// When the generated code calls an external reference we need to catch that in
+// the simulator.  The external reference will be a function compiled for the
+// host architecture.  We need to call that function instead of trying to
+// execute it with the simulator.  We do that by redirecting the external
+// reference to a swi (software-interrupt) instruction that is handled by
+// the simulator.  We write the original destination of the jump just at a known
+// offset from the swi instruction so the simulator knows what to call.
+class Redirection {
+ public:
+  Redirection(void* external_function, bool fp_return)
+      : external_function_(external_function),
+        swi_instruction_(rtCallRedirInstr),
+        fp_return_(fp_return),
+        next_(list_) {
+    list_ = this;
+  }
+
+  void* address_of_swi_instruction() {
+    return reinterpret_cast<void*>(&swi_instruction_);
+  }
+
+  void* external_function() { return external_function_; }
+  bool fp_return() { return fp_return_; }
+
+  static Redirection* Get(void* external_function, bool fp_return) {
+    Redirection* current;
+    for (current = list_; current != NULL; current = current->next_) {
+      if (current->external_function_ == external_function) return current;
+    }
+    return new Redirection(external_function, fp_return);
+  }
+
+  static Redirection* FromSwiInstruction(Instruction* swi_instruction) {
+    char* addr_of_swi = reinterpret_cast<char*>(swi_instruction);
+    char* addr_of_redirection =
+        addr_of_swi - OFFSET_OF(Redirection, swi_instruction_);
+    return reinterpret_cast<Redirection*>(addr_of_redirection);
+  }
+
+ private:
+  void* external_function_;
+  uint32_t swi_instruction_;
+  bool fp_return_;
+  Redirection* next_;
+  static Redirection* list_;
+};
+
+
+Redirection* Redirection::list_ = NULL;
+
+
+void* Simulator::RedirectExternalReference(void* external_function,
+                                           bool fp_return) {
+  Redirection* redirection = Redirection::Get(external_function, fp_return);
+  return redirection->address_of_swi_instruction();
+}
+
+
+// Get the active Simulator for the current thread.
+Simulator* Simulator::current() {
+  Initialize();
+  Simulator* sim = reinterpret_cast<Simulator*>(
+      v8::internal::Thread::GetThreadLocal(simulator_key));
+  if (sim == NULL) {
+    // TODO(146): delete the simulator object when a thread goes away.
+    sim = new Simulator();
+    v8::internal::Thread::SetThreadLocal(simulator_key, sim);
+  }
+  return sim;
+}
+
+
+// Sets the register in the architecture state. It will also deal with updating
+// Simulator internal state for special registers such as PC.
+void Simulator::set_register(int reg, int32_t value) {
+  ASSERT((reg >= 0) && (reg < kNumSimuRegisters));
+  if (reg == pc) {
+    pc_modified_ = true;
+  }
+
+  // zero register always hold 0.
+  registers_[reg] = (reg == 0) ? 0 : value;
+}
+
+void Simulator::set_fpu_register(int fpureg, int32_t value) {
+  ASSERT((fpureg >= 0) && (fpureg < kNumFPURegisters));
+  FPUregisters_[fpureg] = value;
+}
+
+void Simulator::set_fpu_register_double(int fpureg, double value) {
+  ASSERT((fpureg >= 0) && (fpureg < kNumFPURegisters) && ((fpureg % 2) == 0));
+  *v8i::bit_cast<double*, int32_t*>(&FPUregisters_[fpureg]) = value;
+}
+
+
+// Get the register from the architecture state. This function does handle
+// the special case of accessing the PC register.
+int32_t Simulator::get_register(int reg) const {
+  ASSERT((reg >= 0) && (reg < kNumSimuRegisters));
+  if (reg == 0)
+    return 0;
+  else
+    return registers_[reg] + ((reg == pc) ? Instruction::kPCReadOffset : 0);
+}
+
+int32_t Simulator::get_fpu_register(int fpureg) const {
+  ASSERT((fpureg >= 0) && (fpureg < kNumFPURegisters));
+  return FPUregisters_[fpureg];
+}
+
+double Simulator::get_fpu_register_double(int fpureg) const {
+  ASSERT((fpureg >= 0) && (fpureg < kNumFPURegisters) && ((fpureg % 2) == 0));
+  return *v8i::bit_cast<double*, int32_t*>(
+      const_cast<int32_t*>(&FPUregisters_[fpureg]));
+}
+
+// Raw access to the PC register.
+void Simulator::set_pc(int32_t value) {
+  pc_modified_ = true;
+  registers_[pc] = value;
+}
+
+// Raw access to the PC register without the special adjustment when reading.
+int32_t Simulator::get_pc() const {
+  return registers_[pc];
+}
+
+
+// The MIPS cannot do unaligned reads and writes.  On some MIPS platforms an
+// interrupt is caused.  On others it does a funky rotation thing.  For now we
+// simply disallow unaligned reads, but at some point we may want to move to
+// emulating the rotate behaviour.  Note that simulator runs have the runtime
+// system running directly on the host system and only generated code is
+// executed in the simulator.  Since the host is typically IA32 we will not
+// get the correct MIPS-like behaviour on unaligned accesses.
+
+int Simulator::ReadW(int32_t addr, Instruction* instr) {
+  if ((addr & v8i::kPointerAlignmentMask) == 0) {
+    intptr_t* ptr = reinterpret_cast<intptr_t*>(addr);
+    return *ptr;
+  }
+  PrintF("Unaligned read at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+  return 0;
+}
+
+
+void Simulator::WriteW(int32_t addr, int value, Instruction* instr) {
+  if ((addr & v8i::kPointerAlignmentMask) == 0) {
+    intptr_t* ptr = reinterpret_cast<intptr_t*>(addr);
+    *ptr = value;
+    return;
+  }
+  PrintF("Unaligned write at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+}
+
+
+double Simulator::ReadD(int32_t addr, Instruction* instr) {
+  if ((addr & kDoubleAlignmentMask) == 0) {
+    double* ptr = reinterpret_cast<double*>(addr);
+    return *ptr;
+  }
+  PrintF("Unaligned read at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+  return 0;
+}
+
+
+void Simulator::WriteD(int32_t addr, double value, Instruction* instr) {
+  if ((addr & kDoubleAlignmentMask) == 0) {
+    double* ptr = reinterpret_cast<double*>(addr);
+    *ptr = value;
+    return;
+  }
+  PrintF("Unaligned write at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+}
+
+
+uint16_t Simulator::ReadHU(int32_t addr, Instruction* instr) {
+  if ((addr & 1) == 0) {
+    uint16_t* ptr = reinterpret_cast<uint16_t*>(addr);
+    return *ptr;
+  }
+  PrintF("Unaligned unsigned halfword read at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+  return 0;
+}
+
+
+int16_t Simulator::ReadH(int32_t addr, Instruction* instr) {
+  if ((addr & 1) == 0) {
+    int16_t* ptr = reinterpret_cast<int16_t*>(addr);
+    return *ptr;
+  }
+  PrintF("Unaligned signed halfword read at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+  return 0;
+}
+
+
+void Simulator::WriteH(int32_t addr, uint16_t value, Instruction* instr) {
+  if ((addr & 1) == 0) {
+    uint16_t* ptr = reinterpret_cast<uint16_t*>(addr);
+    *ptr = value;
+    return;
+  }
+  PrintF("Unaligned unsigned halfword write at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+}
+
+
+void Simulator::WriteH(int32_t addr, int16_t value, Instruction* instr) {
+  if ((addr & 1) == 0) {
+    int16_t* ptr = reinterpret_cast<int16_t*>(addr);
+    *ptr = value;
+    return;
+  }
+  PrintF("Unaligned halfword write at 0x%08x, pc=%p\n", addr, instr);
+  OS::Abort();
+}
+
+
+uint32_t Simulator::ReadBU(int32_t addr) {
+  uint8_t* ptr = reinterpret_cast<uint8_t*>(addr);
+  return *ptr & 0xff;
+}
+
+
+int32_t Simulator::ReadB(int32_t addr) {
+  int8_t* ptr = reinterpret_cast<int8_t*>(addr);
+  return ((*ptr << 24) >> 24) & 0xff;
+}
+
+
+void Simulator::WriteB(int32_t addr, uint8_t value) {
+  uint8_t* ptr = reinterpret_cast<uint8_t*>(addr);
+  *ptr = value;
+}
+
+
+void Simulator::WriteB(int32_t addr, int8_t value) {
+  int8_t* ptr = reinterpret_cast<int8_t*>(addr);
+  *ptr = value;
+}
+
+
+// Returns the limit of the stack area to enable checking for stack overflows.
+uintptr_t Simulator::StackLimit() const {
+  // Leave a safety margin of 256 bytes to prevent overrunning the stack when
+  // pushing values.
+  return reinterpret_cast<uintptr_t>(stack_) + 256;
+}
+
+
+// Unsupported instructions use Format to print an error and stop execution.
+void Simulator::Format(Instruction* instr, const char* format) {
+  PrintF("Simulator found unsupported instruction:\n 0x%08x: %s\n",
+         instr, format);
+  UNIMPLEMENTED_MIPS();
+}
+
+
+// Calls into the V8 runtime are based on this very simple interface.
+// Note: To be able to return two values from some calls the code in runtime.cc
+// uses the ObjectPair which is essentially two 32-bit values stuffed into a
+// 64-bit value. With the code below we assume that all runtime calls return
+// 64 bits of result. If they don't, the r1 result register contains a bogus
+// value, which is fine because it is caller-saved.
+typedef int64_t (*SimulatorRuntimeCall)(int32_t arg0,
+                                        int32_t arg1,
+                                        int32_t arg2,
+                                        int32_t arg3);
+typedef double (*SimulatorRuntimeFPCall)(double fparg0,
+                                         double fparg1);
+
+
+// Software interrupt instructions are used by the simulator to call into the
+// C-based V8 runtime.
+void Simulator::SoftwareInterrupt(Instruction* instr) {
+  // We first check if we met a call_rt_redirected.
+  if (instr->InstructionBits() == rtCallRedirInstr) {
+    Redirection* redirection = Redirection::FromSwiInstruction(instr);
+    int32_t arg0 = get_register(a0);
+    int32_t arg1 = get_register(a1);
+    int32_t arg2 = get_register(a2);
+    int32_t arg3 = get_register(a3);
+    // fp args are (not always) in f12 and f14.
+    // See MIPS conventions for more details.
+    double fparg0 = get_fpu_register_double(f12);
+    double fparg1 = get_fpu_register_double(f14);
+    // This is dodgy but it works because the C entry stubs are never moved.
+    // See comment in codegen-arm.cc and bug 1242173.
+    int32_t saved_ra = get_register(ra);
+    if (redirection->fp_return()) {
+      intptr_t external =
+          reinterpret_cast<intptr_t>(redirection->external_function());
+      SimulatorRuntimeFPCall target =
+          reinterpret_cast<SimulatorRuntimeFPCall>(external);
+      if (::v8::internal::FLAG_trace_sim) {
+        PrintF("Call to host function at %p with args %f, %f\n",
+               FUNCTION_ADDR(target), fparg0, fparg1);
+      }
+      double result = target(fparg0, fparg1);
+      set_fpu_register_double(f0, result);
+    } else {
+      intptr_t external =
+          reinterpret_cast<int32_t>(redirection->external_function());
+      SimulatorRuntimeCall target =
+          reinterpret_cast<SimulatorRuntimeCall>(external);
+      if (::v8::internal::FLAG_trace_sim) {
+        PrintF(
+            "Call to host function at %p with args %08x, %08x, %08x, %08x\n",
+            FUNCTION_ADDR(target),
+            arg0,
+            arg1,
+            arg2,
+            arg3);
+      }
+      int64_t result = target(arg0, arg1, arg2, arg3);
+      int32_t lo_res = static_cast<int32_t>(result);
+      int32_t hi_res = static_cast<int32_t>(result >> 32);
+      if (::v8::internal::FLAG_trace_sim) {
+        PrintF("Returned %08x\n", lo_res);
+      }
+      set_register(v0, lo_res);
+      set_register(v1, hi_res);
+    }
+    set_register(ra, saved_ra);
+    set_pc(get_register(ra));
+  } else {
+    Debugger dbg(this);
+    dbg.Debug();
+  }
+}
+
+void Simulator::SignalExceptions() {
+  for (int i = 1; i < kNumExceptions; i++) {
+    if (exceptions[i] != 0) {
+      V8_Fatal(__FILE__, __LINE__, "Error: Exception %i raised.", i);
+    }
+  }
+}
+
+// Handle execution based on instruction types.
+void Simulator::DecodeTypeRegister(Instruction* instr) {
+  // Instruction fields
+  Opcode   op     = instr->OpcodeFieldRaw();
+  int32_t  rs_reg = instr->RsField();
+  int32_t  rs     = get_register(rs_reg);
+  uint32_t rs_u   = static_cast<uint32_t>(rs);
+  int32_t  rt_reg = instr->RtField();
+  int32_t  rt     = get_register(rt_reg);
+  uint32_t rt_u   = static_cast<uint32_t>(rt);
+  int32_t  rd_reg = instr->RdField();
+  uint32_t sa     = instr->SaField();
+
+  int32_t fs_reg= instr->FsField();
+
+  // ALU output
+  // It should not be used as is. Instructions using it should always initialize
+  // it first.
+  int32_t alu_out = 0x12345678;
+  // Output or temporary for floating point.
+  double fp_out = 0.0;
+
+  // For break and trap instructions.
+  bool do_interrupt = false;
+
+  // For jr and jalr
+  // Get current pc.
+  int32_t current_pc = get_pc();
+  // Next pc
+  int32_t next_pc = 0;
+
+  // ---------- Configuration
+  switch (op) {
+    case COP1:    // Coprocessor instructions
+      switch (instr->RsFieldRaw()) {
+        case BC1:   // branch on coprocessor condition
+          UNREACHABLE();
+          break;
+        case MFC1:
+          alu_out = get_fpu_register(fs_reg);
+          break;
+        case MFHC1:
+          fp_out = get_fpu_register_double(fs_reg);
+          alu_out = *v8i::bit_cast<int32_t*, double*>(&fp_out);
+          break;
+        case MTC1:
+        case MTHC1:
+          // Do the store in the execution step.
+          break;
+        case S:
+        case D:
+        case W:
+        case L:
+        case PS:
+          // Do everything in the execution step.
+          break;
+        default:
+          UNIMPLEMENTED_MIPS();
+      };
+      break;
+    case SPECIAL:
+      switch (instr->FunctionFieldRaw()) {
+        case JR:
+        case JALR:
+          next_pc = get_register(instr->RsField());
+          break;
+        case SLL:
+          alu_out = rt << sa;
+          break;
+        case SRL:
+          alu_out = rt_u >> sa;
+          break;
+        case SRA:
+          alu_out = rt >> sa;
+          break;
+        case SLLV:
+          alu_out = rt << rs;
+          break;
+        case SRLV:
+          alu_out = rt_u >> rs;
+          break;
+        case SRAV:
+          alu_out = rt >> rs;
+          break;
+        case MFHI:
+          alu_out = get_register(HI);
+          break;
+        case MFLO:
+          alu_out = get_register(LO);
+          break;
+        case MULT:
+          UNIMPLEMENTED_MIPS();
+          break;
+        case MULTU:
+          UNIMPLEMENTED_MIPS();
+          break;
+        case DIV:
+        case DIVU:
+            exceptions[kDivideByZero] = rt == 0;
+          break;
+        case ADD:
+          if (HaveSameSign(rs, rt)) {
+            if (rs > 0) {
+              exceptions[kIntegerOverflow] = rs > (Registers::kMaxValue - rt);
+            } else if (rs < 0) {
+              exceptions[kIntegerUnderflow] = rs < (Registers::kMinValue - rt);
+            }
+          }
+          alu_out = rs + rt;
+          break;
+        case ADDU:
+          alu_out = rs + rt;
+          break;
+        case SUB:
+          if (!HaveSameSign(rs, rt)) {
+            if (rs > 0) {
+              exceptions[kIntegerOverflow] = rs > (Registers::kMaxValue + rt);
+            } else if (rs < 0) {
+              exceptions[kIntegerUnderflow] = rs < (Registers::kMinValue + rt);
+            }
+          }
+          alu_out = rs - rt;
+          break;
+        case SUBU:
+          alu_out = rs - rt;
+          break;
+        case AND:
+          alu_out = rs & rt;
+          break;
+        case OR:
+          alu_out = rs | rt;
+          break;
+        case XOR:
+          alu_out = rs ^ rt;
+          break;
+        case NOR:
+          alu_out = ~(rs | rt);
+          break;
+        case SLT:
+          alu_out = rs < rt ? 1 : 0;
+          break;
+        case SLTU:
+          alu_out = rs_u < rt_u ? 1 : 0;
+          break;
+        // Break and trap instructions
+        case BREAK:
+          do_interrupt = true;
+          break;
+        case TGE:
+          do_interrupt = rs >= rt;
+          break;
+        case TGEU:
+          do_interrupt = rs_u >= rt_u;
+          break;
+        case TLT:
+          do_interrupt = rs < rt;
+          break;
+        case TLTU:
+          do_interrupt = rs_u < rt_u;
+          break;
+        case TEQ:
+          do_interrupt = rs == rt;
+          break;
+        case TNE:
+          do_interrupt = rs != rt;
+          break;
+        default:
+          UNREACHABLE();
+      };
+      break;
+    case SPECIAL2:
+      switch (instr->FunctionFieldRaw()) {
+        case MUL:
+          alu_out = rs_u * rt_u;  // Only the lower 32 bits are kept.
+          break;
+        default:
+          UNREACHABLE();
+      }
+      break;
+    default:
+      UNREACHABLE();
+  };
+
+  // ---------- Raise exceptions triggered.
+  SignalExceptions();
+
+  // ---------- Execution
+  switch (op) {
+    case COP1:
+      switch (instr->RsFieldRaw()) {
+        case BC1:   // branch on coprocessor condition
+          UNREACHABLE();
+          break;
+        case MFC1:
+        case MFHC1:
+          set_register(rt_reg, alu_out);
+          break;
+        case MTC1:
+          // We don't need to set the higher bits to 0, because MIPS ISA says
+          // they are in an unpredictable state after executing MTC1.
+          FPUregisters_[fs_reg] = registers_[rt_reg];
+          FPUregisters_[fs_reg+1] = Unpredictable;
+          break;
+        case MTHC1:
+          // Here we need to keep the lower bits unchanged.
+          FPUregisters_[fs_reg+1] = registers_[rt_reg];
+          break;
+        case S:
+          switch (instr->FunctionFieldRaw()) {
+            case CVT_D_S:
+            case CVT_W_S:
+            case CVT_L_S:
+            case CVT_PS_S:
+              UNIMPLEMENTED_MIPS();
+              break;
+            default:
+              UNREACHABLE();
+          }
+          break;
+        case D:
+          switch (instr->FunctionFieldRaw()) {
+            case CVT_S_D:
+            case CVT_W_D:
+            case CVT_L_D:
+              UNIMPLEMENTED_MIPS();
+              break;
+            default:
+              UNREACHABLE();
+          }
+          break;
+        case W:
+          switch (instr->FunctionFieldRaw()) {
+            case CVT_S_W:
+              UNIMPLEMENTED_MIPS();
+              break;
+            case CVT_D_W:   // Convert word to double.
+              set_fpu_register(rd_reg, static_cast<double>(rs));
+              break;
+            default:
+              UNREACHABLE();
+          };
+          break;
+        case L:
+          switch (instr->FunctionFieldRaw()) {
+            case CVT_S_L:
+            case CVT_D_L:
+              UNIMPLEMENTED_MIPS();
+              break;
+            default:
+              UNREACHABLE();
+          }
+          break;
+        case PS:
+          break;
+        default:
+          UNREACHABLE();
+      };
+      break;
+    case SPECIAL:
+      switch (instr->FunctionFieldRaw()) {
+        case JR: {
+          Instruction* branch_delay_instr = reinterpret_cast<Instruction*>(
+              current_pc+Instruction::kInstructionSize);
+          BranchDelayInstructionDecode(branch_delay_instr);
+          set_pc(next_pc);
+          pc_modified_ = true;
+          break;
+        }
+        case JALR: {
+          Instruction* branch_delay_instr = reinterpret_cast<Instruction*>(
+              current_pc+Instruction::kInstructionSize);
+          BranchDelayInstructionDecode(branch_delay_instr);
+          set_register(31, current_pc + 2* Instruction::kInstructionSize);
+          set_pc(next_pc);
+          pc_modified_ = true;
+          break;
+        }
+        // Instructions using HI and LO registers.
+        case MULT:
+        case MULTU:
+          break;
+        case DIV:
+          // Divide by zero was checked in the configuration step.
+          set_register(LO, rs / rt);
+          set_register(HI, rs % rt);
+          break;
+        case DIVU:
+          set_register(LO, rs_u / rt_u);
+          set_register(HI, rs_u % rt_u);
+          break;
+        // Break and trap instructions
+        case BREAK:
+        case TGE:
+        case TGEU:
+        case TLT:
+        case TLTU:
+        case TEQ:
+        case TNE:
+          if (do_interrupt) {
+            SoftwareInterrupt(instr);
+          }
+          break;
+        default:  // For other special opcodes we do the default operation.
+          set_register(rd_reg, alu_out);
+      };
+      break;
+    case SPECIAL2:
+      switch (instr->FunctionFieldRaw()) {
+        case MUL:
+          set_register(rd_reg, alu_out);
+          // HI and LO are UNPREDICTABLE after the operation.
+          set_register(LO, Unpredictable);
+          set_register(HI, Unpredictable);
+          break;
+        default:
+          UNREACHABLE();
+      }
+      break;
+    // Unimplemented opcodes raised an error in the configuration step before,
+    // so we can use the default here to set the destination register in common
+    // cases.
+    default:
+      set_register(rd_reg, alu_out);
+  };
+}
+
+// Type 2: instructions using a 16 bytes immediate. (eg: addi, beq)
+void Simulator::DecodeTypeImmediate(Instruction* instr) {
+  // Instruction fields
+  Opcode   op     = instr->OpcodeFieldRaw();
+  int32_t  rs     = get_register(instr->RsField());
+  uint32_t rs_u   = static_cast<uint32_t>(rs);
+  int32_t  rt_reg = instr->RtField();  // destination register
+  int32_t  rt     = get_register(rt_reg);
+  int16_t  imm16  = instr->Imm16Field();
+
+  int32_t  ft_reg = instr->FtField();  // destination register
+  int32_t  ft     = get_register(ft_reg);
+
+  // zero extended immediate
+  uint32_t  oe_imm16 = 0xffff & imm16;
+  // sign extended immediate
+  int32_t   se_imm16 = imm16;
+
+  // Get current pc.
+  int32_t current_pc = get_pc();
+  // Next pc.
+  int32_t next_pc = bad_ra;
+
+  // Used for conditional branch instructions
+  bool do_branch = false;
+  bool execute_branch_delay_instruction = false;
+
+  // Used for arithmetic instructions
+  int32_t alu_out = 0;
+  // Floating point
+  double fp_out = 0.0;
+
+  // Used for memory instructions
+  int32_t addr = 0x0;
+
+  // ---------- Configuration (and execution for REGIMM)
+  switch (op) {
+    // ------------- COP1. Coprocessor instructions
+    case COP1:
+      switch (instr->RsFieldRaw()) {
+        case BC1:   // branch on coprocessor condition
+          UNIMPLEMENTED_MIPS();
+          break;
+        default:
+          UNREACHABLE();
+      };
+      break;
+    // ------------- REGIMM class
+    case REGIMM:
+      switch (instr->RtFieldRaw()) {
+        case BLTZ:
+          do_branch = (rs  < 0);
+          break;
+        case BLTZAL:
+          do_branch = rs  < 0;
+          break;
+        case BGEZ:
+          do_branch = rs >= 0;
+          break;
+        case BGEZAL:
+          do_branch = rs >= 0;
+          break;
+        default:
+          UNREACHABLE();
+      };
+      switch (instr->RtFieldRaw()) {
+        case BLTZ:
+        case BLTZAL:
+        case BGEZ:
+        case BGEZAL:
+          // Branch instructions common part.
+          execute_branch_delay_instruction = true;
+          // Set next_pc
+          if (do_branch) {
+            next_pc = current_pc + (imm16 << 2) + Instruction::kInstructionSize;
+            if (instr->IsLinkingInstruction()) {
+              set_register(31, current_pc + kBranchReturnOffset);
+            }
+          } else {
+            next_pc = current_pc + kBranchReturnOffset;
+          }
+        default:
+          break;
+        };
+    break;  // case REGIMM
+    // ------------- Branch instructions
+    // When comparing to zero, the encoding of rt field is always 0, so we don't
+    // need to replace rt with zero.
+    case BEQ:
+      do_branch = (rs == rt);
+      break;
+    case BNE:
+      do_branch = rs != rt;
+      break;
+    case BLEZ:
+      do_branch = rs <= 0;
+      break;
+    case BGTZ:
+      do_branch = rs  > 0;
+      break;
+    // ------------- Arithmetic instructions
+    case ADDI:
+      if (HaveSameSign(rs, se_imm16)) {
+        if (rs > 0) {
+          exceptions[kIntegerOverflow] = rs > (Registers::kMaxValue - se_imm16);
+        } else if (rs < 0) {
+          exceptions[kIntegerUnderflow] =
+              rs < (Registers::kMinValue - se_imm16);
+        }
+      }
+      alu_out = rs + se_imm16;
+      break;
+    case ADDIU:
+      alu_out = rs + se_imm16;
+      break;
+    case SLTI:
+      alu_out = (rs < se_imm16) ? 1 : 0;
+      break;
+    case SLTIU:
+      alu_out = (rs_u < static_cast<uint32_t>(se_imm16)) ? 1 : 0;
+      break;
+    case ANDI:
+        alu_out = rs & oe_imm16;
+      break;
+    case ORI:
+        alu_out = rs | oe_imm16;
+      break;
+    case XORI:
+        alu_out = rs ^ oe_imm16;
+      break;
+    case LUI:
+        alu_out = (oe_imm16 << 16);
+      break;
+    // ------------- Memory instructions
+    case LB:
+      addr = rs + se_imm16;
+      alu_out = ReadB(addr);
+      break;
+    case LW:
+      addr = rs + se_imm16;
+      alu_out = ReadW(addr, instr);
+      break;
+    case LBU:
+      addr = rs + se_imm16;
+      alu_out = ReadBU(addr);
+      break;
+    case SB:
+      addr = rs + se_imm16;
+      break;
+    case SW:
+      addr = rs + se_imm16;
+      break;
+    case LWC1:
+      addr = rs + se_imm16;
+      alu_out = ReadW(addr, instr);
+      break;
+    case LDC1:
+      addr = rs + se_imm16;
+      fp_out = ReadD(addr, instr);
+      break;
+    case SWC1:
+    case SDC1:
+      addr = rs + se_imm16;
+      break;
+    default:
+      UNREACHABLE();
+  };
+
+  // ---------- Raise exceptions triggered.
+  SignalExceptions();
+
+  // ---------- Execution
+  switch (op) {
+    // ------------- Branch instructions
+    case BEQ:
+    case BNE:
+    case BLEZ:
+    case BGTZ:
+      // Branch instructions common part.
+      execute_branch_delay_instruction = true;
+      // Set next_pc
+      if (do_branch) {
+        next_pc = current_pc + (imm16 << 2) + Instruction::kInstructionSize;
+        if (instr->IsLinkingInstruction()) {
+          set_register(31, current_pc + 2* Instruction::kInstructionSize);
+        }
+      } else {
+        next_pc = current_pc + 2 * Instruction::kInstructionSize;
+      }
+      break;
+    // ------------- Arithmetic instructions
+    case ADDI:
+    case ADDIU:
+    case SLTI:
+    case SLTIU:
+    case ANDI:
+    case ORI:
+    case XORI:
+    case LUI:
+      set_register(rt_reg, alu_out);
+      break;
+    // ------------- Memory instructions
+    case LB:
+    case LW:
+    case LBU:
+      set_register(rt_reg, alu_out);
+      break;
+    case SB:
+      WriteB(addr, static_cast<int8_t>(rt));
+      break;
+    case SW:
+      WriteW(addr, rt, instr);
+      break;
+    case LWC1:
+      set_fpu_register(ft_reg, alu_out);
+      break;
+    case LDC1:
+      set_fpu_register_double(ft_reg, fp_out);
+      break;
+    case SWC1:
+      addr = rs + se_imm16;
+      WriteW(addr, get_fpu_register(ft_reg), instr);
+      break;
+    case SDC1:
+      addr = rs + se_imm16;
+      WriteD(addr, ft, instr);
+      break;
+    default:
+      break;
+  };
+
+
+  if (execute_branch_delay_instruction) {
+    // Execute branch delay slot
+    // We don't check for end_sim_pc. First it should not be met as the current
+    // pc is valid. Secondly a jump should always execute its branch delay slot.
+    Instruction* branch_delay_instr =
+      reinterpret_cast<Instruction*>(current_pc+Instruction::kInstructionSize);
+    BranchDelayInstructionDecode(branch_delay_instr);
+  }
+
+  // If needed update pc after the branch delay execution.
+  if (next_pc != bad_ra) {
+    set_pc(next_pc);
+  }
+}
+
+// Type 3: instructions using a 26 bytes immediate. (eg: j, jal)
+void Simulator::DecodeTypeJump(Instruction* instr) {
+  // Get current pc.
+  int32_t current_pc = get_pc();
+  // Get unchanged bits of pc.
+  int32_t pc_high_bits = current_pc & 0xf0000000;
+  // Next pc
+  int32_t next_pc = pc_high_bits | (instr->Imm26Field() << 2);
+
+  // Execute branch delay slot
+  // We don't check for end_sim_pc. First it should not be met as the current pc
+  // is valid. Secondly a jump should always execute its branch delay slot.
+  Instruction* branch_delay_instr =
+    reinterpret_cast<Instruction*>(current_pc+Instruction::kInstructionSize);
+  BranchDelayInstructionDecode(branch_delay_instr);
+
+  // Update pc and ra if necessary.
+  // Do this after the branch delay execution.
+  if (instr->IsLinkingInstruction()) {
+    set_register(31, current_pc + 2* Instruction::kInstructionSize);
+  }
+  set_pc(next_pc);
+  pc_modified_ = true;
+}
+
+// Executes the current instruction.
+void Simulator::InstructionDecode(Instruction* instr) {
+  pc_modified_ = false;
+  if (::v8::internal::FLAG_trace_sim) {
+    disasm::NameConverter converter;
+    disasm::Disassembler dasm(converter);
+    // use a reasonably large buffer
+    v8::internal::EmbeddedVector<char, 256> buffer;
+    dasm.InstructionDecode(buffer,
+                           reinterpret_cast<byte_*>(instr));
+    PrintF("  0x%08x  %s\n", instr, buffer.start());
+  }
+
+  switch (instr->InstructionType()) {
+    case Instruction::kRegisterType:
+      DecodeTypeRegister(instr);
+      break;
+    case Instruction::kImmediateType:
+      DecodeTypeImmediate(instr);
+      break;
+    case Instruction::kJumpType:
+      DecodeTypeJump(instr);
+      break;
+    default:
+      UNSUPPORTED();
+  }
+  if (!pc_modified_) {
+    set_register(pc, reinterpret_cast<int32_t>(instr) +
+                 Instruction::kInstructionSize);
+  }
+}
+
+
+
+void Simulator::Execute() {
+  // Get the PC to simulate. Cannot use the accessor here as we need the
+  // raw PC value and not the one used as input to arithmetic instructions.
+  int program_counter = get_pc();
+  if (::v8::internal::FLAG_stop_sim_at == 0) {
+    // Fast version of the dispatch loop without checking whether the simulator
+    // should be stopping at a particular executed instruction.
+    while (program_counter != end_sim_pc) {
+      Instruction* instr = reinterpret_cast<Instruction*>(program_counter);
+      icount_++;
+      InstructionDecode(instr);
+      program_counter = get_pc();
+    }
+  } else {
+    // FLAG_stop_sim_at is at the non-default value. Stop in the debugger when
+    // we reach the particular instuction count.
+    while (program_counter != end_sim_pc) {
+      Instruction* instr = reinterpret_cast<Instruction*>(program_counter);
+      icount_++;
+      if (icount_ == ::v8::internal::FLAG_stop_sim_at) {
+        Debugger dbg(this);
+        dbg.Debug();
+      } else {
+        InstructionDecode(instr);
+      }
+      program_counter = get_pc();
+    }
+  }
+}
+
+
+int32_t Simulator::Call(byte_* entry, int argument_count, ...) {
+  va_list parameters;
+  va_start(parameters, argument_count);
+  // Setup arguments
+
+  // First four arguments passed in registers.
+  ASSERT(argument_count >= 4);
+  set_register(a0, va_arg(parameters, int32_t));
+  set_register(a1, va_arg(parameters, int32_t));
+  set_register(a2, va_arg(parameters, int32_t));
+  set_register(a3, va_arg(parameters, int32_t));
+
+  // Remaining arguments passed on stack.
+  int original_stack = get_register(sp);
+  // Compute position of stack on entry to generated code.
+  int entry_stack = (original_stack - (argument_count - 4) * sizeof(int32_t)
+                                    - kArgsSlotsSize);
+  if (OS::ActivationFrameAlignment() != 0) {
+    entry_stack &= -OS::ActivationFrameAlignment();
+  }
+  // Store remaining arguments on stack, from low to high memory.
+  intptr_t* stack_argument = reinterpret_cast<intptr_t*>(entry_stack);
+  for (int i = 4; i < argument_count; i++) {
+    stack_argument[i - 4 + kArgsSlotsNum] = va_arg(parameters, int32_t);
+  }
+  va_end(parameters);
+  set_register(sp, entry_stack);
+
+  // Prepare to execute the code at entry
+  set_register(pc, reinterpret_cast<int32_t>(entry));
+  // Put down marker for end of simulation. The simulator will stop simulation
+  // when the PC reaches this value. By saving the "end simulation" value into
+  // the LR the simulation stops when returning to this call point.
+  set_register(ra, end_sim_pc);
+
+  // Remember the values of callee-saved registers.
+  // The code below assumes that r9 is not used as sb (static base) in
+  // simulator code and therefore is regarded as a callee-saved register.
+  int32_t s0_val = get_register(s0);
+  int32_t s1_val = get_register(s1);
+  int32_t s2_val = get_register(s2);
+  int32_t s3_val = get_register(s3);
+  int32_t s4_val = get_register(s4);
+  int32_t s5_val = get_register(s5);
+  int32_t s6_val = get_register(s6);
+  int32_t s7_val = get_register(s7);
+  int32_t gp_val = get_register(gp);
+  int32_t sp_val = get_register(sp);
+  int32_t fp_val = get_register(fp);
+
+  // Setup the callee-saved registers with a known value. To be able to check
+  // that they are preserved properly across JS execution.
+  int32_t callee_saved_value = icount_;
+  set_register(s0, callee_saved_value);
+  set_register(s1, callee_saved_value);
+  set_register(s2, callee_saved_value);
+  set_register(s3, callee_saved_value);
+  set_register(s4, callee_saved_value);
+  set_register(s5, callee_saved_value);
+  set_register(s6, callee_saved_value);
+  set_register(s7, callee_saved_value);
+  set_register(gp, callee_saved_value);
+  set_register(fp, callee_saved_value);
+
+  // Start the simulation
+  Execute();
+
+  // Check that the callee-saved registers have been preserved.
+  CHECK_EQ(callee_saved_value, get_register(s0));
+  CHECK_EQ(callee_saved_value, get_register(s1));
+  CHECK_EQ(callee_saved_value, get_register(s2));
+  CHECK_EQ(callee_saved_value, get_register(s3));
+  CHECK_EQ(callee_saved_value, get_register(s4));
+  CHECK_EQ(callee_saved_value, get_register(s5));
+  CHECK_EQ(callee_saved_value, get_register(s6));
+  CHECK_EQ(callee_saved_value, get_register(s7));
+  CHECK_EQ(callee_saved_value, get_register(gp));
+  CHECK_EQ(callee_saved_value, get_register(fp));
+
+  // Restore callee-saved registers with the original value.
+  set_register(s0, s0_val);
+  set_register(s1, s1_val);
+  set_register(s2, s2_val);
+  set_register(s3, s3_val);
+  set_register(s4, s4_val);
+  set_register(s5, s5_val);
+  set_register(s6, s6_val);
+  set_register(s7, s7_val);
+  set_register(gp, gp_val);
+  set_register(sp, sp_val);
+  set_register(fp, fp_val);
+
+  // Pop stack passed arguments.
+  CHECK_EQ(entry_stack, get_register(sp));
+  set_register(sp, original_stack);
+
+  int32_t result = get_register(v0);
+  return result;
+}
+
+
+uintptr_t Simulator::PushAddress(uintptr_t address) {
+  int new_sp = get_register(sp) - sizeof(uintptr_t);
+  uintptr_t* stack_slot = reinterpret_cast<uintptr_t*>(new_sp);
+  *stack_slot = address;
+  set_register(sp, new_sp);
+  return new_sp;
+}
+
+
+uintptr_t Simulator::PopAddress() {
+  int current_sp = get_register(sp);
+  uintptr_t* stack_slot = reinterpret_cast<uintptr_t*>(current_sp);
+  uintptr_t address = *stack_slot;
+  set_register(sp, current_sp + sizeof(uintptr_t));
+  return address;
+}
+
+
+#undef UNSUPPORTED
+
+} }  // namespace assembler::mips
+
+#endif  // !defined(__mips)
+