diff --git a/src/ia32/assembler-ia32.cc b/src/ia32/assembler-ia32.cc
index e6f1d83cb5..01b366431b 100644
--- a/src/ia32/assembler-ia32.cc
+++ b/src/ia32/assembler-ia32.cc
@@ -730,6 +730,33 @@ void Assembler::xchg_w(Register reg, const Operand& op) {
   emit_operand(reg, op);
 }
 
+void Assembler::lock() {
+  EnsureSpace ensure_space(this);
+  EMIT(0xF0);
+}
+
+void Assembler::cmpxchg(const Operand& dst, Register src) {
+  EnsureSpace ensure_space(this);
+  EMIT(0x0F);
+  EMIT(0xB1);
+  emit_operand(src, dst);
+}
+
+void Assembler::cmpxchg_b(const Operand& dst, Register src) {
+  EnsureSpace ensure_space(this);
+  EMIT(0x0F);
+  EMIT(0xB0);
+  emit_operand(src, dst);
+}
+
+void Assembler::cmpxchg_w(const Operand& dst, Register src) {
+  EnsureSpace ensure_space(this);
+  EMIT(0x66);
+  EMIT(0x0F);
+  EMIT(0xB1);
+  emit_operand(src, dst);
+}
+
 void Assembler::adc(Register dst, int32_t imm32) {
   EnsureSpace ensure_space(this);
   emit_arith(2, Operand(dst), Immediate(imm32));
diff --git a/src/ia32/assembler-ia32.h b/src/ia32/assembler-ia32.h
index c3edacb0ac..f2e7c6cfd4 100644
--- a/src/ia32/assembler-ia32.h
+++ b/src/ia32/assembler-ia32.h
@@ -662,6 +662,14 @@ class Assembler : public AssemblerBase {
   void xchg_b(Register reg, const Operand& op);
   void xchg_w(Register reg, const Operand& op);
 
+  // Lock prefix
+  void lock();
+
+  // CompareExchange
+  void cmpxchg(const Operand& dst, Register src);
+  void cmpxchg_b(const Operand& dst, Register src);
+  void cmpxchg_w(const Operand& dst, Register src);
+
   // Arithmetics
   void adc(Register dst, int32_t imm32);
   void adc(Register dst, const Operand& src);
diff --git a/src/ia32/disasm-ia32.cc b/src/ia32/disasm-ia32.cc
index a87e44b3f9..d776365b59 100644
--- a/src/ia32/disasm-ia32.cc
+++ b/src/ia32/disasm-ia32.cc
@@ -1232,6 +1232,10 @@ static const char* F0Mnem(byte f0byte) {
       return "shrd";  // 3-operand version.
     case 0xAB:
       return "bts";
+    case 0xB0:
+      return "cmpxchg_b";
+    case 0xB1:
+      return "cmpxchg";
     case 0xBC:
       return "bsf";
     case 0xBD:
@@ -1264,6 +1268,9 @@ int DisassemblerIA32::InstructionDecode(v8::internal::Vector<char> out_buffer,
     vex_byte0_ = *data;
     vex_byte1_ = *(data + 1);
     data += 2;
+  } else if (*data == 0xF0 /*lock*/) {
+    AppendToBuffer("lock ");
+    data++;
   }
 
   bool processed = true;  // Will be set to false if the current instruction
@@ -1496,6 +1503,18 @@ int DisassemblerIA32::InstructionDecode(v8::internal::Vector<char> out_buffer,
             } else {
               AppendToBuffer(",%s,cl", NameOfCPURegister(regop));
             }
+          } else if (f0byte == 0xB0) {
+            // cmpxchg_b
+            data += 2;
+            AppendToBuffer("%s ", f0mnem);
+            int mod, regop, rm;
+            get_modrm(*data, &mod, &regop, &rm);
+            data += PrintRightOperand(data);
+            AppendToBuffer(",%s", NameOfByteCPURegister(regop));
+          } else if (f0byte == 0xB1) {
+            // cmpxchg
+            data += 2;
+            data += PrintOperands(f0mnem, OPER_REG_OP_ORDER, data);
           } else if (f0byte == 0xBC) {
             data += 2;
             int mod, regop, rm;
@@ -1612,9 +1631,8 @@ int DisassemblerIA32::InstructionDecode(v8::internal::Vector<char> out_buffer,
           data++;
           int mod, regop, rm;
           get_modrm(*data, &mod, &regop, &rm);
-          AppendToBuffer("xchg_w ");
+          AppendToBuffer("xchg_w %s,", NameOfCPURegister(regop));
           data += PrintRightOperand(data);
-          AppendToBuffer(",%s", NameOfCPURegister(regop));
         } else if (*data == 0x89) {
           data++;
           int mod, regop, rm;
@@ -1884,6 +1902,9 @@ int DisassemblerIA32::InstructionDecode(v8::internal::Vector<char> out_buffer,
                            NameOfXMMRegister(regop),
                            NameOfXMMRegister(rm));
             data++;
+          } else if (*data == 0xB1) {
+            data++;
+            data += PrintOperands("cmpxchg_w", OPER_REG_OP_ORDER, data);
           } else {
             UnimplementedInstruction();
           }
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index 5f8fb6818e..4b9115d43b 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -1016,6 +1016,40 @@ void Assembler::cmpb_al(Immediate imm8) {
   emit(imm8.value_);
 }
 
+void Assembler::lock() {
+  EnsureSpace ensure_space(this);
+  emit(0xf0);
+}
+
+void Assembler::cmpxchgb(const Operand& dst, Register src) {
+  EnsureSpace ensure_space(this);
+  if (!src.is_byte_register()) {
+    // Register is not one of al, bl, cl, dl.  Its encoding needs REX.
+    emit_rex_32(src, dst);
+  } else {
+    emit_optional_rex_32(src, dst);
+  }
+  emit(0x0f);
+  emit(0xb0);
+  emit_operand(src, dst);
+}
+
+void Assembler::cmpxchgw(const Operand& dst, Register src) {
+  EnsureSpace ensure_space(this);
+  emit(0x66);
+  emit_optional_rex_32(src, dst);
+  emit(0x0f);
+  emit(0xb1);
+  emit_operand(src, dst);
+}
+
+void Assembler::emit_cmpxchg(const Operand& dst, Register src, int size) {
+  EnsureSpace ensure_space(this);
+  emit_rex(src, dst, size);
+  emit(0x0f);
+  emit(0xb1);
+  emit_operand(src, dst);
+}
 
 void Assembler::cpuid() {
   EnsureSpace ensure_space(this);
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 77a1a57889..4df1801801 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -421,11 +421,11 @@ class Operand BASE_EMBEDDED {
   friend class Assembler;
 };
 
-
 #define ASSEMBLER_INSTRUCTION_LIST(V) \
   V(add)                              \
   V(and)                              \
   V(cmp)                              \
+  V(cmpxchg)                          \
   V(dec)                              \
   V(idiv)                             \
   V(div)                              \
@@ -445,7 +445,6 @@ class Operand BASE_EMBEDDED {
   V(xchg)                             \
   V(xor)
 
-
 // Shift instructions on operands/registers with kPointerSize, kInt32Size and
 // kInt64Size.
 #define SHIFT_INSTRUCTION_LIST(V)       \
@@ -788,9 +787,15 @@ class Assembler : public AssemblerBase {
   void decb(Register dst);
   void decb(const Operand& dst);
 
+  // Lock prefix.
+  void lock();
+
   void xchgb(Register reg, const Operand& op);
   void xchgw(Register reg, const Operand& op);
 
+  void cmpxchgb(const Operand& dst, Register src);
+  void cmpxchgw(const Operand& dst, Register src);
+
   // Sign-extends rax into rdx:rax.
   void cqo();
   // Sign-extends eax into edx:eax.
@@ -2054,6 +2059,11 @@ class Assembler : public AssemblerBase {
     immediate_arithmetic_op(0x7, dst, src, size);
   }
 
+  // Compare {al,ax,eax,rax} with src.  If equal, set ZF and write dst into
+  // src. Otherwise clear ZF and write src into {al,ax,eax,rax}.  This
+  // operation is only atomic if prefixed by the lock instruction.
+  void emit_cmpxchg(const Operand& dst, Register src, int size);
+
   void emit_dec(Register dst, int size);
   void emit_dec(const Operand& dst, int size);
 
diff --git a/src/x64/disasm-x64.cc b/src/x64/disasm-x64.cc
index 7126b89b99..0f5230a19f 100644
--- a/src/x64/disasm-x64.cc
+++ b/src/x64/disasm-x64.cc
@@ -142,19 +142,18 @@ enum InstructionType {
   SHORT_IMMEDIATE_INSTR
 };
 
-
 enum Prefixes {
   ESCAPE_PREFIX = 0x0F,
   OPERAND_SIZE_OVERRIDE_PREFIX = 0x66,
   ADDRESS_SIZE_OVERRIDE_PREFIX = 0x67,
   VEX3_PREFIX = 0xC4,
   VEX2_PREFIX = 0xC5,
+  LOCK_PREFIX = 0xF0,
   REPNE_PREFIX = 0xF2,
   REP_PREFIX = 0xF3,
   REPEQ_PREFIX = REP_PREFIX
 };
 
-
 struct InstructionDesc {
   const char* mnem;
   InstructionType type;
@@ -1607,6 +1606,8 @@ int DisassemblerX64::TwoByteOpcodeInstruction(byte* data) {
         AppendToBuffer("%s %s,%d", (regop == 6) ? "psllq" : "psrlq",
                        NameOfXMMRegister(rm), *current & 0x7f);
         current += 1;
+      } else if (opcode == 0xB1) {
+        current += PrintOperands("cmpxchg", OPER_REG_OP_ORDER, current);
       } else {
         const char* mnemonic = "?";
         if (opcode == 0x54) {
@@ -1884,6 +1885,12 @@ int DisassemblerX64::TwoByteOpcodeInstruction(byte* data) {
     current += PrintRightOperand(current);
   } else if (opcode == 0x0B) {
     AppendToBuffer("ud2");
+  } else if (opcode == 0xB0 || opcode == 0xB1) {
+    // CMPXCHG.
+    if (opcode == 0xB0) {
+      byte_size_operand_ = true;
+    }
+    current += PrintOperands(mnemonic, OPER_REG_OP_ORDER, current);
   } else {
     UnimplementedInstruction();
   }
@@ -1926,6 +1933,9 @@ const char* DisassemblerX64::TwoByteMnemonic(byte opcode) {
       return "shrd";
     case 0xAF:
       return "imul";
+    case 0xB0:
+    case 0xB1:
+      return "cmpxchg";
     case 0xB6:
       return "movzxb";
     case 0xB7:
@@ -1963,6 +1973,8 @@ int DisassemblerX64::InstructionDecode(v8::internal::Vector<char> out_buffer,
       if (rex_w()) AppendToBuffer("REX.W ");
     } else if ((current & 0xFE) == 0xF2) {  // Group 1 prefix (0xF2 or 0xF3).
       group_1_prefix_ = current;
+    } else if (current == LOCK_PREFIX) {
+      AppendToBuffer("lock ");
     } else if (current == VEX3_PREFIX) {
       vex_byte0_ = current;
       vex_byte1_ = *(data + 1);
diff --git a/test/cctest/test-disasm-ia32.cc b/test/cctest/test-disasm-ia32.cc
index 3ecd7b3256..e0fe9697ba 100644
--- a/test/cctest/test-disasm-ia32.cc
+++ b/test/cctest/test-disasm-ia32.cc
@@ -665,12 +665,30 @@ TEST(DisasmIa320) {
 
   // xchg.
   {
+    __ xchg_b(eax, Operand(eax, 8));
+    __ xchg_w(eax, Operand(ebx, 8));
     __ xchg(eax, eax);
     __ xchg(eax, ebx);
     __ xchg(ebx, ebx);
     __ xchg(ebx, Operand(esp, 12));
   }
 
+  // cmpxchg.
+  {
+    __ cmpxchg_b(Operand(esp, 12), eax);
+    __ cmpxchg_w(Operand(ebx, ecx, times_4, 10000), eax);
+    __ cmpxchg(Operand(ebx, ecx, times_4, 10000), eax);
+  }
+
+  // lock prefix.
+  {
+    __ lock();
+    __ cmpxchg(Operand(esp, 12), ebx);
+
+    __ lock();
+    __ xchg_w(eax, Operand(ecx, 8));
+  }
+
   // Nop instructions
   for (int i = 0; i < 16; i++) {
     __ Nop(i);
diff --git a/test/cctest/test-disasm-x64.cc b/test/cctest/test-disasm-x64.cc
index b855626cee..59ad683ba4 100644
--- a/test/cctest/test-disasm-x64.cc
+++ b/test/cctest/test-disasm-x64.cc
@@ -745,12 +745,31 @@ TEST(DisasmX64) {
 
   // xchg.
   {
+    __ xchgb(rax, Operand(rax, 8));
+    __ xchgw(rax, Operand(rbx, 8));
     __ xchgq(rax, rax);
     __ xchgq(rax, rbx);
     __ xchgq(rbx, rbx);
     __ xchgq(rbx, Operand(rsp, 12));
   }
 
+  // cmpxchg.
+  {
+    __ cmpxchgb(Operand(rsp, 12), rax);
+    __ cmpxchgw(Operand(rbx, rcx, times_4, 10000), rax);
+    __ cmpxchgl(Operand(rbx, rcx, times_4, 10000), rax);
+    __ cmpxchgq(Operand(rbx, rcx, times_4, 10000), rax);
+  }
+
+  // lock prefix.
+  {
+    __ lock();
+    __ cmpxchgl(Operand(rsp, 12), rbx);
+
+    __ lock();
+    __ xchgw(rax, Operand(rcx, 8));
+  }
+
   // Nop instructions
   for (int i = 0; i < 16; i++) {
     __ Nop(i);