diff --git a/native_exec/simple_x86.py b/native_exec/simple_x86.py
index a1db2b6..ac2292f 100644
--- a/native_exec/simple_x86.py
+++ b/native_exec/simple_x86.py
@@ -538,7 +538,7 @@ class Jz(JmpType):
 class Jnz(JmpType):
     encoding = [(RawBits.from_int(8, 0x75), JmpImm8(2)),
                 (RawBits.from_int(16, 0x0f85), JmpImm32(6))]
-                
+
 class Jbe(JmpType):
     encoding = [(RawBits.from_int(8, 0x76), JmpImm8(2)),
                 (RawBits.from_int(16, 0x0f86), JmpImm32(6))]
@@ -574,6 +574,12 @@ class Mov(Instruction):
     encoding = [(RawBits.from_int(8, 0x89), ModRM([ModRM_REG__REG, ModRM_REG__MEM])),
                 (RawBits.from_int(5, 0xb8 >> 3), X86RegisterSelector(), Imm32())]
 
+class Movsb(Instruction):
+    encoding = [(RawBits.from_int(8, 0xa4),)]
+
+class Movsd(Instruction):
+    encoding = [(RawBits.from_int(8, 0xa5),)]
+
 class Lea(Instruction):
     encoding = [(RawBits.from_int(8, 0x8d), ModRM([ModRM_REG__MEM], accept_reverse=False, has_direction_bit=False))]
 
diff --git a/native_exec/test_simple_x86.py b/native_exec/test_simple_x86.py
index 4006cae..90f87e7 100644
--- a/native_exec/test_simple_x86.py
+++ b/native_exec/test_simple_x86.py
@@ -9,8 +9,9 @@ def disas(x):
 
 
 class TestInstr(object):
-    def __init__(self, instr_to_test):
+    def __init__(self, instr_to_test, expected_result=None):
         self.instr_to_test = instr_to_test
+        self.expected_result = expected_result
 
     def __call__(self, *args):
         res = bytes(self.instr_to_test(*args).get_code())
@@ -31,6 +32,12 @@ class TestInstr(object):
         return True
 
     def compare_args(self, args, capres):
+        if self.expected_result is not None:
+            result = "{0} {1}".format(capres.mnemonic, capres.op_str)
+            if result != self.expected_result:
+                raise AssertionError("Bad expected result expect <{0}> got <{1}>".format(self.expected_result, result))
+            return
+
         capres_op = list(capres.operands)
         if len(args) != len(capres_op):
             raise AssertionError("Expected {0} operands got {1}".format(len(args), len(capres_op)))
@@ -105,5 +112,8 @@ TestInstr(Call)('EAX')
 TestInstr(Call)(mem('[EAX + ECX * 8]'))
 TestInstr(Cpuid)()
 
+TestInstr(Movsb, expected_result='movsb byte ptr es:[edi], byte ptr [esi]')()
+TestInstr(Movsd, expected_result='movsd dword ptr es:[edi], dword ptr [esi]')()
+
 TestInstr(Xchg)('EAX', 'ESP')
 assert Xchg('EAX', 'ECX').get_code() == Xchg('ECX', 'EAX').get_code()