diff --git a/stepper.py b/stepper.py
new file mode 100644
index 0000000..008e09c
--- /dev/null
+++ b/stepper.py
@@ -0,0 +1,143 @@
+import ll
+import parser
+
+def TODO(msg):
+    print('TODO, not implemented yet at {}'
+          .format(msg))
+
+def err(msg):
+    print('ERROR: {}'
+          .format(msg))
+
+def step(insns, terminator, blocks, stack_frames, ssa_env, global_env, memory,
+         tdecs, fdecs, call_res):
+    if len(insns) == 0:
+        return terminate(terminator, blocks, stack_frames, ssa_env, global_env, memory)
+    ssa_target, next_insn = insns[0]
+    insns_rest = insns[1:]
+
+    # TODO
+    print('Evaluating {}'
+          .format(ll.insn2s(next_insn)))
+
+    res = None
+    if isinstance(next_insn, ll.Binop):
+        bop = next_insn.bop
+        ty = next_insn.ty
+        left = next_insn.left
+        right = next_insn.right
+        left_v = eval_oper(left, ssa_env, global_env)
+        right_v = eval_oper(right, ssa_env, global_env)
+        res = eval_binop(bop, left_v, right_v)
+
+        # TODO
+        print('{} {}, {}'
+              .format(bop, left_v, right_v))
+    else:
+        err('Unknown LLVM instruction: {}'
+            .format(next_insn))
+
+    if ssa_target is not None:
+        if ssa_target in ssa_env:
+            err('Cannot assign to variable twice: {}'
+                .format(ssa_target))
+        else:
+            # TODO
+            print('%{} <- {}'
+                  .format(ssa_target, res))
+            ssa_env[ssa_target] = res
+
+    return insns_rest, terminator, blocks, stack_frames, ssa_env, memory, None
+
+
+def terminate(terminator, blocks, stack_frames, ssa_env, global_env, memory):
+    if isinstance(terminator, ll.Ret):
+        oper = terminator.oper
+        if oper is None:
+            oper_v = None
+        else:
+            oper_v = eval_oper(oper, ssa_env, global_env)
+        if len(stack_frames) == 0:
+            return [], None, [], [], ssa_env, memory, oper_v
+        else:
+            new_insns, new_terminator, new_blocks, new_ssa_env = stack_frames[0]
+            new_stack_frames = stack_frames[1:]
+            return (new_insns, new_terminator, new_blocks, new_stack_frames,
+                    new_ssa_env, memory, oper_v)
+    else:
+        err('Unknown LLVM terminator: {}'
+            .format(terminator))
+
+
+def eval_oper(operand, ssa_env, global_env):
+    if isinstance(operand, ll.Null):
+        return 0
+    elif isinstance(operand, ll.Const):
+        return operand.val
+    elif isinstance(operand, ll.Gid):
+        TODO('eval_oper Gid')
+    elif isinstance(operand, ll.Id):
+        id = operand.val
+        try:
+            return ssa_env[id]
+        except KeyError:
+            err('Unable to find %{} in environment:\n{}'
+                .format(id, ssa_env))
+
+
+def eval_binop(bop, left, right):
+    if bop == 'add':
+        return left + right
+    else:
+        err('Unknown LLVM Binary operator: {}'
+            .format(bop))
+
+
+
+def gogo():
+    p = parser.LLVMParser()
+    p.build()
+    data = r'''
+define i64 @tigermain (i64 %U_mainSL_8, i64 %U_mainDummy_9) {
+    %a = add i64 3, 5 ; please be 8
+    %b = add i64 %a, %a
+    %c = add i64 %a, %b
+    %d = add i64 100, %c
+    ret i64 %d
+}
+    '''
+
+    ast = p.parse(data)
+
+    tdecs = ast.tdecls
+    fdecs = ast.fdecls
+    global_env = ast.gdecls
+
+    tigermain = ast.fdecls['tigermain']
+    first_block = tigermain.body.first_block
+    blocks = tigermain.body.named_blocks
+    insns = first_block.insns
+    terminator = first_block.terminator
+    stack_frames = []
+    ssa_env = {}
+    # TODO: memory structure has not been decided yet
+    memory = [None]
+    call_res = None
+
+    while True:
+        (insns, terminator, blocks,
+         stack_frames, ssa_env, memory, call_res) = step(insns, terminator, blocks,
+                                                         stack_frames, ssa_env,
+                                                         global_env, memory, tdecs,
+                                                         fdecs, call_res)
+
+        if terminator is None:
+            print('Stepping done! Final ssa_env:\n{}'
+                  .format(ssa_env))
+            print('Program resulted in {}'.
+                  format(call_res))
+            break
+
+
+if __name__ == '__main__':
+    gogo()