diff --git a/demo/bit.cpp b/demo/bit.cpp
deleted file mode 100644
index 956caea..0000000
--- a/demo/bit.cpp
+++ /dev/null
@@ -1,39 +0,0 @@
-#include "../include/tools"
-
-signed main() {
-    [[maybe_unused]]
-    Bit <1> a; // Create a 1-bit object, default to 0
-
-    Bit <10> b(10); // Create a 10-bit object
-
-    auto ref = b.slice <4, 2> (); // A copy of [2, 3, 4] bit
-
-    std::cout << b << std::endl; // 10
-
-    b.set <3, 1> (ref); // Set [1, 2, 3] bit to [2, 3, 4] bit
-
-    std::cout << b << std::endl; // 4
-
-    b.set <0> (~a); // Set the 0-th bit to ~a (= 1 in this case)
-
-    std::cout << b << std::endl; // 5
-
-    auto sec = b.slice <2> (); // A copy of the 2-th bit (= 1 in this case)
-
-    auto d = sec.zero_extend(); // Zero extend (default to 32-bit)
-
-    std::cout << d << std::endl; // 1
-
-    auto c = sec.sign_extend <3> (); // Sign extend (default to 32-bit)
-
-    std::cout << c << std::endl; // 7
-
-    // c + d;   // Error: different size
-    // a += 1;  // Error: no assignment-operation operator
-
-    auto e = b - 1; // normal integer can be assumed as any size
-
-    std::cout << e << std::endl; // 4
-
-    return 0;
-}
diff --git a/demo/hardware.cpp b/demo/hardware.cpp
deleted file mode 100644
index a7d09ae..0000000
--- a/demo/hardware.cpp
+++ /dev/null
@@ -1,38 +0,0 @@
-#include "../include/tools"
-
-struct Input {
-    Wire a;
-    Wire b;
-    std::array <Wire, 2> c;
-};
-
-struct Output {
-    Register d;
-    Register e;
-};
-
-struct Content {
-    Register r;
-};
-
-struct MyModule : Input, Output, private Content {
-    using Tags = SyncTags <Input, Output, Content>;
-    friend class Visitor;
-
-    void demo() { this->d <= this->a + this->b; }
-};
-
-signed main() {
-    MyModule m;
-
-    m.a = []() { return 1; };
-    m.b.assign([&]() { return (target_size_t)m.d; });
-
-    for (int i = 0 ; i < 10 ; ++i) {
-        std::cout << m.d << std::endl;
-        m.demo();
-        std::cout << m.d << std::endl;
-        sync_member(m);
-    }
-    return 0;
-}
diff --git a/include/bit.h b/include/bit.h
index e71e9be..992ecbb 100644
--- a/include/bit.h
+++ b/include/bit.h
@@ -1,20 +1,13 @@
 #pragma once
-#include "target.h"
+#include "concept.h"
 #include "debug.h"
 #include <version>
+#include <climits>
 #include <concepts>
 
-namespace dark::bits {
+namespace dark {
 
-static constexpr std::size_t kMaxLength = 8 * sizeof(target_size_t);
-
-template <int _First>
-constexpr auto int_concat(target_size_t arg) { return arg; }
-
-template <int _First, int ..._Lens>
-constexpr auto int_concat(target_size_t arg, auto ...args) {
-    return (arg << (_Lens + ...)) | int_concat<_Lens...>(args...);
-}
+static constexpr std::size_t kMaxLength = std::numeric_limits<max_size_t>::digits;
 
 template <std::size_t _Nm>
 struct Bit {
@@ -22,117 +15,38 @@ struct Bit {
     static_assert(0 < _Nm && _Nm <= kMaxLength,
         "Bit: _Nm out of range. Should be in [1, kMaxLength]");
  
-    target_size_t _M_data : _Nm; // Real storage
+    max_size_t _M_data : _Nm; // Real storage
+
+    template <std::size_t _Hi, std::size_t _Lo>
+    static constexpr void _M_range_check();
 
   public:
     static constexpr std::size_t _Bit_Len = _Nm;
 
-    constexpr Bit(target_size_t data = {}) : _M_data(data) {}
+    constexpr Bit(max_size_t data = 0) : _M_data(data) {}
 
-    template <std::size_t ..._Lens>
-    constexpr Bit(Bit<_Lens> ...args) requires ((_Lens + ...) == _Nm) :
-        _M_data(int_concat<_Lens...>(args...)) {}
+    constexpr explicit operator max_size_t() const { return this->_M_data; }
 
-    constexpr Bit(const Bit &val) = default;
+    template <concepts::bit_type ..._Tp>
+    requires ((_Tp::_Bit_Len + ...) == _Nm)
+    constexpr Bit(const _Tp &...args);
 
-    template <std::size_t _Len>
-    constexpr Bit(const Bit <_Len> &val) requires (_Len != _Nm) {
-        static_assert(_Len == _Nm, "Bit: bit length mismatch");
-    }
+    template <concepts::bit_match <Bit> _Tp>
+    constexpr Bit &operator=(const _Tp &val);
 
-    constexpr Bit &operator=(const Bit &val) = default;
-
-    template <std::size_t _Len> requires (_Len != _Nm)
-    constexpr Bit &operator=(const Bit <_Len> &val) {
-        static_assert(_Len == _Nm, "Bit: bit length mismatch");
-        return *this;
-    }
-
-    constexpr Bit &operator=(target_size_t val) {
-        this->_M_data = val;
-        return *this;
-    }
-
-    constexpr operator target_size_t() const { return this->_M_data; }
+    template <std::size_t _Hi, std::size_t _Lo = _Hi, concepts::bit_match <Bit> _Tp>
+    constexpr void set(const _Tp &val);
 
     template <std::size_t _Hi, std::size_t _Lo = _Hi>
-    constexpr auto set(Bit <_Hi - _Lo + 1> val) {
-        static_cast <void> (this->slice<_Hi, _Lo>());
-        const auto mask = // Mask those bit in the middle
-            (target_size_t(1) << (_Hi + 1)) - (target_size_t(1) << _Lo);
-        const auto data = // Set those bit in the middle
-            static_cast <target_size_t> (val) << _Lo;
-        this->_M_data = (this->_M_data & ~mask) | data;
-    }
-
-    template <std::size_t _Hi, std::size_t _Lo = _Hi>
-    constexpr auto slice() const -> Bit <_Hi - _Lo + 1> {
-        static_assert(_Lo <= _Hi, "Bit::slice: _Lo should be no greater than _Hi");
-        static_assert(_Hi < _Nm, "Bit::slice: _Hi should be less than _Nm");
-        return Bit<_Hi - _Lo + 1>(this->_M_data >> _Lo);
-    }
+    constexpr auto range() const -> Bit <_Hi - _Lo + 1>;
 
     template <std::size_t _Len = 1>
-    constexpr auto at(target_size_t pos) const -> Bit <_Len> {
-        static_assert(_Len != 0, "Bit::at: _Len should be greater than 0");
-        debug::assert(pos + _Len <= _Nm, "Bit::at: pos out of range");
-        return Bit <_Len> (this->_M_data >> pos);
-    }
+    constexpr auto slice(std::size_t pos) const -> Bit <_Len>;
 
-    constexpr auto operator [](target_size_t pos) const -> Bit <1> { return this->at(pos); }
-
-    template <std::size_t _New = kMaxLength>
-    constexpr auto zero_extend() const -> Bit<_New>;
-
-    template <std::size_t _New = kMaxLength>
-    constexpr auto sign_extend() const -> Bit<_New>;
+    constexpr Bit <1> operator [](std::size_t pos) const { return this->slice(pos); }
 };
 
-template <typename _Tp>
-static constexpr bool is_bit_v = false;
-template <std::size_t _Nm>
-static constexpr bool is_bit_v<Bit<_Nm>> = true;
-
-template <typename _Tp>
-concept BitType = is_bit_v<_Tp>;
-
-template <BitType ..._Tp>
+template <concepts::bit_type ..._Tp>
 Bit(_Tp...) -> Bit<(_Tp::_Bit_Len + ...)>;
 
-template <std::size_t _Old, std::size_t _New = kMaxLength>
-constexpr auto sign_extend(target_size_t val) {
-    static_assert(_Old < _New, "sign_extend: _Old should be less than _New");
-    struct { target_ssize_t _M_data : _Old; } tmp;
-    return Bit<_New>(tmp._M_data = val);
-}
-
-template <std::size_t _New = kMaxLength, BitType _Tp>
-constexpr auto sign_extend(_Tp val) {
-    return sign_extend<_Tp::_Bit_Len, _New>(static_cast<target_size_t>(val));
-}
-
-template <std::size_t _Old, std::size_t _New = kMaxLength>
-constexpr auto zero_extend(target_size_t val) {
-    static_assert(_Old < _New, "zero_extend: _Old should be less than _New");
-    struct { target_size_t _M_data : _Old; } tmp;
-    return Bit<_New>(tmp._M_data = val);
-}
-
-template <std::size_t _New = kMaxLength, BitType _Tp>
-constexpr auto zero_extend(_Tp val) {
-    return zero_extend<_Tp::_Bit_Len, _New>(static_cast<target_size_t>(val));
-}
-
-template <std::size_t _Nm>
-template <std::size_t _New>
-constexpr auto Bit<_Nm>::sign_extend() const -> Bit<_New> {
-    return ::dark::bits::sign_extend<_New> (*this);
-}
-
-template <std::size_t _Nm>
-template <std::size_t _New>
-constexpr auto Bit<_Nm>::zero_extend() const -> Bit<_New> {
-    return ::dark::bits::zero_extend<_New> (*this);
-}
-
-} // namespace dark::bits
+} // namespace dark
diff --git a/include/bit_impl.h b/include/bit_impl.h
new file mode 100644
index 0000000..a62f15d
--- /dev/null
+++ b/include/bit_impl.h
@@ -0,0 +1,94 @@
+#pragma once
+#include "bit.h"
+
+namespace dark {
+
+template <int _First>
+constexpr auto int_concat(max_size_t arg) { return arg; }
+
+template <int _First, int ..._Lens>
+constexpr auto int_concat(max_size_t arg, auto ...args) {
+    return (arg << (_Lens + ...)) | int_concat<_Lens...>(args...);
+}
+
+template <std::size_t _Old, std::size_t _New = kMaxLength>
+constexpr auto sign_extend(max_size_t val) {
+    static_assert(_Old < _New, "sign_extend: _Old should be less than _New");
+    struct { max_ssize_t _M_data : _Old; } tmp;
+    return Bit<_New>(tmp._M_data = val);
+}
+
+template <std::size_t _New = kMaxLength, concepts::bit_type _Tp>
+constexpr auto sign_extend(const _Tp & val) {
+    return sign_extend<_Tp::_Bit_Len, _New>(static_cast<max_size_t>(val));
+}
+
+template <std::size_t _Old, std::size_t _New = kMaxLength>
+constexpr auto zero_extend(max_size_t val) {
+    static_assert(_Old < _New, "zero_extend: _Old should be less than _New");
+    struct { max_size_t _M_data : _Old; } tmp;
+    return Bit<_New>(tmp._M_data = val);
+}
+
+template <std::size_t _New = kMaxLength, concepts::bit_type _Tp>
+constexpr auto zero_extend(const _Tp &val) {
+    return zero_extend<_Tp::_Bit_Len, _New>(static_cast<max_size_t>(val));
+}
+
+template <std::size_t _Nm>
+template <concepts::bit_type ..._Tp>
+requires ((_Tp::_Bit_Len + ...) == _Nm)
+constexpr Bit<_Nm>::Bit(const _Tp &...args)
+    : _M_data(int_concat<_Tp::_Bit_Len...>(static_cast<max_size_t>(args)...)) {}
+
+template <std::size_t _Nm>
+template <concepts::bit_match <Bit <_Nm>> _Tp>
+constexpr Bit<_Nm> &Bit<_Nm>::operator=(const _Tp &val) {
+    this->_M_data = static_cast<max_size_t>(val);
+    return *this;
+}
+
+template <std::size_t _Nm>
+template <std::size_t _Hi, std::size_t _Lo>
+constexpr void Bit<_Nm>::_M_range_check() {
+    static_assert(_Lo <= _Hi, "Bit::range_check: _Lo should be no greater than _Hi");
+    static_assert(_Hi < _Nm,  "Bit::range_check: _Hi should be less than _Nm");
+}
+
+template <std::size_t _Nm>
+template <std::size_t _Hi, std::size_t _Lo, concepts::bit_match <Bit<_Nm>> _Tp>
+constexpr void Bit<_Nm>::set(const _Tp &val) {
+    this->_M_range_check<_Hi, _Lo>();
+    auto data = static_cast<max_size_t>(val);
+    constexpr auto _Length = _Hi - _Lo + 1;
+    if constexpr (_Length == kMaxLength) {
+        this->_M_data = data;
+    } else {
+        auto mask = ((1 << _Length) - 1) << _Lo;
+        this->_M_data = (this->_M_data & ~mask) | ((data << _Lo) & mask);
+    }
+}
+
+template <std::size_t _Nm>
+template <std::size_t _Hi, std::size_t _Lo>
+constexpr auto Bit<_Nm>::range() const -> Bit <_Hi - _Lo + 1> {
+    this->_M_range_check<_Hi, _Lo>();
+    constexpr auto _Length = _Hi - _Lo + 1;
+    return Bit<_Length>(this->_M_data >> _Lo);
+}
+
+template <std::size_t _Nm>
+template <std::size_t _Len>
+constexpr auto Bit<_Nm>::slice(std::size_t pos) const -> Bit <_Len> {
+    static_assert(_Len <= _Nm, "Bit::slice: _Len should be no greater than _Nm");
+    debug::assert(pos <= _Nm - _Len, "Bit::slice: pos should be less than _Nm - _Len");
+    if constexpr (_Len == kMaxLength) {
+        return *this;
+    } else {
+        return Bit<_Len>(this->_M_data >> pos);
+    }
+}
+
+
+
+} // namespace dark
diff --git a/include/bit_op.h b/include/bit_op.h
new file mode 100644
index 0000000..6f70f09
--- /dev/null
+++ b/include/bit_op.h
@@ -0,0 +1 @@
+#pragma once
diff --git a/include/bitop.h b/include/bitop.h
deleted file mode 100644
index 27d5db3..0000000
--- a/include/bitop.h
+++ /dev/null
@@ -1,32 +0,0 @@
-#pragma once
-#include "bit.h"
-
-namespace dark::bits {
-
-template <typename _Tp>
-concept NonBit = !BitType<_Tp>;
-
-template <typename _Tp>
-auto cast(_Tp &&val) -> target_size_t {
-    return static_cast<target_size_t>(val);
-}
-
-template <BitType _Lhs, BitType _Rhs>
-constexpr auto operator + (_Lhs lhs, _Rhs rhs) {
-    static_assert(_Lhs::_Bit_Len == _Rhs::_Bit_Len,
-        "operator +: lhs and rhs should have the same length");
-    return _Lhs { cast(lhs) + cast(rhs) };
-}
-
-template <BitType _Lhs, NonBit _Rhs>
-constexpr auto operator + (_Lhs lhs, _Rhs &&rhs) {
-    return _Lhs { cast(lhs) + cast(rhs) };
-}
-
-template <NonBit _Lhs, BitType _Rhs>
-constexpr auto operator + (_Lhs &&lhs, _Rhs rhs) {
-    return _Rhs { cast(lhs) + cast(rhs) };
-}
-
-
-} // namespace dark::bits
diff --git a/include/concept.h b/include/concept.h
new file mode 100644
index 0000000..6d47846
--- /dev/null
+++ b/include/concept.h
@@ -0,0 +1,41 @@
+#pragma once
+#include <cstdint>
+#include <concepts>
+
+namespace dark {
+
+using max_size_t = std::uint32_t;
+using max_ssize_t = std::int32_t;
+
+} // namespace dark
+
+namespace dark::concepts {
+
+template <typename _Tp>
+using func_t = void(*)(_Tp);
+
+template <typename _From, typename _To>
+concept implicit_convertible_to = requires(_From &a, func_t <_To> b) {
+    b(a); // Can implicitly convert
+};
+
+template <typename _From, typename _To>
+concept explicit_convertible_to =
+    !implicit_convertible_to <_From, _To>
+&&   std::constructible_from <_To, _From>;
+
+template <typename _Tp>
+concept has_length = requires { _Tp::_Bit_Len; };
+
+template <typename _Tp>
+concept bit_type =  has_length <_Tp> && explicit_convertible_to <_Tp, max_size_t>;
+
+template <typename _Tp>
+concept int_type = !has_length <_Tp> && implicit_convertible_to <_Tp, max_size_t>;
+
+template <typename _Lhs, typename _Rhs>
+concept bit_match =
+    (bit_type <_Lhs> && bit_type <_Rhs> && _Lhs::_Bit_Len == _Rhs::_Bit_Len)
+||  (int_type <_Lhs> || int_type <_Rhs>);
+
+} // namespace dark::concepts
diff --git a/include/hardware.h b/include/hardware.h
index 1ab1c32..e10a48b 100644
--- a/include/hardware.h
+++ b/include/hardware.h
@@ -1,7 +1,8 @@
 #pragma once
 #include "debug.h"
-#include "target.h"
+#include "concept.h"
 #include <memory>
+#include <cstdint>
 #include <concepts>
 
 namespace dark::hardware {
@@ -13,7 +14,7 @@ struct Register;
 template <typename _Fn>
 concept WireFunction =
     !std::same_as <Wire, std::decay_t <_Fn>> &&
-    requires(_Fn &&__f) { { __f() } -> std::convertible_to <target_size_t>; };
+    requires(_Fn &&__f) { { __f() } -> std::convertible_to <max_size_t>; };
 
 struct WireBase {
     using _Ret_t = int;
@@ -46,7 +47,7 @@ struct Wire {
     using _Manage_t = WireBase;
 
     std::unique_ptr <_Manage_t>     _M_impl;
-    mutable target_size_t           _M_cache;
+    mutable max_size_t           _M_cache;
     mutable bool                    _M_holds;
 
     [[no_unique_address]]
@@ -65,7 +66,6 @@ struct Wire {
     }
 
   public:
-
     Wire() : _M_impl(new EmptyWire), _M_cache(), _M_holds(), _M_dirty() {}
 
     Wire(Wire &&) = delete;
@@ -90,7 +90,7 @@ struct Wire {
         this->_M_holds = false;
     }
 
-    operator target_size_t() const {
+    operator max_size_t() const {
         if (this->_M_holds == false) {
             this->_M_cache = this->_M_impl->call();
             this->_M_holds = true;
@@ -103,8 +103,8 @@ struct Register {
   private:
     friend struct Visitor;
 
-    target_size_t _M_new;
-    target_size_t _M_old;
+    max_size_t _M_new;
+    max_size_t _M_old;
 
     [[no_unique_address]]
     debug::DebugValue <bool, false> _M_dirty;
@@ -116,15 +116,16 @@ struct Register {
         }
     }
 
-    void set_value(target_size_t value) {
+    void set_value(max_size_t value) {
         this->_M_new = value;
         debug::assert(!this->_M_dirty, "Register is already assigned in this cycle.");
         this->_M_dirty = true;
     }
 
-    auto get_value() const -> target_size_t { return this->_M_old; }
+    auto get_value() const -> max_size_t { return this->_M_old; }
 
   public:
+
     Register() : _M_new(), _M_old(), _M_dirty() {}
 
     Register(Register &&) = delete;
@@ -132,12 +133,12 @@ struct Register {
     Register &operator=(Register &&) = delete;
     Register &operator=(const Register &rhs) = delete;
 
-    template <std::convertible_to <target_size_t> _Int>
+    template <std::convertible_to <max_size_t> _Int>
     void operator <= (_Int &&value) {
-        this->set_value(static_cast <target_size_t>(value));
+        this->set_value(static_cast <max_size_t>(value));
     }
 
-    operator target_size_t() const { return this->get_value(); }
+    operator max_size_t() const { return this->get_value(); }
 };
 
 } // namespace dark::hardware
diff --git a/include/target.h b/include/target.h
deleted file mode 100644
index 8daf92c..0000000
--- a/include/target.h
+++ /dev/null
@@ -1,9 +0,0 @@
-#pragma once
-#include <cstdint>
-
-namespace dark {
-
-using target_size_t = std::uint32_t;
-using target_ssize_t = std::int32_t;
-
-} // namespace dark
diff --git a/include/tools b/include/tools
deleted file mode 100644
index b0817d9..0000000
--- a/include/tools
+++ /dev/null
@@ -1,17 +0,0 @@
-#pragma once
-#include "target.h"
-#include "hardware.h"
-#include "synchronize.h"
-#include "bit.h"
-#include "bitop.h"
-
-using ::dark::target_size_t;
-using ::dark::target_ssize_t;
-
-using ::dark::hardware::Register;
-using ::dark::hardware::Wire;
-using ::dark::hardware::Visitor;
-using ::dark::hardware::SyncTags;
-using ::dark::hardware::sync_member;
-
-using ::dark::bits::Bit;