diff --git a/day5/d4.cpp b/day5/d4.cpp
new file mode 100644
index 0000000..6bc2f6b
--- /dev/null
+++ b/day5/d4.cpp
@@ -0,0 +1,12 @@
+// 测试负数的二进制表示：补码方式存储
+#include <iostream>
+#include <bitset>
+
+using namespace std;
+
+int main()
+{
+    int x = -123;
+    cout << bitset<8>(x) << endl;
+    return 0;
+}