diff --git a/src/test/java/TestCpu.java b/src/test/java/TestCpu.java
index f06b1449e241a7cd5f6b168f44b64a465b2acc96..1f168ba80bade18395c1b2c37a4f2288839e5df6 100644
--- a/src/test/java/TestCpu.java
+++ b/src/test/java/TestCpu.java
@@ -47,6 +47,13 @@ public class TestCpu {
     }
 
 
-
+    private static String substring(String src, int start_idx, int end_idx) {
+        byte[] b = src.getBytes();
+        StringBuilder tgt = new StringBuilder();
+        for (int i = start_idx; i <= end_idx; i++) {
+            tgt.append((char) b[i]);
+        }
+        return tgt.toString();
+    }
 
 }