diff --git a/Concurrent Systems/notes/3.md b/Concurrent Systems/notes/3.md
index 34e568d..4f0d343 100644
--- a/Concurrent Systems/notes/3.md	
+++ b/Concurrent Systems/notes/3.md	
@@ -25,4 +25,23 @@ X.test&set() :=
 
 ###### How do we use it?
 ```
-lock()
\ No newline at end of file
+lock() :=
+	wait X.test&set() = 0
+	return
+
+
+unlock() :=
+	X <- 0
+	return
+```
+
+
+#### Swap
+```
+X general register
+
+X.swap(v) :=
+	tmp <- X
+	X <- v
+	return tmp
+	
\ No newline at end of file