diff --git a/Concurrent Systems/notes/3.md b/Concurrent Systems/notes/3.md
index e80771b..34e568d 100644
--- a/Concurrent Systems/notes/3.md	
+++ b/Concurrent Systems/notes/3.md	
@@ -6,3 +6,23 @@ We can strenghten the model by adding specialized HW primitives, that essentiall
 Usually, every operating system provides at least one specilized HW primitive.
 
 ##### Most common ones:
+- **Test&set:** atomic read+write of a boolean register
+- **Swap:** atomic read+write of a general register (generalization of the above)
+- **Fetch&add:** atomic read+increase of an integer register
+- **Compare&swap:** tests the value of a general register, returns a boolean (result of the comparison, true if it is the same).
+
+#### Test&Set
+```
+Let X be a boolean register
+
+X.test&set() :=
+	tmp <- X
+	X <- 1
+	return tmp
+
+(the function is implemented in an atomic way by the hardware by suspending the interruptions!)
+```
+
+###### How do we use it?
+```
+lock()
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