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Concurrent Systems/notes/4 - Semaphores.md

@@ -13,7 +13,7 @@ Main use: **prevent busy waiting**: suspend processes that cannot perform *down*
 **Binary semaphore:** if it is at most 1 (also *up* are blocking).
 
 A semaphore needs two underlying objects:
-- a counter initialized at s0 that can also become negative
+- a counter initialized at s0 that can also become negative (see the implementation below to understand)
 - a data structure (typically a queue), initially empty, to store suspended processes.
 
 #### Ideal implementation
@@ -29,4 +29,30 @@ S.up() :=
 	S.counter++
 	if S.counter <= 0 then
 		activate a proc from S.queue
-	return 
+	return
+```
+>[!note] note
+>if S.counter ≥ 0, then this is the value of the semaphore; otherwise, S.counter tells you how many processes are suspended in S
+>
+>all operations are in MUTEX
+
+
+#### Actual implementation
+```
+Let t be a test&set register initialized at 0
+
+S.down() :=
+	Disable interrupts
+	wait S.t.test&set() = 0
+	S.counter--
+	if S.counter < 0 then
+		enter into S.queue
+		S.t <- 0
+		SUSPEND
+	return
+
+S.up() :=
+	S.counter++
+	if S.counter <= 0 then
+		activate a proc from S.queue
+	return