diff --git a/.obsidian/workspace.json b/.obsidian/workspace.json
index 17dc80e..dba74c0 100644
--- a/.obsidian/workspace.json
+++ b/.obsidian/workspace.json
@@ -34,9 +34,9 @@
               "type": "pdf",
               "state": {
                 "file": "Concurrent Systems/slides/class 4.pdf",
-                "page": 3,
+                "page": 4,
                 "left": -23,
-                "top": 298,
+                "top": 342,
                 "zoom": 0.652019002375297
               },
               "icon": "lucide-file-text",
@@ -217,10 +217,10 @@
   },
   "active": "6edd4157a160e462",
   "lastOpenFiles": [
+    "Concurrent Systems/slides/class 4.pdf",
+    "Concurrent Systems/notes/4 - Semaphores.md",
     "Concurrent Systems/notes/3a - Hardware primitives & Lamport Bakery algorithm.md",
     "Concurrent Systems/notes/3b - Aravind's algorithm and improvements.md",
-    "Concurrent Systems/notes/4 - Semaphores.md",
-    "Concurrent Systems/slides/class 4.pdf",
     "Concurrent Systems/notes/images/Pasted image 20250310172134.png",
     "Concurrent Systems/notes/1b - Peterson algorithm.md",
     "Concurrent Systems/slides/class 3.pdf",
diff --git a/Concurrent Systems/notes/4 - Semaphores.md b/Concurrent Systems/notes/4 - Semaphores.md
index b758a9b..81fe07e 100644
--- a/Concurrent Systems/notes/4 - Semaphores.md	
+++ b/Concurrent Systems/notes/4 - Semaphores.md	
@@ -48,11 +48,27 @@ S.down() :=
 	if S.counter < 0 then
 		enter into S.queue
 		S.t <- 0
+		Enable interrupts
 		SUSPEND
+	else
+		S.t <- 0
+		Enable interrupts
 	return
 
 S.up() :=
+	Disable interrupts
+	wait S.t.test&set() = 0
 	S.counter++
 	if S.counter <= 0 then
 		activate a proc from S.queue
+	S.t <- 0
+	Enable interrupts
 	return
+```
+Same as before but we use test&set to actually ensure MUTEX (of course we could have used any other hardware MUTEX implementation seen so far).
+
+#### (Single) Producer/Consumer
+It's a shared FIFO buffer of size k.
+- `BUF[0,…,k-1]`: generic registers (not even safe) accessed in MUTEX
+- `IN/OUT` : two variables pointing to locations in `BUF` to (circularly) insert/remove items, both initialized at 0
+- `FREE/BUSY`: two semaphores that count thew number of free/busy cells of BUF, initialized at k and 0 respectively.
\ No newline at end of file