vault backup: 2025-03-12 10:49:27

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.