From 5df57ffe73283777c16ddbc400c8ea57d08ce516 Mon Sep 17 00:00:00 2001
From: Marco Realacci <marco@marcorealacci.me>
Date: Wed, 12 Mar 2025 11:39:27 +0100
Subject: [PATCH] vault backup: 2025-03-12 11:39:27

---
 .obsidian/workspace.json                   |  4 +--
 Concurrent Systems/notes/4 - Semaphores.md | 39 ++++++++++++++++++++++
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/.obsidian/workspace.json b/.obsidian/workspace.json
index bfe8edc..299ac83 100644
--- a/.obsidian/workspace.json
+++ b/.obsidian/workspace.json
@@ -34,9 +34,9 @@
               "type": "pdf",
               "state": {
                 "file": "Concurrent Systems/slides/class 4.pdf",
-                "page": 8,
+                "page": 10,
                 "left": -23,
-                "top": 187,
+                "top": 122,
                 "zoom": 0.652019002375297
               },
               "icon": "lucide-file-text",
diff --git a/Concurrent Systems/notes/4 - Semaphores.md b/Concurrent Systems/notes/4 - Semaphores.md
index d66924b..7d4d5a3 100644
--- a/Concurrent Systems/notes/4 - Semaphores.md	
+++ b/Concurrent Systems/notes/4 - Semaphores.md	
@@ -180,3 +180,42 @@ begin_write() :=
 end_write() :=
 	GLOB_MUTEX.up()
 	return
+```
+
+##### Strong priority to Readers
+When a writer terminates, it activates the first reader, if there is any, or the first writer, otherwise.
+
+```
+GLOB_MUTEX and R_MUTEX and W_MUTEX semaphores init. at 1
+R a shared register init. at 0
+
+begin_read() :=      end_read() :=
+	come prima           come prima
+
+begin_write() :=
+	W_MUTEX.down()
+	GLOB_MUTEX.down()
+	return
+
+end_write() :=
+	GLOB_MUTEX.up()
+	W_MUTEX.up()
+	return
+```
+
+##### Weak priority to Writers
+```
+GLOB_MUTEX, PRIO_MUTEX, R_MUTEX and W_MUTEX semaphores init. at 1 R and W shared registers init. at 0
+
+begin_read() :=
+	PRIO_MUTEX.down()
+	R_MUTEX.down()
+	R++
+	if R = 1 then
+		GLOB_MUTEX.down()
+	R_MUTEX.up()
+	PRIO_MUTEX.up()
+	return
+
+end_read() :=
+