From ec1b4706d8b9fe106501e96ec6eedd1d3ccdb2b3 Mon Sep 17 00:00:00 2001
From: Marco Realacci <marco@marcorealacci.me>
Date: Wed, 12 Mar 2025 11:24:27 +0100
Subject: [PATCH] vault backup: 2025-03-12 11:24:27

---
 .obsidian/workspace.json                   |  2 +-
 Concurrent Systems/notes/4 - Semaphores.md | 28 +++++++++++++++++++++-
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/.obsidian/workspace.json b/.obsidian/workspace.json
index 9499de3..bfe8edc 100644
--- a/.obsidian/workspace.json
+++ b/.obsidian/workspace.json
@@ -36,7 +36,7 @@
                 "file": "Concurrent Systems/slides/class 4.pdf",
                 "page": 8,
                 "left": -23,
-                "top": 421,
+                "top": 187,
                 "zoom": 0.652019002375297
               },
               "icon": "lucide-file-text",
diff --git a/Concurrent Systems/notes/4 - Semaphores.md b/Concurrent Systems/notes/4 - Semaphores.md
index 0d586b2..d66924b 100644
--- a/Concurrent Systems/notes/4 - Semaphores.md	
+++ b/Concurrent Systems/notes/4 - Semaphores.md	
@@ -153,4 +153,30 @@ conc_write() :=
 
 ##### Weak priority to Readers
 - If a reader arrives during a read, it can surpass possible writers already suspended (risk of starvation for the writes)
-- When a writer terminates, it activates the first suspended process, irrispectively of whether it is a reader or a writer (so, the priority to readers is said «weak»)
\ No newline at end of file
+- When a writer terminates, it activates the first suspended process, irrispectively of whether it is a reader or a writer (so, the priority to readers is said «weak»)
+
+```
+GLOB_MUTEX and R_MUTEX semaphores init. at 1
+R a shared register init. at 0
+
+begin_read() :=
+	R_MUTEX.down()
+	R++ # currently active readers
+	if R = 1 then GLOB_MUTEX.down()
+	R_MUTEX.up()
+	return
+
+end_read() :=
+	R_MUTEX.down()
+	R--
+	if R = 0 then GLOB_MUTEX.up()
+	R_MUTEX.up()
+	return
+
+begin_write() :=
+	GLOB_MUTEX.down()
+	return
+
+end_write() :=
+	GLOB_MUTEX.up()
+	return