diff --git a/common/system.cpp b/common/system.cpp
index ef41f7c234e..ce9ba67c3cd 100644
--- a/common/system.cpp
+++ b/common/system.cpp
@@ -83,3 +83,37 @@ bool OSystem::setGraphicsMode(const char *name) {
 
 	return false;
 }
+
+#pragma mark -
+
+
+namespace Common {
+
+StackLock::StackLock(OSystem::MutexRef mutex, OSystem *syst, const char *mutexName)
+	: _mutex(mutex), _syst(syst), _mutexName(mutexName) {
+	if (syst == 0)
+		_syst = g_system;
+	lock();
+}
+
+StackLock::~StackLock() {
+	unlock();
+}
+
+void StackLock::lock() {
+	assert(_syst);
+	if (_mutexName != NULL)
+		debug(6, "Locking mutex %s", _mutexName);
+	
+	_syst->lockMutex(_mutex);
+}
+
+void StackLock::unlock() {
+	assert(_syst);
+	if (_mutexName != NULL)
+		debug(6, "Unlocking mutex %s", _mutexName);
+
+	_syst->unlockMutex(_mutex);
+}
+
+}	// End of namespace Common
diff --git a/common/system.h b/common/system.h
index 3afe5196173..3ddf4982d70 100644
--- a/common/system.h
+++ b/common/system.h
@@ -25,6 +25,8 @@
 
 #include "common/scummsys.h"
 #include "common/savefile.h"
+#include "common/util.h"
+#include "common/rect.h"
 
 
 /**
@@ -479,7 +481,19 @@ public:
 
 
 
-	/** @name Mutex handling */
+	/**
+	 * @name Mutex handling
+	 * Historically, the OSystem API used to have a method which allowed
+	 * creating threads. Hence mutex support was needed for thread syncing.
+	 * To ease portability, though, we decided to remove the threading API.
+	 * Instead, we now use timers (see setTimerCallback() and Common::Timer).
+	 * But since those may be implemented using threads (and in fact, that's
+	 * how our primary backend, the SDL one, does it on many systems), we
+	 * still have to do mutex syncing in our timer callbacks.
+	 *
+	 * Hence backends which do not use threads to implement the timers simply
+	 * can use dummy implementations for these methods.
+	 */
 	//@{
 
 	typedef struct Mutex *MutexRef;
@@ -570,5 +584,24 @@ public:
 /** The global OSystem instance. Inited in main(). */
 #define g_system	(OSystem::instance())
 
+namespace Common {
+
+/**
+ * Auxillary class to (un)lock a mutex on the stack.
+ */
+class StackLock {
+	OSystem::MutexRef _mutex;
+	OSystem *_syst;
+	const char *_mutexName;
+
+	void lock();
+	void unlock();
+public:
+	StackLock(OSystem::MutexRef mutex, OSystem *syst = 0, const char *mutexName = NULL);
+	~StackLock();
+};
+
+}	// End of namespace Common
+
 
 #endif 
diff --git a/common/util.cpp b/common/util.cpp
index 811140618ef..bb7c4ed9bab 100644
--- a/common/util.cpp
+++ b/common/util.cpp
@@ -100,36 +100,6 @@ uint RandomSource::getRandomNumberRng(uint min, uint max) {
 	return getRandomNumber(max - min) + min;
 }
 
-#pragma mark -
-
-
-StackLock::StackLock(OSystem::MutexRef mutex, OSystem *syst, const char *mutexName)
-	: _mutex(mutex), _syst(syst), _mutexName(mutexName) {
-	if (syst == 0)
-		_syst = g_system;
-	lock();
-}
-
-StackLock::~StackLock() {
-	unlock();
-}
-
-void StackLock::lock() {
-	assert(_syst);
-	if (_mutexName != NULL)
-		debug(6, "Locking mutex %s", _mutexName);
-	
-	_syst->lockMutex(_mutex);
-}
-
-void StackLock::unlock() {
-	assert(_syst);
-	if (_mutexName != NULL)
-		debug(6, "Unlocking mutex %s", _mutexName);
-
-	_syst->unlockMutex(_mutex);
-}
-
 
 #pragma mark -
 
diff --git a/common/util.h b/common/util.h
index b602f3ef852..68c47a1fb50 100644
--- a/common/util.h
+++ b/common/util.h
@@ -22,7 +22,6 @@
 #define COMMON_UTIL_H
 
 #include "common/scummsys.h"
-#include "common/system.h"
 
 template<typename T> inline T ABS (T x)			{ return (x>=0) ? x : -x; }
 template<typename T> inline T MIN (T a, T b)	{ return (a<b) ? a : b; }
@@ -75,21 +74,6 @@ public:
 	uint getRandomNumberRng(uint min, uint max);
 };
 
-/**
- * Auxillary class to (un)lock a mutex on the stack.
- */
-class StackLock {
-	OSystem::MutexRef _mutex;
-	OSystem *_syst;
-	const char *_mutexName;
-
-	void lock();
-	void unlock();
-public:
-	StackLock(OSystem::MutexRef mutex, OSystem *syst = 0, const char *mutexName = NULL);
-	~StackLock();
-};
-
 /**
  * List of language ids.
  * @note The order and mappings of the values 0..8 are *required* to stay the