diff --git a/userland/arch/x86_64/dec.S b/userland/arch/x86_64/dec.S
index fba8112..40eed85 100644
--- a/userland/arch/x86_64/dec.S
+++ b/userland/arch/x86_64/dec.S
@@ -1,4 +1,7 @@
-/* Decrement: i--. */
+/* Decrement: i--.
+ *
+ * https://github.com/cirosantilli/linux-kernel-module-cheat#x86-binary-arithmetic-instructions
+ */
 
 #include <lkmc.h>
 
diff --git a/userland/arch/x86_64/inc.S b/userland/arch/x86_64/inc.S
index 263ef16..ea46c0b 100644
--- a/userland/arch/x86_64/inc.S
+++ b/userland/arch/x86_64/inc.S
@@ -1,4 +1,7 @@
-/* Increment: i++. */
+/* Increment: i++.
+ *
+ * https://github.com/cirosantilli/linux-kernel-module-cheat#x86-binary-arithmetic-instructions
+ */
 
 #include <lkmc.h>
 
diff --git a/userland/arch/x86_64/sub.S b/userland/arch/x86_64/sub.S
index 16996ca..a62b816 100644
--- a/userland/arch/x86_64/sub.S
+++ b/userland/arch/x86_64/sub.S
@@ -1,4 +1,7 @@
-/* Subtraction. */
+/* Subtraction.
+ *
+ * https://github.com/cirosantilli/linux-kernel-module-cheat#x86-binary-arithmetic-instructions
+ */
 
 #include <lkmc.h>