diff --git a/MC6809/MC6809.cs b/MC6809/MC6809.cs
index 5d10304..87bff11 100644
--- a/MC6809/MC6809.cs
+++ b/MC6809/MC6809.cs
@@ -10,6 +10,7 @@ namespace MC6809
     // http://www.cpu-world.com/Arch/6809.html
     // https://web.archive.org/web/20200817115822/http://atjs.mbnet.fi/mc6809/
     // https://colorcomputerarchive.com/repo/Documents/Books/Motorola%206809%20and%20Hitachi%206309%20Programming%20Reference%20(Darren%20Atkinson).pdf
+    // https://github.com/aladur/flexemu/blob/master/src/tools/cputest.txt
 
     /*
     |---------------|-----------------------------------|