diff --git a/C64_4joy_adapter/README.md b/C64_4joy_adapter/README.md
index 308920e..cf10136 100644
--- a/C64_4joy_adapter/README.md
+++ b/C64_4joy_adapter/README.md
@@ -22,3 +22,8 @@ out	0x05, r24
 sei
 ``` 
 375ns
+
+## register as variable
+- https://forum.arduino.cc/index.php?topic=43760.5
+- http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_regbind
+- volatile register unsigned char my_register asm("r2");