diff --git a/C64_4joy_adapter/README.md b/C64_4joy_adapter/README.md
index e22aafa..64fddcf 100644
--- a/C64_4joy_adapter/README.md
+++ b/C64_4joy_adapter/README.md
@@ -32,6 +32,7 @@ sei
 - out can write from register to io
 - only one io/data address space access within one instruction?
 - lds/sts takes 2 cycles and in/out takes 1 cycle? ldi takes 1 cycle?
+- lds/sts: \_SFR_MEM_ADDR(GPIOR2), in/out: \SFR_IO_ADDR(GPIOR0)
 
 ## avr asm
 - http://www.nongnu.org/avr-libc/user-manual/inline_asm.html