diff --git a/C64_4joy_adapter/README.md b/C64_4joy_adapter/README.md
new file mode 100644
index 0000000..e915036
--- /dev/null
+++ b/C64_4joy_adapter/README.md
@@ -0,0 +1,6 @@
+  PORTB = *ptr; // is this atomic? probably, because ptr is 6-bit pointer.
+ f98:	e0 91 26 01 	lds	r30, 0x0126	; 0x800126 <__data_end>
+ f9c:	f0 91 27 01 	lds	r31, 0x0127	; 0x800127 <__data_end+0x1>
+ fa0:	80 81       	ld	r24, Z
+ fa2:	85 b9       	out	0x05, r24	; 5
+