diff --git a/src/dfu.rs b/src/dfu.rs
index 9ac6727..95a4085 100644
--- a/src/dfu.rs
+++ b/src/dfu.rs
@@ -3,6 +3,7 @@ use cortex_m_rt::{pre_init};
 const DFU_TRIG_MSG: u32 = 0xDECAFBAD;
 
 extern "C" {
+    // This symbol comes from memory.x
     static mut _dfu_msg: u32;
 }