diff --git a/tools/speclib.c b/tools/speclib.c
index 41b73a77c8b1ae24b70a44eacd54e32645025e7f..611383e50d63a878f95af924a11e7ac3b62d7a8f 100644
--- a/tools/speclib.c
+++ b/tools/speclib.c
@@ -20,7 +20,9 @@
 #include "wb_uart.h"
 
 struct spec_private {
+	/* BAR0 maps the local bus (i.e. we can access FGPA register) */
 	void *bar0;
+	/* BAR4 maps the local GN412x register */
 	void *bar4;
 	uint32_t vuart_base;
 };