diff --git a/include/_6522.h b/include/_6522.h
index c68d268da..d5dbc5f75 100644
--- a/include/_6522.h
+++ b/include/_6522.h
@@ -35,7 +35,11 @@
 
 
 
-/* Define a structure with the via register offsets */
+/* Define a structure with the 6522 register offsets.
+ * NOTE: The timer registers are not declared as 16 bit registers, because
+ * the order in which the two 8 bit halves are written is important, and
+ * the compiler doesn't guarantee any order when writing 16 bit values.
+ */
 struct __6522 {
     unsigned char	prb;		/* Port register B */
     unsigned char	pra;		/* Port register A */
diff --git a/include/_6526.h b/include/_6526.h
index 2e72aec48..c04b26d68 100644
--- a/include/_6526.h
+++ b/include/_6526.h
@@ -38,7 +38,11 @@
 
 
 
-/* Define a structure with the 6526 register offsets */
+/* Define a structure with the 6526 register offsets.
+ * NOTE: The timer registers are not declared as 16 bit registers, because
+ * the order in which the two 8 bit halves are written is important, and
+ * the compiler doesn't guarantee any order when writing 16 bit values.
+ */
 struct __6526 {
     unsigned char	pra;		/* Port register A */
     unsigned char	prb;		/* Port register B */