diff --git a/src/sim/core.c b/src/sim/core.c
index bc0d05c..8690752 100644
--- a/src/sim/core.c
+++ b/src/sim/core.c
@@ -174,8 +174,15 @@
 	return NULL;
 }
 
-
-
+/* create an APDU header
+ * APDU format as defined in ISO/IEC 7816-4:2005(E) §5.1
+ * - cla: CLASS byte
+ * - ins: INSTRUCTION byte
+ * - p1: Parameter 1 byte
+ * - p2: Parameter 2 byte
+ * - lc: number of bytes in the command data field Nc, which will encoded in 0, 1 or 3 bytes into Lc
+ * - le: maximum number of bytes expected in the response data field,  which will encoded in 0, 1, 2 or 3 bytes into Le
+ */
 struct msgb *osim_new_apdumsg(uint8_t cla, uint8_t ins, uint8_t p1,
 			      uint8_t p2, uint16_t lc, uint16_t le)
 {