diff --git a/disas/nanomips.cpp b/disas/nanomips.cpp
index 6f15821403..23556b4fc6 100644
--- a/disas/nanomips.cpp
+++ b/disas/nanomips.cpp
@@ -389,6 +389,27 @@ uint64 NMD::decode_gpr_gpr3_src_store(uint64 d)
 }
 
 
+/*
+ * NMD::decode_gpr_gpr1() - decoder for 'gpr1' gpr encoding type
+ *
+ *   Map a 1-bit code to the 5-bit register space according to this pattern:
+ *
+ *                                  1 0
+ *                                  | |
+ *                                  | |
+ *                                  | └---------------------┐
+ *                                  └---------------------┐ |
+ *                                                        | |
+ *                                                        | |
+ *                                                        | |
+ *                                                        | |
+ *    1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
+ *      3                   2                   1                   0
+ *
+ *   Used in handling following instruction:
+ *
+ *     - MOVE.BALC
+ */
 uint64 NMD::decode_gpr_gpr1(uint64 d)
 {
     static uint64 register_list[] = {  4,  5 };