diff --git a/src/mainboard/asrock/b85m_pro4/bootblock.c b/src/mainboard/asrock/b85m_pro4/bootblock.c
index 9b3746c521..0cedcd6e47 100644
--- a/src/mainboard/asrock/b85m_pro4/bootblock.c
+++ b/src/mainboard/asrock/b85m_pro4/bootblock.c
@@ -9,6 +9,41 @@
 #define SERIAL_DEV PNP_DEV(0x2e, NCT6776_SP1)
 #define ACPI_DEV   PNP_DEV(0x2e, NCT6776_ACPI)
 
+/*
+ * Asrock B85M Pro4 Super I/O GPIOs
+ *
+ * +------+-----+---------------------------+
+ * | GPIO | Pin | Description               |
+ * +------+-----+---------------------------+
+ * | GP00 |   2 | N/C                       |
+ * | GP01 |   4 | CPU Fan 4-pin tach enable |
+ * | GP02 |   5 | CPU Fan 3-pin tach enable |
+ * | GP03 |   8 | CPU Fan 3-pin FON# signal |
+ * | GP04 |   9 | N/C                       |
+ * | GP05 |  11 | N/C     (+1.05V_PCH_GPIO) |
+ * | GP06 |  12 | N/C      (+1.5V_PCH_GPIO) |
+ * | GP07 |  13 | N/C                       |
+ * +------+-----+---------------------------+
+ * | GP10 | 123 | N/C            (VCCM_OV1) |
+ * | GP11 | 122 | N/C            (VCCM_OV2) |
+ * | GP12 | 121 | N/C            (VCCM_OV3) |
+ * | GP13 | 120 | N/C           (VCCM_STEP) |
+ * | GP14 | 119 | Assert HDA_SDO (SIO_GP14) |
+ * | GP15 | 118 | N/C        (PWM_THROTTLE) |
+ * | GP16 | 117 | OTP for VCORE (OTE_GATE1) |
+ * | GP17 | 116 | N/C           (IMON_GPIO) |
+ * +------+-----+---------------------------+
+ * | GP70 |  93 | PWR_FANIN                 |
+ * | GP71 |  92 | N/C                       |
+ * | GP72 |  91 | N/C           (SIO_PIN91) |
+ * | GP73 |  90 | CHA2_FANIN                |
+ * | GP74 |  89 | N/C           (SIO_PIN89) |
+ * | GP75 |  88 | N/C           (SIO_PIN88) |
+ * | GP76 |  87 | HDA reset gate     (GP76) |
+ * | GP77 |  86 | HDD_LED gate              |
+ * +------+-----+---------------------------+
+ */
+
 void mainboard_config_superio(void)
 {
 	nuvoton_pnp_enter_conf_state(GLOBAL_DEV);