diff --git a/hw/sd/sd.c b/hw/sd/sd.c
index d4e3d07..a48010c 100644
--- a/hw/sd/sd.c
+++ b/hw/sd/sd.c
@@ -315,6 +315,8 @@
     return shift_reg;
 }
 
+/* Operation Conditions register */
+
 #define OCR_POWER_DELAY_NS      500000 /* 0.5ms */
 
 FIELD(OCR, VDD_VOLTAGE_WINDOW,          0, 24)
@@ -364,6 +366,8 @@
     }
 }
 
+/* SD Configuration register */
+
 static void sd_set_scr(SDState *sd)
 {
     sd->scr[0] = 0 << 4;        /* SCR structure version 1.0 */
@@ -386,6 +390,8 @@
     sd->scr[7] = 0x00;
 }
 
+/* Card IDentification register */
+
 #define MID     0xaa
 #define OID     "XY"
 #define PNM     "QEMU!"
@@ -411,6 +417,8 @@
     sd->cid[15] = (sd_crc7(sd->cid, 15) << 1) | 1;
 }
 
+/* Card-Specific Data register */
+
 #define HWBLOCK_SHIFT   9        /* 512 bytes */
 #define SECTOR_SHIFT    5        /* 16 kilobytes */
 #define WPGROUP_SHIFT   7        /* 2 megs */
@@ -480,6 +488,8 @@
     sd->csd[15] = (sd_crc7(sd->csd, 15) << 1) | 1;
 }
 
+/* Relative Card Address register */
+
 static void sd_set_rca(SDState *sd)
 {
     sd->rca += 0x4567;
@@ -493,6 +503,8 @@
     return 0;
 }
 
+/* Card Status register */
+
 FIELD(CSR, AKE_SEQ_ERROR,               3,  1)
 FIELD(CSR, APP_CMD,                     5,  1)
 FIELD(CSR, FX_EVENT,                    6,  1)
@@ -623,6 +635,8 @@
     sect = sd_addr_to_wpnum(size) + 1;
 
     sd->state = sd_idle_state;
+
+    /* card registers */
     sd->rca = 0x0000;
     sd->size = size;
     sd_set_ocr(sd);
@@ -1055,6 +1069,7 @@
     return sd_illegal;
 }
 
+/* CMD0 */
 static sd_rsp_type_t sd_cmd_GO_IDLE_STATE(SDState *sd, SDRequest req)
 {
     if (sd->state != sd_inactive_state) {
@@ -1065,6 +1080,7 @@
     return sd_is_spi(sd) ? sd_r1 : sd_r0;
 }
 
+/* CMD1 */
 static sd_rsp_type_t spi_cmd_SEND_OP_COND(SDState *sd, SDRequest req)
 {
     sd->state = sd_transfer_state;
@@ -1072,6 +1088,7 @@
     return sd_r1;
 }
 
+/* CMD2 */
 static sd_rsp_type_t sd_cmd_ALL_SEND_CID(SDState *sd, SDRequest req)
 {
     switch (sd->state) {
@@ -1083,6 +1100,7 @@
     }
 }
 
+/* CMD3 */
 static sd_rsp_type_t sd_cmd_SEND_RELATIVE_ADDR(SDState *sd, SDRequest req)
 {
     switch (sd->state) {
@@ -1097,6 +1115,7 @@
     }
 }
 
+/* CMD19 */
 static sd_rsp_type_t sd_cmd_SEND_TUNING_BLOCK(SDState *sd, SDRequest req)
 {
     if (sd->spec_version < SD_PHY_SPECv3_01_VERS) {
@@ -1113,6 +1132,7 @@
     return sd_r1;
 }
 
+/* CMD23 */
 static sd_rsp_type_t sd_cmd_SET_BLOCK_COUNT(SDState *sd, SDRequest req)
 {
     if (sd->spec_version < SD_PHY_SPECv3_01_VERS) {