diff --git a/bbl/bbl.c b/bbl/bbl.c
index 36bbaad..e54e0d4 100644
--- a/bbl/bbl.c
+++ b/bbl/bbl.c
@@ -19,42 +19,55 @@
 #include <sys/mman.h>
 
 
-#define	PDDATS	0x10010314
-#define	PDDATC	0x10010318
-#define	PDFUNC	0x10010348
-#define	PDDIRS	0x10010364
-#define	PDDIRC	0x10010368
+#define	GPIO_BASE		0x10010000
 
-#define	LEDS		10
+#define	REG(n)	(*(uint32_t *) (mem+(n)-GPIO_BASE))
 
-#define	LED_MASK	0x3f00
-#define	LED_SEL		11
+/*
+ * The XBurst CPUs use an interesting concept for setting GPIOs: instead of
+ * writing the value of a pin (0 or 1) directly to an output latch, the latch
+ * is set by writing a 1 to a "set" register and cleared by writing a 1 to a
+ * "clear" register. This way, only an atomic store but no atomic
+ * read-modify-write operations are necessary for changing only part of the
+ * bits on a port.
+ *
+ * The same set/clear mechanism is also used for the other writable registers
+ * affecting GPIOs, e.g., the direction (input/output) and function (GPIO or
+ * MMC) registers.
+ */
+
+#define	PDDATS	REG(0x10010314)	/* port D data set */
+#define	PDDATC	REG(0x10010318)	/* port D data clear */
+#define	PDFUNC	REG(0x10010348)	/* port D function clear */
+#define	PDDIRS	REG(0x10010364)	/* port D direction set */
+#define	PDDIRC	REG(0x10010368)	/* port D direction clear */
+
+#define	LEDS		10	/* number of LEDs */
+
+#define	GROUP_SEL	11	/* PD11, DAT1 - LED group selection */
+
+#define	LED_MASK	0x3f00	/* all the LED groups and GROUP_SEL */
 
 #define	DELAY_US	50000
 
 #define PAGE_SIZE 4096
-#define	GPIO_BASE		0x10010000
-#define	REG(n)	(*(uint32_t *) (mem+(n)-GPIO_BASE))
 
 static volatile void *mem;
 
 
-/* ----- Command-line parsing ---------------------------------------------- */
-
-
-static int map(int g)
+static int map(int group)
 {
-	switch (g) {
+	switch (group) {
 	case 0:
-		return 12;
+		return 12;	/* PD12, DAT2 */
 	case 1:
-		return 13;
+		return 13;	/* PD13, DAT3/CD */
 	case 2:
-		return 8;
+		return 8;	/* PD08, CMD */
 	case 3:
-		return 9;
+		return 9;	/* PD09, CLK */
 	case 4:
-		return 10;
+		return 10;	/* PD10, DAT0 */
 	default:
 		abort();
 	}
@@ -65,18 +78,18 @@ static void set(int n)
 {
 	
 	if (n & 1) {
-		REG(PDDATC) = LED_MASK;
-		REG(PDDATS) = 1 << map(n >> 1);
+		PDDATC = LED_MASK;
+		PDDATS = 1 << map(n >> 1);
 	} else {
-		REG(PDDATS) = LED_MASK;
-		REG(PDDATC) = 1 << map(n >> 1);
+		PDDATS = LED_MASK;
+		PDDATC = 1 << map(n >> 1);
 	}
 }
 
 
 static void die(int sig)
 {
-	REG(PDDIRC) = LED_MASK;
+	PDDIRC = LED_MASK;	/* make all MMC pins inputs again */
 	_exit(0);
 }
 
@@ -99,8 +112,8 @@ int main(int argc, char **argv)
 
 	signal(SIGINT, die);
 
-	REG(PDFUNC) = LED_MASK;
-	REG(PDDIRS) = LED_MASK;
+	PDFUNC = LED_MASK;	/* make all MMC pins GPIOs */
+	PDDIRS = LED_MASK;	/* make all MMC pins outputs */
 
 	while (1) {
 		for (i = 0; i != LEDS-1; i++) {