refine rtc.s

1. Add description of used register to remove TODO.
2. Refine all registers addr to HEX format.

Author: Junming Liu

rtc.S

@@ -1,20 +1,35 @@
 /* https://github.com/cirosantilli/x86-bare-metal-examples#rct */
 
-/* TODO what do those numbers mean? Where is this all documented? */
+/*
+ * Reference: https://wiki.osdev.org/CMOS
+	Register  Contents            Range
+	0x00      Seconds             0–59
+	0x02      Minutes             0–59
+	0x04      Hours               0–23 in 24-hour mode, 
+				      1–12 in 12-hour mode, highest bit set if pm
+	0x07      Day of Month        1–31
+	0x08      Month               1–12
+	0x09      Year                0–99
+
+	0x0A      Status Register A
+	RTC has an "Update in progress" flag (bit 7 of Status Register A). 
+	To read the time and date properly you have to wait until 
+	the "Update in progress" flag goes from "set" to "clear". 
+*/ 
 .equ RTCaddress, 0x70
 .equ RTCdata, 0x71
 
 #include "common.h"
 BEGIN
 update_in_progress:
-    mov $10, %al
+    mov $0x0A, %al
     out %al, $RTCaddress
     in $RTCdata, %al
     testb $0x80, %al
     jne update_in_progress
 
     /* Second. */
-    mov $0, %al
+    mov $0x00, %al
     out %al, $RTCaddress
     in $RTCdata, %al