Commented the PIT-initialization code to make it easier to understand.

arch/x86/kernel/timer.c

@@ -127,6 +127,9 @@ int timer_wait(unsigned int ticks)
 }
 
 #define LATCH(f)	((CLOCK_TICK_RATE + f/2) / f)
+#define WAIT_SOME_TIME() do { uint64_t start = rdtsc(); \
+			      while(rdtsc() - start < 1000000) ; \
+			} while (0)
 
 /* 
  * Sets up the system clock by installing the timer handler
@@ -134,10 +137,6 @@ int timer_wait(unsigned int ticks)
  */
 int timer_init(void)
 {
-#ifndef CONFIG_ROCKCREEK
-	uint64_t start;
-#endif
-
 	/* 
 	 * Installs 'timer_handler' for the PIC and APIC timer,
 	 * only one handler will be later used.
@@ -150,16 +149,27 @@ int timer_init(void)
 	 * Therefore, we have not to configure the PIC timer.
 	 */
 #ifndef CONFIG_ROCKCREEK 
+	/*
+	 * Port 0x43 is for initializing the PIT:
+	 *
+	 * 0x34 means the following:
+	 * 0b...     (step-by-step binary representation)
+	 * ...  00  - channel 0
+	 * ...  11  - write two values to counter register: 
+	 * 	      first low-, then high-byte
+	 * ... 010  - mode number 2: "rate generator" / frequency divider
+	 * ...   0  - binary counter (the alternative is BCD)
+	 */
 	outportb(0x43, 0x34);
-	/* before we write to 0x40, we wait some time */
+
-	start = rdtsc();
+	WAIT_SOME_TIME();
-	while(rdtsc() - start < 1000000)
+
-		;
+	/* Port 0x40 is for the counter register of channel 0 */
+
 	outportb(0x40, LATCH(TIMER_FREQ) & 0xFF);   /* low byte  */
-	/* before we write to 0x40, we wait some time */
+
-	start = rdtsc();
+	WAIT_SOME_TIME();
-	while(rdtsc() - start < 1000000)
+
-		;
 	outportb(0x40, LATCH(TIMER_FREQ) >> 8);     /* high byte */
 #endif