diff -Naur xnu-1228.0.2.orig/osfmk/i386/misc_protos.h xnu-1228.0.2/osfmk/i386/misc_protos.h
--- xnu-1228.0.2.orig/osfmk/i386/misc_protos.h	2008-01-05 13:34:12.000000000 +1100
+++ xnu-1228.0.2/osfmk/i386/misc_protos.h	2008-10-03 11:24:51.000000000 +1000
@@ -126,7 +126,7 @@
 extern void	rtc_clock_stepped(
 			uint32_t new_frequency,
 			uint32_t old_frequency);
-extern void	rtc_clock_napped(uint64_t);
+extern void	rtc_clock_napped(uint64_t, uint64_t);
 
 extern void     x86_lowmem_free(void);
 
diff -Naur xnu-1228.0.2.orig/osfmk/i386/rtclock.c xnu-1228.0.2/osfmk/i386/rtclock.c
--- xnu-1228.0.2.orig/osfmk/i386/rtclock.c	2008-01-05 13:34:12.000000000 +1100
+++ xnu-1228.0.2/osfmk/i386/rtclock.c	2008-10-03 11:25:07.000000000 +1000
@@ -107,6 +107,28 @@
 rtc_nanotime_t	rtc_nanotime_info = {0,0,0,0,1,0};
 
 
+/*
+ * tsc_to_nanoseconds:
+ *
+ * Basic routine to convert a raw 64 bit TSC value to a
+ * 64 bit nanosecond value.  The conversion is implemented
+ * based on the scale factor and an implicit 32 bit shift.
+ */
+static inline uint64_t
+_tsc_to_nanoseconds(uint64_t value)
+{
+    asm volatile("movl	%%edx,%%esi	;"
+		 "mull	%%ecx		;"
+		 "movl	%%edx,%%edi	;"
+		 "movl	%%esi,%%eax	;"
+		 "mull	%%ecx		;"
+		 "addl	%%edi,%%eax	;"	
+		 "adcl	$0,%%edx	 "
+		 : "+A" (value) : "c" (rtc_nanotime_info.scale) : "esi", "edi");
+
+    return (value);
+}
+
 static uint32_t
 deadline_to_decrementer(
 	uint64_t	deadline,
@@ -234,26 +256,31 @@
 /*
  * rtc_clock_napped:
  *
- * Invoked from power manangement when we have awoken from a nap (C3/C4)
- * during which the TSC lost counts.  The nanotime data is updated according
- * to the provided value which indicates the number of nanoseconds that the
- * TSC was not counting.
- *
- * The caller must guarantee non-reentrancy.
+ * Invoked from power management when we exit from a low C-State (>= C4)
+ * and the TSC has stopped counting.  The nanotime data is updated according
+ * to the provided value which represents the new value for nanotime.
  */
 void
-rtc_clock_napped(
-	uint64_t		delta)
+rtc_clock_napped(uint64_t base, uint64_t tsc_base)
 {
 	rtc_nanotime_t	*rntp = &rtc_nanotime_info;
-	uint32_t	generation;
+	uint64_t	oldnsecs;
+	uint64_t	newnsecs;
+	uint64_t	tsc;
 
 	assert(!ml_get_interrupts_enabled());
-	generation = rntp->generation;
-	rntp->generation = 0;
-	rntp->ns_base += delta;
-	rntp->generation = ((generation + 1) != 0) ? (generation + 1) : 1;
-	rtc_nanotime_set_commpage(rntp);
+	tsc = rdtsc64();
+	oldnsecs = rntp->ns_base + _tsc_to_nanoseconds(tsc - rntp->tsc_base);
+	newnsecs = base + _tsc_to_nanoseconds(tsc - tsc_base);
+	
+	/*
+	 * Only update the base values if time using the new base values
+	 * is later than the time using the old base values.
+	 */
+	if (oldnsecs < newnsecs) {
+	    _rtc_nanotime_store(tsc_base, base, rntp->scale, rntp->shift, rntp);
+	    rtc_nanotime_set_commpage(rntp);
+	}
 }
 
 void
diff -Naur xnu-1228.0.2.orig/osfmk/i386/tsc.c xnu-1228.0.2/osfmk/i386/tsc.c
--- xnu-1228.0.2.orig/osfmk/i386/tsc.c	2008-01-05 13:34:12.000000000 +1100
+++ xnu-1228.0.2/osfmk/i386/tsc.c	2008-10-03 11:38:13.000000000 +1000
@@ -160,13 +160,16 @@
 	 * Get the TSC increment.  The TSC is incremented by this
 	 * on every bus tick.  Calculate the TSC conversion factors
 	 * to and from nano-seconds.
+	 * The tsc granularity is also called the "bus ratio". If the N/2 bit
+	 * is set this indicates the bus ration is 0.5 more than this - i.e.
+	 * that the true bus ratio is (2*tscGranularity + 1)/2.
 	 */
 	if (cpuid_info()->cpuid_family == CPU_FAMILY_PENTIUM_M) {
 		uint64_t	prfsts;
 
 		prfsts = rdmsr64(IA32_PERF_STS);
 		tscGranularity = (uint32_t)bitfield(prfsts, 44, 40);
-		N_by_2_bus_ratio = prfsts & bit(46);
+		N_by_2_bus_ratio = (prfsts & bit(46)) != 0;
 
 	} else {
 		panic("rtclock_init: unknown CPU family: 0x%X\n",
@@ -174,20 +177,20 @@
 	}
 
 	if (N_by_2_bus_ratio)
-		tscFCvtt2n = busFCvtt2n * 2 / (uint64_t)tscGranularity;
+		tscFCvtt2n = busFCvtt2n * 2 / (1 + 2*tscGranularity);
 	else
-		tscFCvtt2n = busFCvtt2n / (uint64_t)tscGranularity;
+		tscFCvtt2n = busFCvtt2n / tscGranularity;
 
 	tscFreq = ((1 * Giga)  << 32) / tscFCvtt2n;
 	tscFCvtn2t = 0xFFFFFFFFFFFFFFFFULL / tscFCvtt2n;
 
 	kprintf(" TSC: Frequency = %6d.%04dMHz, "
-			"cvtt2n = %08X.%08X, cvtn2t = %08X.%08X, gran = %lld\n",
+			"cvtt2n = %08X.%08X, cvtn2t = %08X.%08X, gran = %lld%s\n",
 			(uint32_t)(tscFreq / Mega),
 			(uint32_t)(tscFreq % Mega), 
 			(uint32_t)(tscFCvtt2n >> 32), (uint32_t)tscFCvtt2n,
 			(uint32_t)(tscFCvtn2t >> 32), (uint32_t)tscFCvtn2t,
-			tscGranularity);
+			tscGranularity, N_by_2_bus_ratio ? " (N/2)" : "");
 
 	/*
 	 * Calculate conversion from BUS to TSC