diff --git a/kernel/clock.c b/kernel/clock.c
new file mode 100644
index 0000000..8d7c504
--- /dev/null
+++ b/kernel/clock.c
@@ -0,0 +1,80 @@
+/*
+Copyright (C) 2015-2019 The University of Notre Dame
+This software is distributed under the GNU General Public License.
+See the file LICENSE for details.
+*/
+
+#include "interrupt.h"
+#include "clock.h"
+#include "ioports.h"
+#include "process.h"
+
+// Minimum PIT frequency is 18.2Hz.
+#define CLICKS_PER_SECOND 20
+
+#define TIMER0		0x40
+#define TIMER_MODE	0x43
+#define SQUARE_WAVE     0x36
+#define TIMER_FREQ	1193182
+#define TIMER_COUNT	(((unsigned)TIMER_FREQ)/CLICKS_PER_SECOND)
+
+static uint32_t clicks = 0;
+static uint32_t seconds = 0;
+
+static struct list queue = { 0, 0 };
+
+static void clock_interrupt(int i, int code)
+{
+	clicks++;
+	process_wakeup_all(&queue);
+	if(clicks >= CLICKS_PER_SECOND) {
+		clicks = 0;
+		seconds++;
+		process_preempt();
+	}
+}
+
+clock_t clock_read()
+{
+	clock_t result;
+	result.seconds = seconds;
+	result.millis = 1000 * clicks / CLICKS_PER_SECOND;
+	return result;
+}
+
+clock_t clock_diff(clock_t start, clock_t stop)
+{
+	clock_t result;
+	if(stop.millis < start.millis) {
+		stop.millis += 1000;
+		stop.seconds -= 1;
+	}
+	result.seconds = stop.seconds - start.seconds;
+	result.millis = stop.millis - start.millis;
+	return result;
+}
+
+void clock_wait(uint32_t millis)
+{
+	clock_t start, elapsed;
+	uint32_t total;
+
+	start = clock_read();
+	do {
+		process_wait(&queue);
+		elapsed = clock_diff(start, clock_read());
+		total = elapsed.millis + elapsed.seconds * 1000;
+	} while(total < millis);
+}
+
+void clock_init()
+{
+	outb(SQUARE_WAVE, TIMER_MODE);
+	outb((TIMER_COUNT & 0xff), TIMER0);
+	outb((TIMER_COUNT >> 8) & 0xff, TIMER0);
+
+	interrupt_register(32, clock_interrupt);
+	interrupt_enable(32);
+
+	printf("clock: ticking\n");
+}