diff --git a/12_day/bootpack.h b/12_day/bootpack.h
index f7a1a01..d42a313 100644
--- a/12_day/bootpack.h
+++ b/12_day/bootpack.h
@@ -176,7 +176,7 @@ struct TIMER {
 	unsigned char data;
 };
 struct TIMERCTL {
-	unsigned int count;
+	unsigned int count, next;
 	struct TIMER timer[MAX_TIMER];
 };
 extern struct TIMERCTL timerctl;
diff --git a/12_day/timer.c b/12_day/timer.c
index 77ad20c..e936ffb 100644
--- a/12_day/timer.c
+++ b/12_day/timer.c
@@ -16,6 +16,7 @@ void init_pit(void)
 	io_out8(PIT_CNT0, 0x9c);
 	io_out8(PIT_CNT0, 0x2e);
 	timerctl.count = 0;
+	timerctl.next = 0xffffffff; /* 因为最初没有正在运行的定时器 */
 	for (i = 0; i < MAX_TIMER; i++) {
 		timerctl.timer[i].flags = 0; /* 未使用 */
 	}
@@ -51,6 +52,10 @@ void timer_settime(struct TIMER *timer, unsigned int timeout)
 {
 	timer->timeout = timeout + timerctl.count;
 	timer->flags = TIMER_FLAGS_USING;
+	if (timerctl.next > timer->timeout) {
+		/* 更新下一次的时刻 */
+		timerctl.next = timer->timeout;
+	}
 	return;
 }
 
@@ -59,12 +64,22 @@ void inthandler20(int *esp)
 	int i;
 	io_out8(PIC0_OCW2, 0x60); /* 把IRQ-00信号接收完了的信息通知给PIC */
 	timerctl.count++;
+	if (timerctl.next > timerctl.count) {
+		return; /* 还不到下一个时刻，所以结束*/
+	}
+	timerctl.next = 0xffffffff;
 	for (i = 0; i < MAX_TIMER; i++) {
 		if (timerctl.timer[i].flags == TIMER_FLAGS_USING) {
 			if (timerctl.timer[i].timeout <= timerctl.count) {
+				 /* 超时 */
 				timerctl.timer[i].flags = TIMER_FLAGS_ALLOC;
 				fifo8_put(timerctl.timer[i].fifo, timerctl.timer[i].data);
 			}
+		} else {
+			 /* 还没有超时 */
+			if (timerctl.next > timerctl.timer[i].timeout) {
+				timerctl.next = timerctl.timer[i].timeout;
+			}
 		}
 	}
 	return;