diff --git a/15_day/bootpack.c b/15_day/bootpack.c
index 98b7bc4..42ea912 100644
--- a/15_day/bootpack.c
+++ b/15_day/bootpack.c
@@ -278,5 +278,24 @@ void make_textbox8(struct SHEET *sht, int x0, int y0, int sx, int sy, int c)
 
 void task_b_main(void)
 {
-	for (;;) { io_hlt(); }
+	struct FIFO32 fifo;
+	struct TIMER *timer;
+	int i, fifobuf[128];
+	fifo32_init(&fifo, 128, fifobuf);
+	timer = timer_alloc();
+	timer_init(timer, &fifo, 1);
+	timer_settime(timer, 500);
+
+	for (;;) {
+		io_cli();
+		if (fifo32_status(&fifo) == 0) {
+			io_stihlt();
+		} else {
+			i = fifo32_get(&fifo);
+			io_sti();
+			if (i == 1) { /*超时时间为5秒 */
+				taskswitch3(); /*返回任务A */
+			}
+		}
+	}
 }
diff --git a/15_day/bootpack.h b/15_day/bootpack.h
index 7028583..5868890 100644
--- a/15_day/bootpack.h
+++ b/15_day/bootpack.h
@@ -28,6 +28,7 @@ void asm_inthandler21(void);
 void asm_inthandler27(void);
 void asm_inthandler2c(void);
 unsigned int memtest_sub(unsigned int start, unsigned int end);
+void taskswitch3(void);
 void taskswitch4(void);
 
 /* fifo.c */
diff --git a/15_day/naskfunc.nas b/15_day/naskfunc.nas
index 0a1e37f..d7194a7 100644
--- a/15_day/naskfunc.nas
+++ b/15_day/naskfunc.nas
@@ -16,7 +16,7 @@
 		GLOBAL	_asm_inthandler20, _asm_inthandler21
 		GLOBAL	_asm_inthandler27, _asm_inthandler2c
 		GLOBAL	_memtest_sub
-		GLOBAL	_taskswitch4
+		GLOBAL	_taskswitch3, _taskswitch4
 		EXTERN	_inthandler20, _inthandler21
 		EXTERN	_inthandler27, _inthandler2c
 
@@ -207,6 +207,10 @@ mts_fin:
 		POP		EDI
 		RET
 
+_taskswitch3:  ; void taskswitch3(void);
+		JMP    3*8:0
+		RET
+
 _taskswitch4:	; void taskswitch4(void);
 		JMP		4*8:0
 		RET