nachos.machine.AssertionFailureError: Unexpected exception
	at nachos.machine.Lib.assertTrue(Lib.java:89)
	at nachos.machine.Lib.assertNotReached(Lib.java:106)
	at nachos.userprog.UserProcess.handleException(UserProcess.java:814)
	at nachos.userprog.UserKernel.exceptionHandler(UserKernel.java:94)
	at nachos.userprog.UserKernel$1.run(UserKernel.java:38)
	at nachos.machine.Processor$MipsException.handle(Processor.java:603)
	at nachos.machine.Processor.run(Processor.java:101)
	at nachos.userprog.UThread.runProgram(UThread.java:31)
	at nachos.userprog.UThread.access$0(UThread.java:27)
	at nachos.userprog.UThread$1.run(UThread.java:20)
	at nachos.threads.KThread.runThread(KThread.java:162)
	at nachos.threads.KThread.access$1(KThread.java:160)
	at nachos.threads.KThread$1.run(KThread.java:151)
	at nachos.machine.TCB.threadroot(TCB.java:235)
	at nachos.machine.TCB.access$3(TCB.java:210)
	at nachos.machine.TCB$1.run(TCB.java:93)
	at java.lang.Thread.run(Thread.java:680)