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CS 2733/2731  Computer Organization II  Spring 2004  Take-home Quiz    Exceptions  Due Wednesday, 28 April 2004,    (start of class)

% spim -notrap -file quiz_exception.s

Below is the code (line numbers to the left are not part of the code). Here is the source of the above code without line numbers: quiz_exception.s

     1	# Program for Take-home Quiz: Exception Handling
     2	main:   la      $t1, Data
     3	        lw      $t4, 0($t1)
     4	        li      $v0, 1          # syscall 4 (print_int)
     5	        add     $a0, $0, $t4
     6	        syscall
     7	        add     $t5, $0, $0
     8	        lw      $t6, 0($t5)
     9	        li      $v0, 4          # syscall 4 (print_str)
    10	        la      $a0, All        # print "That's all!"
    11	        syscall
    12	        jr      $ra             # return from main
    13	        .data
    14	Data:   .word   314159265
    15	All:    .asciiz "\nTh-th-th-that's all folkes!\n"
    16	############# Start of Trap Handler ######################
    17	        .kdata  
    18	Duhh:   .asciiz "\nDuhh-hhhhh!\n"
    19	        .ktext 0x80000080
    20	        li $v0 4        # syscall 4 (print_str)
    21	        la $a0 Duhh     # print "Duhh-hhhhh!"
    22	        syscall
    23	        mfc0 $k0 $14    
    24	        rfe             
    25	        addiu $k0 $k0 4 
    26	        jr $k0          
    27	# Standard startup code.  Invoke the routine main with no arguments.
    28	        .text
    29	        .globl __start
    30	__start: jal main       # start up main
    31	        li $v0 10       
    32	        syscall         # syscall 10 (exit)

1. What will happen if the above code is run without lines 27 through 32 and without the -notrap option, that is, with % spim -file quiz_exception0.s? This code is given below and in quiz_exception0.s:

   
   # Program for Review Quiz: Exception Handling
   main:   la      $t1, Data
           lw      $t4, 0($t1)
           li      $v0, 1          # syscall 4 (print_int)
           add     $a0, $0, $t4
           syscall
           add     $t5, $0, $0
           lw      $t6, 0($t5)
           li      $v0, 4          # syscall 4 (print_str)
           la      $a0, All        # print "That's all!"
           syscall
           jr      $ra             # return from main
           .data
   Data:   .word   314159265
   All:    .asciiz "\nTh-th-th-that's all folkes!\n"
   

2. Assume now that the entire program is run with the -notrap option. Starting with line 2, describe what is printed when this code segment executes lines 2 through 12. Will Duhh-hhhhh! be printed and if so, why?

3. What (if anything) is line 26 (jr $k0) doing? What would happen if line 25 (addiu $k0 $k0 4) were omitted?