Why does my MIPS running my code freeze Mars simulator?

Question

Running my code freezes Mars.

At first my error message was saying that my second while label was already declared, so I changed its name to while2.

I've changed my t registers in the second while loop from t0 to t1, created separate print integer procedures to display both arrays, changed my t6 register to t7 in the second array to load the element value for printing.

Here is what I have so far:

.data
    
    #declaring array with 12 bytes to hold 3 integers
    #myArray: .space 12 #uninitialized
    
    #declaring and initializing a 3 element long array with 100 as value
    #default value of each element is 100
    myArray: .word 100:3 #3 elements long, each element is 100
    myArray2: .word 24:5 #5 elements long, each element is 24
    
    #print a new line
    newLine: .asciiz "\n"
    #main title
    mainTitle: .asciiz "This program displays array elements that have already been initialized when the array was created.\n"
    exitMessage: .asciiz "This is the end. "

.text
    
    #main procedure
    main: 
    
        #displaying main title
        li $v0, 4
        la $a0, mainTitle
        syscall
    
        #calling function to display 3 element array
        jal displayMyArray
        
        jal printNL
        
        #calling function to display 5 element array 
        jal displayMyArray2
    
    #closing statement for main procedure
    li $v0, 10
    syscall
    
    #function to display integer for first array
    printInt1:
        li $v0, 1
        addi $a0, $t6, 0
        syscall
    #closing statement
    jr $ra
    
    #function to display integer for second array
    printInt2:
        li $v0, 1
        move $a0, $t7
        syscall
    #closing statement
    jr $ra
    
    #function to display new line
    printNL:
        li $v0, 4
        la $a0, newLine
        syscall
    #closing statement
    jr $ra
    
    #function to display 3 element array of 100 
    displayMyArray:
        #setting t0 register to value of 0
        addi $t0, $zero, 0
        
        #while loop to display all the elements of myArray
        while:
            #test condition
            beq $t0, 12, exit #if t0 = 12, then break out of while loop
            
            #load current element to register t0
            lw $t6, myArray ($t0)
            
            #updating offset
            addi $t0, $t0, 4
            
            #calling printInt to display current number
            jal printInt1 
            #calling printNL to print new line
            jal printNL
            
            j while #iterator
            
        exit:
            li $v0, 4
            la $a0, exitMessage
            syscall
    #closing statement
    jr $ra
    
    #function to display 5 element array
    displayMyArray2:
        #setting t0 register to value of 0
        addi $t1, $zero, 0
        
        #while loop to display all elements of myArray2
        while2:
            #test condition
            beq $t1, 20, exit2 #if t0 = 20, then break out of while loop
            
            #load current element to register t0
            lw $t7, myArray2 ($t1)
            
            #updating offset 
            addi $t1, $t1, 4
            
            #printing current number
            jal printInt2
            #printing new line
            jal printNL
            
            j while #iterator
        exit2:
            li $v0, 4
            la $a0, exitMessage
            syscall
    #closing statement
    jr $ra

Erik Eidt Erik Eidt · Accepted Answer · 2020-07-11T21:09:10

It isn't hanging the simulator, you have an infinite loop.

As you single step, you should notice that the control flow is messed up.

Several of your functions don't return to where they should (when they do jr $ra) because you've changed the $ra value by using jal without first preserving $ra.

When one function, f, calls another function, g, using the jal instruction, which is the right thing to use to call a function, that call/jal to g repurposes the $ra register, so if you were planning to return from the one function, f, back to its caller, you have to have preserved that value for later use.

Why does my MIPS running my code freeze Mars simulator?

1 Answers