Question

Use MARS to write and simulate a MIPS assembly language program to implement the strlen function. The program should ask for a user's input and print the length of the user's input. Write the main function to call strlen.

The main function should:
- Prompt the user for input
- Pass that input to strlen using registers $a0.
- Preserve (i.e. push onto the stack) any T registers that the main function uses. The strlen function should:
- Preserve any S registers it uses.
- Determine the length of the string.
- Return the length of the string to main using register $v0.

Answer 1

Code:

.data
        # reserve 100 bytes for user input
        inputString: .space 100
.text
.globl main

main:
        # load the address of the space where string is to be stored
        la $a0, inputString
        # load how many characters to read
        li $a1, 100
        # syscall code to read a string
        li $v0, 8
        syscall
        
        # load address of the string for passing to strlen function
        la $a0, inputString
        # call strlen function
        jal strlen
        
        # load the value returned by strlen from $v0 to $a0 for printing
        add $a0, $zero, $v0
        # syscall code to print an integer
        li $v0, 1
        syscall
        
        # syscall code to exit the program
        li $v0, 10
        syscall
# strlen function takes string in $a0, returns it's length in $v0
strlen:
        # initialize our counter $t0 to -1, so that null character is not counted
        li $t0, -1
        # loop through the string
        loop:
                # load the byte at address $a0 to $t1
                lb $t1, 0($a0)
                # if $t1 is '\0' i.e null character return 
                beqz $t1, return
                # else increment the address in $a0 by 1
                addi $a0, $a0, 1
                # increment the counter by 1
                addi $t0, $t0, 1
                # loop again
                j loop
        # return the value of $t0 and exit the function
        return:
                # load the value to $t0 to $v0
                add $v0, $zero, $t0
                # return back to the main
                jr $ra

                

OUTPUT: