MIPS Assembly language compare predefined string with user input

Question

I am new to mips, how do I compare a predefined string with user input?

Below is my code asking user to continue or not (Y/N) . If Y then jump back to start, else go to the end. At the end, $t1 is 10010000 , $t2 is 10010046 if I enter Y .

Where is the problem?

.data

# Create some null terminated strings which are to be used in the program
buffer:         .space 10
strAgain:       .asciiz "Continue (Y/N)? "
strY:           .asciiz "Y\n"
strN:           .asciiz "N"

.text
.globl main

main:

    ...

    li $v0, 4                  
    la $a0, strAgain           
    syscall                    

    li $v0, 8                  
    la $a0, buffer
    li $a1, 10                 
    syscall                    
    move $t1, $a0              

    la $t2, strY               
    bne $t1, $t2, end
    j main

end:
    li $v0,10       # Exit
    syscall         

Answer 1

You are attempting to compare strings by comparing addresses, which is a flawed approach.

What you have written is equivalent to the following C code:

char *strY = "Y\n";
char buffer[10];

int main() {
    for (;;) {
        printf("Continue (Y/N)? ");
        scanf("%s", buffer);
        if (buffer != strY)
            break;
    }
    exit();
}

Hopefully it's clear that the error in the C code is that you should be using strcmp to compare strings, not checking for pointer equality.

So, you need to write the strcmp function in MIPS assembly:

int
strcmp(const char *s1, const char *s2)
{
    for ( ; *s1 == *s2; s1++, s2++)
        if (*s1 == '\0')
            return 0;
    return ((*(unsigned char *)s1 < *(unsigned char *)s2) ? -1 : +1);
}