Question

I can’t quite figure out what I’m doing wrong in my assembly code. I’m trying to write a program that compares two null terminated strings that are inputted as $a0, and $a1 sometime during the “main:” section and then calls

jal hamming

to start the program.

Basically for this section I want two strings to be compared char by char until one string hits the null terminating char. Then the program stops and returns how many chars were different until termination.

I think it has to do with the jumps that I am using, but I’m not quite sure. The program is sort of long so I took the main piece that I think is the problem (hence ignore variables like $a3, that has been initialized and defined already):

diffchar:
        li $t4, 0
        li $t5, 1

        beq $a0, $a1, samechars
        move $v0, $t5
        j diffcharend

samechars:
        move $v0, $t4

diffcharend:
        jr $ra

hamming:

absvaluedone:
        li $a2, 0
        #li $v0, 0

        move $t0, $a0
        move $t1, $a1

hammingloopbegin:
        lb $t2, 0($t0)
        lb $t3, 0($t1)
        beq $t2, $0, hammingdone
        beq $t3, $0, hammingdone

        la $a0, 0($t0)
        la $a1, 0($t1)
        jal diffchar **#this is the line that causes me problems, if I take this out it is fine**
        beq $v0, $0, next
        addiu $a2, $a2, 1

next:
        addiu $t0, $t0, 1
        addiu $t1, $t1, 1
        j hammingloopbegin

hammingdone:
        add $v0, $a2, $a3
        jr $ra

When I run my program my output looks like an infinite loop that keeps saying:

Exception occurred at PC=0x00400144
  Bad address in data/stack read: 0x10021226
  Exception 7  [Bad address in data/stack read]  occurred and ignored
Exception occurred at PC=0x00400140
  Bad address in data/stack read: 0x1002121b
  Exception 7  [Bad address in data/stack read]  occurred and ignored

I think there’s something wrong with diffchar or the process I use to jump around to diffchar. This is my first time writing assembly code so I think theres something really basic I am missing thats making this mess up. Any pointers would be great.

Thanks for the help

Answer 1

You do not save the return address before jal diffchar. It returns and $ra still holds the new value. When you jr $ra again in hammingdone, you jump back to right after the diffchar call. You need to store $ra somewhere before calling diffchar and restore it after the call returns.

Here is a good explanation of nested procedures in MIPS, which describes the problem you are experiencing and a solution using the runtime stack:

When the jal B instruction is executed, the return address in register
$ra for procedure A will be overwritten with the return address for
procedure B. Procedure B will return correctly to A, but when
procedure A executes the jr instruction, it will return again to the
return address for B, which is the next instruction after jal B in
procedure A. This puts procedure A in an infinite loop.

…

The system stack is commonly used to save return addresses. They can
be pushed on the stack when a procedure is called and popped off to
execute a return instruction.

A return address in register $ra can be pushed onto the system stack
with the following MAL code:

    sw      $ra, ($sp)
    add     $sp, -4

The following code pops a return address from the top of the stack and
returns it in register $ra:

    add     $sp, 4
    lw      $ra, ($sp)