add1sir

.data
    str: .asciiz "abcdefghijklmnopqrstwyz"
.text
main:
    li $t0,0
    lb $t1,str($t0)
    loop:
        beqz $t1,exit
        addi $t1,1
        sb $t1,str($t0)
        addi $t0,1
        lb $t1,str($t0)
        j loop
    exit:
        la $a0,str
        li $v0,4
        syscall
        li $v0,10
        syscall