You are using $a0
as a character, such as here:
slti $t2, $a0, 91
but it is never filled with the character. At the moment, it contains a memory address, not a character.
You should load the character using lb
and store it back after making it upper/lowercase using sb
.
Feel free to add a comment if you want a code example.
Edit: the changes in the relevant part of the code:
...
li $t0, 0 # t0 = i = 0
for_loop:
slti $t1, $t0, 80 # t1 = 1 if and only if t0 < 80
beq $t1, $0, for_loop_done
lb $t4, 0($a0)
beqz $t4, for_loop_done
beq $t4, 10, for_loop_done
slti $t2, $t4, 91
li $t3, 1
beq $t2, $t3, upper #if the character value is less than 91 branch to upper addition
bne $t2, $t3, lower
upper:
addi $t4, $t4, 32 #adds 32 to the character value to lowercase it
j done
lower:
addi $t4, $t4, -32 #subtracts 32 from the character value to capitalize it
done:
addi $t0, $t0, 1
sb $t4, 0($a0)
addi $a0, $a0, 1
j for_loop
for_loop_done:
#Prints the result string
...