Counting number of elements divisible by 4 in an array using MIPS - arrays

Question

enter code here I'm trying to count the number of elements in this array that are divisible by four, currently it prints out the number of times the program loops. For some reason the program calls mod every time. Can anyone identify why this might be happening?
.data
arr: .word 5, 4, 8,12, 13, 16, 18, 20, 24,23,0
msg: .asciiz "Elements Divisible By 4 : "
four: .word 4
.text
lb $s4, four
.globl main
main:
addi $t0, $0, 0 # clear i
addi $t1, $0, 0 # clear sum
ori $t2, $0, 11 # Initializing t2 to its constant value 10
la $t3, arr # load address of array into t3
loop:
slt $t4, $t0, $t2 # compare, $t4 = i < sum ? 1 : 0
beq $t4, $0, end # if i is not < 11, exit the loop
lw $t4, 0($t3) # load current array element into t4
andi $t4, $t4, 3
beq $t4, $zero, mod
j loop
mod: # if-else exit
add $t1, $t1, 1 # add it to sum
add $t0, $t0, 1 # increment i
add $t3, $t3, 4 # increment current array element pointer
j loop
end:
addi $v0, $0, 4 # Now we print out result: string
la $a0, msg
syscall
addi $v0, $0, 1 # followed by the actual sum (which is in t1)
add $a0, $t1, $0
la $t5, exit
syscall
exit:
j exit
Working version
.data
arr: .word 12, 4, 8, 12, 13, 16, 18, 20, 24, 23, 0
msg: .asciiz "Counter "
fourMod: .word 4
.text
lb $s1, fourMod
.globl main
main:
addi $t0, $0, 0 # clear i
addi $t1, $0, 0 # clear sum
ori $t2, $0, 10 # Initializing t2 to its constant value 10
la $t3, arr # load address of array into t3
loop:
slt $t4, $t0, $t2 # compare, $t4 = i < sum ? 1 : 0
beq $t4, $0, end # if i is not < 10, exit the loop
lw $t4, 0($t3) # load current array element into t4
andi $t4, $t4, 3
beq $t4, $zero, mod
add $t0, $t0, 1 # increment i
add $t3, $t3, 4 # increment current array element pointer
j loop
mod:
#add to the divisible by 4 counter?
add $s2, $s2, 1
add $t0, $t0, 1 # increment i
add $t3, $t3, 4 # increment current array element pointer
j loop
end:
addi $v0, $0, 4 # Now we print out result: string
la $a0, msg
syscall
addi $v0, $0, 1 # followed by the actual sum (which is in t1)
add $a0, $s2, $0
la $t5, exit
syscall
exit:
j exit

Answer 1

You never tell your program not to execute the code after mod::
add $t3, $t3, 4 # increment current array element pointer
<--- There's nothing here to prevent the execution to continue with the below add
mod:
add $s2, $s2, 1
Use a branch instruction if you want to skip over some code. Or in your case what you want is probably a j loop.
Another thing is that you're doing an unnecessary division, which often is a relatively slow operation. Checking whether an integer is a multiple of four can be done by testing the two least significant bits:
andi $t4, $t4, 3 # Isolate the two least significant bits
beq $t4, $zero, mod # We've got a multiple of 4 if those two bits are zero
Edit: With your updated code you're still incrementing your "is divisible" counter on every iteration. It would probably be best if you inverted your branch condition and changed that part of the code into:
andi $t4, $t4, 3
bne $t4, $zero, not_divisible
add $t1, $t1, 1 # add it to sum
not_divisible: # if-else exit
add $t0, $t0, 1 # increment i
.... # omitted for brevity