Jump to Address after Branching in For Loop in MIPS - c

Question

I am trying to code a program that checks if the 16 bits in an integer is a one or zero. I chose to implement this by shifting right one bit 15 times and checking if the first bit in each shift is a zero or non zero. Then, if the first bit is a 1, I increment an integer.
I made some code in C that represents a non-user input version of my code.
int j = 100;
int checker = 0;
int count = 0;
for (i=0; i<16; i++) {
checker = j & 0x1;
if (checker > 0)
count++;
j = (j >> 1);
}
My code in MIPS:
.data
userprompt: .asciiz "Enter positive integer: "
newline: .asciiz "\n"
.text
.globl main
main:
li $v0, 4 # System call: Display string
la $a0, userprompt # Load string userprompt for output
syscall
li $v0, 5 # System call: Read integer
syscall
move $s0, $v0 # Store integer from v0 to s0
move $s1, $s0 # s1 = s0
li $t0, 0 # t0 = 0
jal chk_zeros # Run function: chk_zeroes
li $v0, 1 # System call: Read integer
move $a0, $t2 # Store integer from t2 to a0
syscall
li $v0, 10 # System call: quit
syscall
chk_zeros:
bgt $t0, 15, exitchk # t0 <= 15
addi $t0, $t0, 1 # Add one to t0
andi $t1, $s1, 0x1 # Check if first bit is non-zero, store in t1
bgtz $t1, chk_zerosadd # If t1 >= 0
j chk_zeros
chk_zerosadd:
addi $t2, $t2, 1 # Add one to t2
jr $ra # Return to after the if statement (does not work!)
exitchk:
jr $ra
What I am having trouble with is making chk_zerosadd return to after the branching statement. jr $ra seems to return me to my main function in chk_zerosadd.

Answer 1

bgtz doesn't place the next PC address into the return address register, so jr $ra won't return to the instruction after the branching statement. You can either use bgtzal (branch if greater than zero and link), which will give you the behaviour you are looking for, or else you can re-arrange your code so that you branch over the add, instead of branching to it, like this:
andi $t1, $s1, 0x1 # Check if first bit is non-zero, store in t1
beq $t1, chk_zerosskipadd # Jump if $t1 is zerp
addi $t2, $t2, 1 # Add one to t2
chk_zerosskipadd:
# continue execution...
srl $s1, $s1, 1 # j = (j >> 1);
j chk_zeros