MIPS Recursive Fibonacci Sequence - recursion

Question

I'm having trouble dealing with stacks recursively in MIPS. I get the concept, but my program isn't reacting as I mean it to.
My goal is to take user input as n and print the Fibonacci number at n. What I have so far is below.
(I'm fairly certain the problem is in the actual calculation of the number in the fib function.) Thanks for any help! :)
.text
main:
# Prompt user to input non-negative number
la $a0,prompt
li $v0,4
syscall
li $v0,5
syscall
move $t2,$v0
# Call function to get fibonnacci #n
move $a0,$t2
move $v0,$t2
jal fib
move $t3,$v0
# Output message and n
la $a0,result
li $v0,4
syscall
move $a0,$t2
li $v0,1
syscall
la $a0,result2
li $v0,4
syscall
move $a0,$t3
li $v0,1
syscall
la $a0,endl
li $v0,4
syscall
# End program
li $v0,10
syscall
fib:
# Compute and return fibonacci number
beqz $a0,zero
beq $a0,1,one
sub $sp,$sp,4
sw $ra,0($sp)
sub $a0,$a0,1
jal fib
lw $ra,0($sp)
add $sp,$sp,4
sub $t8,$v0,2 # n - 2
sub $t9,$v0,1 # n - 1
add $v0,$t8,$t9 # add n-2,n-1
jr $ra # decrement/next in stack
zero:
li $v0,0
jr $ra
one:
li $v0,1
jr $ra
.data
prompt: .asciiz "Enter a non-negative number: "
result: .asciiz "F_"
result2: .asciiz " = "
endl: .asciiz "\n"
Example runs:
Enter a non-negative number: 5
F_5 = -29
Enter a non-negative number: 6
F_6 = -61
Correct runs:
Enter a non-negative number: 5
F_5 = 5
Enter a non-negative number: 6
F_6 = 8

Answer 1

Here is a properly working code:
.text
main:
# Prompt user to input non-negative number
la $a0,prompt
li $v0,4
syscall
li $v0,5 #Read the number(n)
syscall
move $t2,$v0 # n to $t2
# Call function to get fibonnacci #n
move $a0,$t2
move $v0,$t2
jal fib #call fib (n)
move $t3,$v0 #result is in $t3
# Output message and n
la $a0,result #Print F_
li $v0,4
syscall
move $a0,$t2 #Print n
li $v0,1
syscall
la $a0,result2 #Print =
li $v0,4
syscall
move $a0,$t3 #Print the answer
li $v0,1
syscall
la $a0,endl #Print '\n'
li $v0,4
syscall
# End program
li $v0,10
syscall
fib:
# Compute and return fibonacci number
beqz $a0,zero #if n=0 return 0
beq $a0,1,one #if n=1 return 1
#Calling fib(n-1)
sub $sp,$sp,4 #storing return address on stack
sw $ra,0($sp)
sub $a0,$a0,1 #n-1
jal fib #fib(n-1)
add $a0,$a0,1
lw $ra,0($sp) #restoring return address from stack
add $sp,$sp,4
sub $sp,$sp,4 #Push return value to stack
sw $v0,0($sp)
#Calling fib(n-2)
sub $sp,$sp,4 #storing return address on stack
sw $ra,0($sp)
sub $a0,$a0,2 #n-2
jal fib #fib(n-2)
add $a0,$a0,2
lw $ra,0($sp) #restoring return address from stack
add $sp,$sp,4
#---------------
lw $s7,0($sp) #Pop return value from stack
add $sp,$sp,4
add $v0,$v0,$s7 # f(n - 2)+fib(n-1)
jr $ra # decrement/next in stack
zero:
li $v0,0
jr $ra
one:
li $v0,1
jr $ra
.data
prompt: .asciiz "This program calculates Fibonacci sequence with recursive functions.\nEnter a non-negative number: "
result: .asciiz "F_"
result2: .asciiz " = "
endl: .asciiz "\n"
Hope to be usefull
Adel Zare
adel.zare.63 [at] gmail [dot] com

Answer 2

You appear to have misunderstood the algorithm (or just implemented it incorrectly). What you're doing is this:
int fib(int n) {
if (n == 0)
return 0;
else if (n == 1)
return 1;
int ret = fib(n - 1);
return (ret - 2) + (ret - 1);
}
What you should be doing is this:
int fib(int n) {
if (n == 0)
return 0;
else if (n == 1)
return 1;
return fib(n - 1) + fib(n - 2);
}