Lecture Notes for Computer Architecture II - St. Cloud State University

Example
Write the MIPS code to compute the nth Fibonacci number F(n) where
F(n) = 0, if n =0
= 1, if n =1
= F(n-1) + F (n-2), otherwise
Base your algorithm on the straightforward procedure below.
int fib( int n) {
if (n == 0) return 0;
else if (n ==1) return 1;
else return fib(n-1) + fib(n-2);
}
Solution
n fib(n-1) fib(n-2)
$a0 $t1 $t2
Stack use
Push return address, n, before calling fib(n-1)
Pop n
Push n, fib(n-1), before calling fib(n-2)
Pop fib (n-1), n, return address
If n==0 return 0
Fib:
bne $a0, $zero, fibne0
Add $v0, $zero $zero
Jr $31
If n==1 return 1
Fibne0: addi $v0, $zero 1
Bne $a0, $v0, fibne1
Jr $31
Compute fib (n-1)
Fibne1: addi $sp $sp –8
Sw $ra 4($sp)
Sw $a0 0($sp)
Addi $a0 $a0 –1
Jal fib
add $t1, $v0 $zero
Lw $a0 0$(sp)
Addi $sp $sp 4
Compute Fibn-2
addi $sp $sp –8
Sw $a0 4($sp)
Sw $t1 0($sp)
Page | 55
Addi $a0 $a0 –2
Jal fib
Move $t2, $v0
Lw $t1 0$(sp)
Lw $a0 4$(sp)
Lw $ra 8$(sp)
Addi $sp $sp 12
Return fib(n-1) + fib(n-2)
Add $v0 $t1 $t2
Jr $31