Use of Frame Pointer MIPS

Question

i need to convert my code from stack pointer to only use frame pointer, how can i do so? i am very new to MIPS.

i have this recursion C code and its MIPS code below. i am using stack pointer , how can i change it to use frame pointer?

here is my C Code

int fact(int n)
{
    if(n!=1)
     return n*factorial(n-1);
}

    int comb (int n, int k)
    {
    return fact (n) / fact (k) / fact (n - k);
    }

here my MIPS code

comb:           
 sub $sp, $sp, 16
 sw $ra , 0($sp)
 sw $s0, 4($sp) 
 sw $a0, 8($sp) 
 sw $a1, 12($sp)
 jal fact       
 move $s0, $v0  
 lw $a0, 12($sp) 
 jal fact       
 div $s0, $s0, $v0 
 lw $a0, 8($sp) 
 lw $a1, 12($sp) 
 sub $a0, $a0, $a1 
 jal fact       
 div $s0, $s0, $v0 
 move $v0, $s0  
 lw  $ra, 0($sp) 
 lw  $s0, 4($sp) 
 addi $sp, $sp, 16 
 jr $ra         

Answer 1

I found @markgz's comments to be interesting. His link to Wikipedia includes the quote:

The frame pointer ($30) is optional and in practice rarely used except when the stack allocation in a function is determined at runtime, for example, by calling alloca().

I always kinda thoght that $fp seemed superfluous, but I always used it anyway because that's how I was taught to do it.

Anyway, if you are still interested, here is how I used the frame pointer:

#save $ra $s0, $a0 on stack
addi $sp $sp -4
sw   $fp 0($sp)
move $fp $sp
addi $sp $sp -12
sw   $ra  -4($fp)
sw   $a0  -8($fp)
sw   $s0 -12($fp)

...

#restore and shrink stack
lw $s0 -12($fp)
lw $ra  -4($fp)
lw $fp   0($fp)
addi $sp $sp 16

jr $ra

So each time the stack is expanded, I use the stack-pointer to save the old value of the frame pointer, and then I restore the old value of the frame pointer when shrinking the stack.

Mostly I just copy and paste this code every time I write a new function.