MIPS Assembly Language Programming

3.1. FUNCTION ENVIRONMENTS AND LINKAGE 45
(a) Put the return value, if any, into register $v0.
(b) Restore callee-saved registers.
(c) Jump back to $ra, using the jr instruction.
5. To clean up after a function call, the caller must:
(a) Restore the caller-saved registers.
(b) If any arguments were passed on the stack (instead of in $a0-$a3), pop
them off of the stack.
(c) Extract the return value, if any, from register $v0.
The convention used by the programs in this document is that a function stores
$fp at the top of its stack frame, followed by $ra, then any of the callee-saved registers
($s0 - $s7), and finally any of the caller-saved registers ($t0 - $t9) that need to be
preserved.
3.1.1 Computing Fibonacci Numbers
The Fibonacci sequence has the following recursive definition: let F (n) be the nth
element (where n ≥ 0) in the sequence:
• If n < 2, then F (n) ≡ 1. (the base case)
• Otherwise, F (n) = F (n − 1) + F (n − 2). (the recursive case)
This definition leads directly to a recursive algorithm for computing the nth Fibonacci
number. As you may have realized, particularly if you’ve seen this sequence
before, there are much more efficient ways to compute the nth Fibonacci number.
Nevertheless, this algorithm is often used to demonstrate recursion– so here we go
again.
In order to demonstrate a few different aspects of the MIPS function calling conventions,
however, we’ll implement the fib function in a few different ways.
3.1.1.1 Using Saved Registers: fib-s.asm
The first way that we’ll code this will use callee-saved registers to hold all of the local
variables.