Who saves what: caller-saved and callee-saved

A call may destroy a register

When jal returns, the callee has run, and it used registers to do its work. Any register it wrote is now clobbered from the caller’s point of view. The convention decides which registers a callee may clobber freely and which it must restore. Here is the table from §2.2 again, read for its last column:

Two duties follow:

The word saved names the side that pays. A callee saves the $s registers it touches; a caller saves the $t values it wants back.

Watch the rule break, then hold

The runner keeps a value across a call twice: first in $t0, then in $s0. It pauses after the first call to ask what will print.

▮

fib keeps a result in $s0

Why would a function want a callee-saved register? To hold a value across a call. fib(n) computes fib(n−1), then must keep that result safe while it calls fib(n−2). A $t register would be clobbered by the second call. So it uses $s0, and because $s0 is callee-saved, fib must save its caller’s $s0 first.

1int fib(int n) {
2    if (n < 3) return 1;
3    return fib(n - 1) + fib(n - 2);
4}

bfc0027csw    s0,20(sp)
bfc002b0jal   bfc00270 <fib>
bfc002b8move  s0,v0
bfc002c8jal   bfc00270 <fib>
bfc002d0addu  v0,s0,v0
bfc002e0lw    s0,20(sp)

$ra: clobbered by the call, saved by the callee

$ra sits in the caller-saved list, and that is correct. After jal f, the caller’s $ra holds f’s return address, not its own. So who saves it? The callee, in its prologue, for its own sake. Its inner jals will overwrite it, and it needs its own way home at the end. A leaf has no inner jal and skips the save (§6.1).

Can you now explain it?

Now try to explain this one yourself: main in hello.c saves $ra and $s8 but not $s0. fib saves $ra, $s8 and $s0. Explain both lists from the rule.

Next: §6.3 — stack frames and recursion: fib step by step.