This next problem will test your understanding of stack frames. It is based on the following  recursive C function:
int silly(int n, int *p)
{
int val, val2;
if (n > 0)
val2 = silly(n << 1, &val);
else
val = val2 = 0;
*p = val + val2 + n;
return val + val2;
}
This yields the following machine code:
silly:
pushl %ebp
movl %esp,%ebp
subl $20,%esp
pushl %ebx
movl 8(%ebp),%ebx
testl %ebx,%ebx
jle .L3
addl $-8,%esp
leal -4(%ebp),%eax
pushl %eax
leal (%ebx,%ebx),%eax
pushl %eax
call silly
jmp .L4
.p2align 4,,7
.L3:
xorl %eax,%eax
movl %eax,-4(%ebp)
.L4:
movl -4(%ebp),%edx
addl %eax,%edx
movl 12(%ebp),%eax
addl %edx,%ebx
movl %ebx,(%eax)
movl -24(%ebp),%ebx
movl %edx,%eax
movl %ebp,%esp
popl %ebp
ret
Page 11 of 13
A. (a) Is the variable val stored on the stack?
(b) If so, at what byte oset(relative to %ebp) is it stored?
(c) Why is it necessary to store it on the stack?
B. (a) Is the variable val2 stored on the stack?
(b) If so, at what byte oset(relative to %ebp) is it stored?
(c) Why is it necessary to store it on the stack?
C. (a) What (if anything) is stored at -24(%ebp)?
(b) If something is stored there, why is it necessary to store it?
D. (a) What (if anything) is stored at -8(%ebp)?
(b) If something is stored there, why is it necessary to store it?