>Dr. Plank,
>
>I had a student ask me to clarify your answer for midterm question 1, but
>I was a little baffled myself, so I couldn't. What I am not clear on is
>why the first byte of s is located at fp-16 in your example.
>
>I would think that the chars would be organized from low to high
>addresses, with fp-19 being the first and fp-4 being the last. However, I
>could believe the compiler uses some other ordering such that fp-16 is the
>first element, folled by 17, 18, 19, and then 12, 13 etc. I could also
>believe that I am just confused. Could you clear this up for me a bit?



So, suppose the program was:

main(int argc, char **argv)
{
  char s[4];
  char *x;

  x = strcpy(s, argv[1]);
  puts(x);
}

Then it's pretty clear that you would start with ``push #8'', and
if x is at fp, then s must start at fp-4:


       |                       |
       |                       |
       |                       | <----- sp
fp-4   |s[0] |s[1] |s[2] |s[3] |
fp     |          x            | <----- fp

Ok -- now suppose it were s[8].  We'd start with ``push #12'' and 
now we have s starting with fp-8:


       |                       |
       |                       |
       |                       | <----- sp
fp-8   |s[0] |s[1] |s[2] |s[3] |
fp-4   |s[4] |s[5] |s[6] |s[7] |
fp     |          x            | <----- fp

So, with s[16] -- We start with ``push #20'' and get:

       |                       |
       |                       |
       |                       | <----- sp
fp-16  |s[0] |s[1] |s[2] |s[3] |
fp-12  |s[4] |s[5] |s[6] |s[7] |
fp-8   |s[8] |s[9] |s[10]|s[11]|
fp-4   |s[12]|s[13]|s[14]|s[15]|
fp     |          x            | <----- fp

Jim Plank