CS 14 -- Lab 4

In today's lab, we will work on procedure calls in assembly. We will work both with non-recursive and recursive procedures.

Simple exponentiation

In this lab, we're going to calculate exponents in a straightforward manner. We are going to assume, for simplicity, that both the base and exponents that we're using are non-negative integers.

If we consider, when y > 0, that x^y = x * x^y-1, then we can write a recursive procedure that will rely on itself to calculate x^y-1, and multiply that result by x. When y = 0, we simply observe that x^y = 1, irrespective of the value of x. So, in Java pseudo-code, the procedure would be:

  public static int calcExponent (int x, int y) {
    if (y == 0) {
      return 1;
    } else {
      return x * calcExponent(x, y-1);
    }
  }
    

An instruction you will need

In order to multiply numbers, you can use the mul instruction, which is an R-type instruction that is used like so:

mul $t0, $t1, $t2 # $t0 = $t1 * $t2

The assignment itself

1. Login to the romulus via VNC and use the following command to copy a file into your directory:

   cp ~sfkaplan/public/cs14/calcexponent.s .

2. Bring up your copy of the calcexponent.s file in xemacs. Examine both the main procedure as well as the printvalue procedure. Note that this latter procedure uses some magic instructions to open a little output window and print numbers into it. It is a simple procedure that doesn't use any of the callee-preserved registers, and so it needs no activation frame.
3. Find the comment labeled TASK 1. Insert two procedure calls to printvalue, such that the base is printed, and then the exponent is printed. The value to be printed must be passed in register $a0.
4. Load the simulator and test your program. Is the printvalue procedure called? Does it emit the base and the exponent as expected. Do you see how the procedure call is working? Observe the register $ra when the jal instruction is executed.
5. Find the comment labeled TASK 2. Write the calcexponent procedure. This procedure should expect the base value to have been passed by the caller in register $a0, and the exponent value to have been passed in register $a1. It should calculate the exponent, as described above, and return its result in register $v0. Be sure to preserve any callee-preserved registers that the method uses!
6. Find the comment labeled TASK 3. Insert two procedure calls; the first should be to calcexponent to perform the exponentiation, and the second should be to printvalue, where the result of the exponentiation should be emitted.

Finishing the lab

Email your code to me like so:

mail -s "CS 14 lab-4" sfkaplan@cs.amherst.edu < calcexponent.s

Be sure to exit xemacs and logout.