IC220 Spring 2011                   Project #1: Pernicious Pell?          Due Fri Feb 11

This project must be completed independently (unlike regular assignments).

This means collaboration between students is not permitted. Online sources (aside from IC220’s website) are also prohibited.  You may consult your notes, textbook, and the instructor. See the instructor for help or clarification.

Background: You have already had experience with writing simple MIPS programs and testing them with SPIM.  In this project you will review and integrate your understanding of:

1.       General MIPS programming

2.       Function calls

3.       Recursion

Task:  Write a recursive MIPS program that computes a term in the “Pell number” sequence, a famous mathematical sequence.  This sequence of numbers (0, 1, 2, 5, 12, 29, 70, 169, 408, 985, 2378, 5741, 13860, 33461, …) can be computed with the following recursive function:

int pell (int N) {

if (N == 0)

return 0;

else if (N == 1)

return 1;

else

return 2*pell(N-1) + pell(N-2);

}

Thus, pell(0) = 0, pell(1) = 1, pell(2) = 2, pell(3) = 5, pell(4) = 12, …. pell(10) = 2378.  Be sure you aren’t off by 1 in the sequence!

There are much more efficient ways to compute the Pell series, but to solidify your understanding of recursion and the stack you are required to have a recursive solution.

Specifics: Starter code is provided: ProjectStarterCode.asm. (right-click and use Save As) Start with this!!!! You should do the following with this code:

1. Modify the main function to:

Print a welcome message that includes your name

Call the pell procedure to compute the Pell term requested by the user

(NOTE: see the sample code at the end of this writeup – shows complete program with functions. This code is NOT the same as the starter code that is given to you.)

Print the calculated term after it has been returned to the main function

2. Write a procedure called pell that:

Recursively calculates the Nth Pell term, where N comes from the argument passed in register \$a0.

Returns the value in \$v0.

You do not have to write this function from scratch – just translate the C code given above!

3. Test your code with SPIM.

Sample screen shot of completed program:

(more on back)
Tips:

1.      Start early!  Recursion can be tricky.  Allow time to get help from the instructor if needed.

2.      But don’t make this harder than it needs to be! Your primary task is to simply translate the C code above to a MIPS function.  While this function is recursive, really this is just another example of a nested procedure – follow the rules for nested procedures and recursion will take care of itself.

3.      You will need to use the stack.  Before you begin, make sure you understand how to do this and what will need to be saved and restored with the stack.

4.      This assignment requires you to write only about 30-40 carefully chosen assembly instructions.  If you have many more than that, you probably are misunderstanding what should be done.

5.      The following material may be helpful to you:

·         Lecture notes on MIPS procedure calls, and SPIM

·         Homework, especially exercises 2-35 thru 2-37

·         Lab #1, where you first used SPIM

·         SPIM reference material: http://www.usna.edu/Users/cs/lmcdowel/courses/ic220/spim/

6.      Use Alt-PrintScreen to generate a screenshot.

7.      An error message like “Can’t add 8 bytes to Lstack” means you are stuck in infinite recursion.

8.      Use the starter code!

9.      In SPIM, the value of the registers are displayed by default in hex. You can change this to decimal via the menu option:   Simulator->Settings, uncheck "General registers in hexadecimal"

10.  Debugging: if it doesn’t work, try running your code for very simple cases (N=0,N=1,N=2,N=3,N=4,etc.).  Then try single-stepping through the simplest case that doesn’t work – what is wrong?

11.  The next page contains another useful example, demonstrating how to do function calls.  Understanding this before you start will save you time!

Deliverables: (1 through 3 handed in, stapled together in the following order)

1)      A cover sheet with your feedback.                                                              (10 pts)

2)      Screenshot showing your program computing pell(15)                                             (10 pts)

3)      Printout of your recursive assembly code                                                     (60 pts)

4)      .asm file saved to your X drive                                                                                (20 pts)

Place this file in your IC220 directory.  Name it Project1-section-lastname.asm

(e.g., Project1-1001-smith.asm).

Extra credit (up to 5 pts each):

1. Write a new program to calculate the Pell numbers in a non-recursive manner.  Turn in this program and a screenshot showing the calculation of Pell(30).

2. Read ahead about floating point registers, then write a new program to calculate the Pell numbers using the floating point registers to keep track of the calculations (you can assume that you only have to deal with numbers with no fractional part).  Why might you want to do this?

# Example code demonstrating how to call a function as part of

# a complete SPIM program.

.data

str_1:  .asciiz "Enter a number=> "

str_2:  .asciiz "Result=> "

.text

.globl main

main:

# Print the prompt

li      \$v0, 4                  # syscall #4 = print string

syscall

# Read an integer from the user

li      \$v0, 5                  # syscall #5 = read integer

syscall                         # read int, result goes in \$v0

# Call a function to add that number together with 13

addi \$a0, \$zero, 13             # Set up first argument to function  (13)

move \$a1, \$v0                   # Set up second argument (the number entered by user)

move \$s0, \$v0                   # save result for later (b/c v0 clobbered below)

# Print string announcing the result

li      \$v0, 4

syscall

# Print actual result

li      \$v0, 1                  # syscall #1 = print integer

move    \$a0, \$s0                # tell syscall what # to print

syscall

# terminate the program

li \$v0, 10

syscall

# Define the (very simple) function we use

# Notice that this goes OUTSIDE of main

jr \$ra                          # return