CS 273

Homework 4

Due Friday, February 23, 2001

No reading assignment this week. We're going to get some practice with things we've already seen.

From the Book

• 4-13 (10 points)

Not From the Book

Background

The Fibonacci series is defined recursively as follows:

• F₀ = 0
• F₁ = 1
• F_i = F_i-2 + F_i-1

A good way to calculate a Fibonacci number F_n is as follows:

1. Maintain three variables, which we'll call Fn, Fn1, and Fn2, for ``Fibonacci of n,'' ``Fibonacci of n+1,'' and``Fibonacci of n+2,'' respectively.
2. The initial values of Fn, Fn1, and Fn2 are 0, 1, and 1, respectively.
3. To compute F₀, you've already got the answer in Fn.
4. To compute any other F_n, execute the following loop n times:
   1. Set Fn2 to Fn + Fn1
   2. Set Fn to Fn1
   3. Set Fn1 to Fn2
   When you've finished the loop, your result is in Fn.

Assignment (25 points)

Write an assembly program to calculate F_n, for some n. Your program should:

• Be well-structured. It should be possible to easily break it down into the three fundamental structures described in the book.
• Use rmb to declare variables fn, fn1, and fn2 in memory locations 00, 01, and 02, respectively.
• Use fcb to place n in location $f800.
• Use equ to define symbols for the start of RAM, the start of EEPROM, and the RESET interrupt vector location.
• Use appropriate comments to describe what your program is doing. Writing proper comments is an art; you need to describe what your code is doing, not restate the lines of code. So, to look at a likely fragment of code that might turn up in your program, here's a good example of commenting:

  	  
  	  ldaa    fn1  * copy fn1...
  	  staa    fn   * ...into fn
  	  
  	  

  and here's a bad example:

  	  
  	  ldaa    fn1  * load fn1 into accumulator a
  	  staa    fn   * store accumulator into fn
  	  
  	  

You may (in fact, you probably will) find it helpful to start by writing the program in some imperative high-level language of your choice, and translate that program into assembly code. But that's not an explicit part of the assignment.