Machine Problem 3, due October 11

Submit the source file mp3.a for your solution using the submit command, documented at:
-- http://www.divms.uiowa.edu/help/msstart/submit.html

The Problem

Given a rectangular area, print ax x in the middle. Then, do the same thing for the rectangular areas making up each quadrant of the area, until your area is just a single x. If the initial area starts out a perfect rectangle of just the right size, you'll get this:

        x   x   x   x   x   x   x   x     
          x       x       x       x
        x   x   x   x   x   x   x   x     
              x               x
        x   x   x   x   x   x   x   x
          x       x       x       x
        x   x   x   x   x   x   x   x
                      x
        x   x   x   x   x   x   x   x     
          x       x       x       x
        x   x   x   x   x   x   x   x     
              x               x
        x   x   x   x   x   x   x   x
          x       x       x       x
        x   x   x   x   x   x   x   x

For other initial area sizes, you'll get less symmetrical results, but elements of this pattern will show up.

In a high level language, the algorithm that produces this is as follows:

plot( int xlow, xhigh, ylow, yhigh ){
    int xmid = (xhigh + xlow)/2;
    int ymid = (yhigh + ylow)/2;
    dspat(xmid,ymiddle);
    dspch('x');
    if ((xlow < xmid) && (ylow < ymid)) {
        plot( xlow, xmid-1, ylow, ymid-1 );
        plot( xmid+1, xhigh, ylow, ymid-1 );
        plot( xlow, xmid-1, ymid+1, yhigh );
        plot( xmid+1, xhigh, ymid+1, yhigh );
    }
}

The main program should start by calling plot(0,width-1,0,height-1) using the width and height of the screen.

Your problem is to write this code in the SMAL Hawk assembly language and test it with a variety of window sizes. You'll find that it produces good looking output only for windows that are sized carefully. Just for fun, what are the good window sizes?

WARNING: Significant parts of the first midterm, which is the Friday before this assignment is due, will be based on this machine problem. Work you do on this before the exam should pay off, so long as you don't ignore other issues we've studied such as binary representations and machine code instruction formats.

Grading Critereia

• 2.5 points for the source code, with penalties up to and including:
  • -1 point for the program title, which must identify the file as mp3.a, and must document it as Machine Problem 3 by yourself, with your full name, in the form it appears on your ID card. All of this should be in the program title.
  • -1 point for failure to use of white space appropriately.
  • -1 point for failure to systematically use of identifiers.
  • -1 point for unclear, excessive or misleading clear comments.

• 2.5 points for program function, with penalties up to and including:
  • -1 point for off by one errors leading to almost correct output.
  • -1.5 points for one-dimensional displays that ought to be two dimensional.
  • -2 points for partial sensible output before a bus trap or other error.
  • -2.5 points for bus trap or similar fatal errors before any output.