Program Specification

The program must start at address x$3000.$

The program will start by copying a picture of a cow from location x$5000$ into the center of the screen.

The image of the cow is a rectangle $30$ pixels wide by $25$ pixels high. This cow image is provided to you

as a separate cow.obj file to be loaded into Pennsim. This cow image data is stored in row order. For

example, the first row of $30$ pixels is stored at x$5000-$x$501$D$,$ the second row is stored at x$501$E$-503$B$,$

and so on$.$

$2$

The user will then draw $10-20$ blades of $$grass$$ colored green on the sides of the cow. These blades of

grass should be $2$ pixels wide, start at the same pixel row as the bottom of the initial cow image, and be

as tall as the cow image. These grass blades do not move with the cow moves or tips. The grass is not

redrawn when it is stomped by the cow.

The PennSim graphics display $($the $$screen$)$ is $128$ by $124$ pixels. We use an $($x$,$ y$)$ coordinate system

to describe a location on the screen. Location $(0,0)$ is the top left corner. The x coordinate increases as

we move to the right, and the y coordinate increases as we move down. In other words, $(1,0)$ is one

pixel to the right of $(0,0),$ and location $(0,1)$ is one pixel below $(0,0).$ Location $(127,123)$ is the

bottom right corner of the screen.

The user interacts with the program using one$-$character commands. The commands are typed on the

keyboard, but are not echoed to the console display. Nothing will be printed to the console during the

execution of this program. The program will wait for a key press, perform the corresponding command,

and repeat. If the keystroke does not correspond to a legal command, it will have no effect.

There are four commands for changing the location of the Pen. We use the WASD scheme of

navigation, used by various computer games. $3$

Details

The PennSim graphics display is bit$-$mapped, meaning that each pixel has a corresponding memory

location. The content of that memory location controls the color of the pixel.

Pixel Addresses

Addresses xC$000$ through xFDFF are assigned to the graphics display. The low address corresponds to

the top left corner $(0,0).$ Moving one pixel to the right adds one to the address, and it $$wraps around$$ to

the next row when it gets to the right edge. Since the display is $128$ pixels wide, this means that moving

down one pixel is equivalent to adding $128$ to the address.

The address of point $($x$,$ y$)$ can be calculated as: xC$000+$ x $+128$y$.$

For this assignment, you will not need to calculate arbitrary pixel addresses, except to figure out where

the initial location is$.$ You will be moving left $(-4),$ right $(+4),$ up $(-512)$ or down $(+512)$ from the

current address.

You will, however, need to recognize when the Cow is at an edge of the display, so that you don$$t go

beyond the edge.