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The Pac-Man Dossier
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The Pac-Man Dossier

February 23, 2009 Article Start Previous Page 8 of 10 Next

Chapter 5: Deciphering The Split Screen

"This is the way the world ends
This is the way the world ends
This is the way the world ends
Not with a bang but with a whimper."
—T. S. Eliot

On The Edge Of Forever

Pac-Man was always meant to be a game with no ending. The developers at Namco mistakenly assumed the game's increasing difficulty was sufficient to prevent anyone from playing indefinitely. Of course, within a few years of Pac-Man's release, players had discovered that every level beyond the 21st was identical.

Patterns were quickly created to exploit this fact and, for any player able to get past the first 20 levels, the game now became a test of endurance to see how many points you could rack up before losing focus and making a mistake. High scores soared into the millions and most players agreed the game simply went on forever.

Eventually, a few highly-skilled players were able to complete 255 consecutive levels of play (scoring over three million points and taking several hours to accomplish) and found a surprise waiting for them on level 256. It was a surprise no one knew about-not even the developers at Namco.

The 256th level displays the left half of the maze correctly, but the right half is a jumbled mess of randomly colored letters, numbers, and symbols. Notice the bonus counter in the lower-right of the screen is also malfunctioning.

The left side of the maze plays normally, but the right side is a different story. Although both the player and the ghosts can navigate through the right half of the screen, the original maze walls no longer apply.

Instead, Pac-Man must be guided through a confusing series of open areas, tunnels, one-way intersections, lone walls, and pass-throughs-all invisible to the player-while four ghosts are in hot pursuit.

Why does this broken level happen in the first place? The culprit is the routine responsible for drawing the bonus symbols along the bottom edge of the screen. Here's what happens: when level 256 is reached, the internal level counter is incremented to 255 (the level counter starts at zero - not one) and the routine for drawing the bonus symbols is called.

The routine loads the current level counter value (255) into a CPU register and increments that register by one. Unfortunately, 255 is the largest number that can fit in a single byte which is the size of the Z-80 CPU registers, so when the value is incremented the overflow is discarded leaving a zero in the register instead of the expected value of 256. This zero value leads the routine to believe this is an early level since its value is less than seven.

The routine starts drawing bonus symbols using the confused register as a counter. At the end of every drawing loop, the register is decreased by one and then checked to see if it is zero (the signal for the routine to stop drawing symbols). Since the register already has a zero in it to start, the first decrement will roll the value back to 255. It will keep decrementing the register and drawing symbols until the register is reduced to zero again, causing the loop to run a total of 256 times.

This means that memory locations outside the bounds of the bonus symbol table are drawn to the screen at increasing locations in video memory. This half-broken level was named the "split screen" by players; developers refer to it as a "kill screen".

Playing The Level

There are 114 dots on the left half of the screen, nine dots on the right, and one bonus key, totaling 6,310 points. When all of the dots have been cleared, nothing happens. The game does not consider a level to be completed until 244 dots have been eaten, so there is nothing left to do but sacrifice Pac-Man to a hungry ghost.

Interestingly, every time a life is lost, the nine dots on the right half of the screen get reset and can be eaten again, resulting in an additional 90 points per extra man. In the best-case scenario (five extra men), 6,760 points is the maximum score possible, but only 168 dots can be harvested-not enough to change levels-so we are stuck. There are no more dots to gobble or energizers to eat.

There is no final victory waiting for Pac-Man, only an empty half-maze full of ghosts. The game has an ending after all-just not a very happy or exciting one.

Four of the nine dots on the right half of the screen are invisible, but can be heard when eaten. The picture on the left shows all nine dot locations. Dots 1, 5, 6, and 9 are invisible; the rest can be seen but some are a different color than normal.

Anyone reaching this level quickly realized: to safely map out the right side of the screen something had to be done about the ghosts. After much tinkering, it was discovered that a ghost would get "trapped" on the right edge of the screen if he got too close to it.

Once trapped, a ghost can only move up or down but never right or left again. By leading ghosts near the edge of the screen, a skilled player could eventually get the ghosts out of the way and concentrate on exploring the right half of the maze and collecting the dots.

There are many methods for trapping the ghosts. One of the easiest ways to trap the three important ghosts is shown in the picture to the right. The yellow line shows Pac-Man's path from the start of the level to a spot near the bottom-right.

The exact instructions are as follows: begin by going right until you reach a blue letter 'N', then go down. Keep going down until you reach a blue letter 'F', then go right. Keep going right until you reach a yellow 'B', then go down again. When executed properly, Pac-Man will hit an invisible wall almost immediately after the last turn is made. Now we wait.

The red ghost will get stuck first. The pink ghost follows a few seconds later. The blue ghost will continue to move freely for several moments until the next scatter mode occurs. At that point, it will try to reach some location near the right edge of the screen and get stuck with the pink and red ghost instead.

Now the orange ghost is the only one still on the loose (bottom-right). Clyde is no real threat, however, since he runs to his corner whenever Pac-Man gets close (see Chapter 4), making it relatively easy to clean up all the dots. Be sure to take care around the lower-left corner of the maze-the orange ghost will have nowhere left to run to and will be much more aggressive.

Click on the YouTube video below to watch this ghost-trapping method in action:

Article Start Previous Page 8 of 10 Next

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