This article is part of a series exploring the reverse engineered inner workings of Downland, a game for the Tandy Color Computer, released in 1983, written by Michael Aichlmayr.
Downland has a very barren ecosystem. Other than the unrelenting acid drops of death there are two other beings roaming and lurking in the depths of the game’s chambers: the ball and the bird.
The Ball Boulder
In Downland the main active enemy you face other than the drops is what I call the ball, even though it’s probably a boulder. You know, a boulder that squashes and stretches when it bounces across the screen.
I can only guess at its inspiration. Was it inspired by Indiana Jones rolling boulder scene? I thought maybe it was inspired from the bouncing springs in Donkey Kong’s elevator level, but an email from Mr. Aichlmayr’s disproves this.
In any case, in the reverse engineered assembly code and the C version I call it the ball. It’s the name that came to most naturally to me. I told myself I can name it anything I wanted as long as it’s consistent. I could’ve named it Jennifer.
Every Day The Same Again
The first thing that struck me about the ball behavior is that no matter what room it appears in, it always spawns in the exact same spot. It spawns at X=101 and Y=116 in logical game space, appearing at X=202 in screen space. I never realized this playing the game, even after all these years. Props to Mr. Aichlmayr for making the chambers’ geometry varied enough to mask that fact.
Do The Locomotion
The ball’s position uses 16 bit fixed point values. Eight bits are for the integer part and the other eight bits are the fractional part. The ball moves along the X axis 88 fractional units to the left. In comparison, the player moves 56 fractional units. The player is constantly getting mowed down by the ball. And even if the player manages to dodge it, its repeating nature just means they’ll only have a few seconds before it comes back.
When the ball is falling its speed is increased by 18 fractional units every frame with a maximum speed of 256. When it hits the ground, it sticks to it for 4 frames (showing the squished animation frame), then bounces back up with a speed of -256 and a gravity of 10. When bouncing up, it counts 10 frames and then sets the vertical speed to 0, letting the falling gravity take effect again. This gives the effect of the ball falling faster than going up, squishing hard on the ground.
When part of the ball overlaps a blue pixel on the background, it restarts. Here I changed the rope color to blue.
When it collides with the terrain, the ball resets its XY position, but doesn’t reset its vertical and horizontal speeds. This has the effect of changing the ball’s trajectory as it can be in the middle of jumping up or falling down. This makes it so the player can sometimes find temporary safe spots in certain chambers, like here on the bottom stairs in chamber two:
On every other cycle, the ball bounces clear over the player, providing a brief respite.
Room With a View Ball
In theory the ball can be used in any chamber. The only thing that limits this is a table that specifies which chamber it appears in. The table contains these values:
roomsWithBouncingBall[10] = 0, 2, 5, 6, 10, 11, 12, 13, 14, 0xff
When the chamber initializes, it goes through the values of the table to find the current chamber number. If the current chamber’s number is found, then the ball is spawned. If it’s not, it reaches 0xff and skips the ball initialization.
Cut Content?
The most fascinating thing about the table above is the set of values 10, 11, 12, 13, 14. As the values are chamber numbers, could this be evidence of cut chambers? Did at one time the game contain five more chambers? Or are these just place holder values? We’ll likely never know. As far as I can tell, this is the only existence of unused data I’ve found in the whole ROM. It’s fun to think about what they could’ve looked like if they had existed.
It’s a Bird, It’s a Bat. It’s Bat Bird.
Drops are random but easily avoidable. The ball is hard to dodge but predictable. When the chamber’s timer hits zero, however, all hell breaks loose. The bouncy zigzagging winged terror of doom is unleashed. Don’t let its simple bird-like appearance fool you. If you see it and you’re not anywhere near a door, consider yourself dead in advance.
Like the ball, no matter which chamber, the bird always spawns at the same spot. It starts at X=35 and Y=26. It has a base X and Y speed of 256 fractional units and then a random speed between 0 and 255 is added on each axis. That means it’ll always spawn going down and to the right but at different angles.
Unlike the ball, the bird doesn’t try to detect the chamber’s terrain. It just flies straight in a direction until it hits an invisible wall and then reflects off. The walls are defined by a bounding box with screen coordinates (14, 16) for the top left and (240, 177) for the bottom right.
It continues to bounce around until you change rooms or die. If you die for any reason, the bird disappears and the timer is reset to 2048, giving you a little more time to escape.
Next Time
With all these hazards going around, I haven’t yet explained how the game detects collisions between them and the player. I hope to cover that in the next installment.
Leave a Reply