Step 6: Design Your Game
Now we're getting to the fun part- game design. Granted, the elements you are going to be given are very limited, but that's where creativity comes into the picture. There are six built-in elements that will be used to create everything on your screen:
2.) Sprite 0
3.) Sprite 1
4.) Missile 0
5.) Missile 1
Advanced programming will allow you to exploit the characteristics of each of these elements. For example, the flashing ghosts on PacMan allow the same sprites to be repeated and show up in multiple places on the screen, creating more characters. This will be explained in far better detail in the tutorials, but here are the basics.
Playfield- stored in 3 registers, with 4,8, and 8 pixels available in each. This will draw half the screen from left to right, then the screen can be repeated or mirrored. You can also re-write to the registers after drawing the first half to create a screen that is different all the way across. Basically you have 40 pixels across the screen. These could be changed on each scanline, but are normally coded in blocks of about 8 scanlines to save memory. The playfield in my exampled is mirrored, meaning the right half of the screens reflects, not repeats, the left half.
Sprites- For ease of explanation sprites are limited to 8 pixels wide. There are techniques of combining two sprites to act like one and even repeat them horizontally to get up to 48 (I believe) pixels across. For a beginner, just getting a simple sprite on the screen will be quite a task. The hard limit to sprite height is the entire screen, up to 192 pixels in NTSC systems. However, you have to keep in mind a character that takes up the entire screen is hard to design a game around. This is where design and creativity in keeping sprites smaller while still conveying the idea comes across. For reference the Robot design is 8x22 pixels shown here, while PacMan is 8x8. Sometimes you have to decide between size and more detail.
Missiles- There are also two built in missiles. These are normally used as straight lines or dots. This is what is shot in the game combat. These (I believe offhand) are what would be used as lasers in games like Space Invaders or Vanguard. They can also serve as design elements (the rails in Pole Position). You are only limited by your imagination, and of course the severe limitations of the 2600. The missile colors are often matched to the sprite of the same number.
Ball- Really, the ball is just like the missiles are far as I can tell. The only real difference I can tell is that the ball matches the color of the playfield. Designers of the game Adventure exploited this to actually make the ball the main character, which is why it shows up as a block instead of with more detail.