Asteroids (video Game) - Technical Description

Technical Description

The Asteroids arcade machine is a vector game. This means that the game graphics are composed entirely of lines which are drawn on a vector monitor. The hardware consists primarily of a standard MOS 6502 CPU, which executes the game program, and the Digital Vector Generator (DVG), vector processing circuitry developed by Atari themselves. As the 6502 by itself was too slow to control both the game play and the vector hardware at the same time, the latter task was delegated to the DVG.

The original design concepts of the DVG came out of Atari's off-campus research lab in Grass Valley, California, in 1978. The prototype was given to engineer Howard Delman, who refined it, productized it, and then added additional features for Atari's first vector game, Lunar Lander. When it was decided that Asteroids would be a vector game as well, Delman modified a Lunar Lander circuit board for Ed Logg. More memory was added, as was the circuitry for the many sounds in the game. That original Asteroids prototype board still exists, and is currently in Delman's personal collection.

For each picture frame, the 6502 writes graphics commands for the DVG into a defined area of RAM (the vector RAM), and then asks the DVG to draw the corresponding vector image on the screen. The DVG reads the commands and generates appropriate signals for the vector monitor. There are DVG commands for positioning the cathode ray, for drawing a line to a specified destination, calling a subroutine with further commands, and so on.

Asteroids also features various sound effects, each of which is implemented by its own circuitry. There are seven distinct audio circuits, designed by Howard Delman. The CPU activates these audio circuits (and other hardware components) by writing to special memory addresses (memory mapped ports). The inputs from the player's controls (buttons) are also mapped into the CPU address space

The main Asteroids game program uses only 6 KB of ROM code. Another 2 KB of vector ROM contains the descriptions of the main graphical elements (rocks, saucer, player's ship, explosion pictures, letters, and digits) in the form of DVG commands.