How does the machine know what the ships look like. There were no dedicated rom chips in the machine. It had a diode matrix that operates as it’s rom. If you look at the schematic of th player memory boards you can see the rom images. (Link at bottom of post to get pdf of user manual.) Above the 74150 chips, which are used to access the memory, you can see the bitmap of the ship. It’s a 16 bit high by 12 bit wide picture. There are 9 seperate images, which is over 800 bytes of memory stored by diodes.
The 74151 chip is a decoder, or demultiplexor. It takes the 4 lines of input and enables one of it’s 16 output lines, this enables the row of bytes we are interested in. Enabling the output, means that we place 5 volts on that line. The 74150 is a multiplexor. The 4 control lines pick which of the inputs to choose. Such that if the control lines were set to 4, the output would be whatever was at input number 4. This allows us to choose which bit to look at. The 74151 choose which line, or you could say which byte, and the 74150 chooses the individual bit from that byte. The 74151 provides 5 volts, however if the line selected by the 74150 does not have a diode, there will not be 5 volts on it’s output.
Unlike a modern machine, because of system constraints, we only use one bit at a time. This is because there is no ram, to hold th entire picture that will be displayed all at once. Later we will look closer at th video display process. 10 points for anyone that knows why there are 2 diodes on the top of each picture. That is a little bit of a cheat. If you follow the lines in the full pdf, you will see it goes to the thrust circuitry. These diodes put dots under the ship when thrust is being pressed. This way they didn’t have to make another set of images with thrust shown.
Why doesn’t it make a big line all the way down the screen. That’s because of the logic gates, that only enable those two diodes on the proper lines. But they also pulse the thrust, to give it the impression of movement. That’s what the 4 inverters are doing. The capacitor accross the second one turns them into a waveform that is generated only when thrust is applied. The movement illusion, is because the two pixels alternate, while the ship is repositioned. A pretty neat trick. The schematics are in pdf form at the killer list of video games inside the owners manual.