The Atari 2600 is able to output 4-bit sampled audio by setting one of the 2 audio channels to have a constant-value waveform, and having the CPU update the volume for every sample. This game spends a little over half of its time generating samples programmatically to emulate 4 square wave channels. Because there are no interrupts, the entirety of the gameplay code has to be interleaved with calls to update the audio at regular intervals.