In 1978, the 76477 Complex Sound Generation chip was foundational to creating the sound effects in many popular games, notably Space Invaders; it was also popular with hobbyists who could buy the chip at Radio Shack — it could do minor miracles, tweaking a white noise generator to produce everything from drums to explosions, using an integrated digital mixer to layer and sequence these sounds.
The thing that makes it all so amazing is how much the 76477 did with so little by way of hardware, built around the now-abandoned "Integration Injection Logic" system, a strange Rube Goldberg once heralded as a revolutionary new direction for integrated electronic design.
Ken Shirriff explanation of the 76477 is a great tour through paleocomputing history, and a tribute to the ingenuity of designers and programmers who figured out how to do rather a lot with very, very little.
Compared to TTL, I2L is also constructed "backwards". The transistors in I2L have multiple collectors, while the transistors in TTL have multiple emitters.3 It may seem strange to think of transistors with multiple collectors, but the diagram below shows how they are constructed. Each collector has an N region (brown) with a P region (green) below for the base, and another N region at the bottom, forming an NPN transistor. The multiple collector is built by creating multiple N regions. Note that the transistor's emitter is the grounded substrate. Also note that the injector PNP transistor is just a P region, reusing the emitter and base's N and P regions; this make the injector more compact than a "full" transistor.
Inside the 76477 Space Invaders sound effect chip: digital logic implemented with I2L [Ken Shirriff's blog]
