The way people used to program games isn't much like how they program them now.

The original Game Boy, released in 1989, was a hugely popular handheld gaming device -- but it was tricky to program for. 

A new video showcases exactly how the device's processor worked, and how programming for the system required developers to understand that to make anything to appear on its screen. 

It's a fascinating look at one of the most successful handheld consoles ever, and it follows on from another video that takes a look inside its hardware.