So the other day I found myself firing up an emulator to play some of the ’80s ZX Spectrum games that were a staple of my childhood. I ran through the classics—Manic Miner, Skool Daze, Rambo, Bruce Lee—but there’s always one that stands out: Jetpac. Masterpiece. You’re an 80’s astronaut, lazering aliens while scavenging fuel that falls from the sky, using it to refill your ship so it can take you to a another planet that looks the same… Where did that fuel come from? Why does Jetman teleport to the opposite edge when he hits the screen border? Mysteries yet unsolved. At age five, I was rubbish at Jetpac; decades later I still am. Unable to reach level 4, I am bested by a game that fits in 48 KB of RAM.

About once every five years I fire up Premier Manager 2 — THE soccer management game before Championship Manager monopolised the genre. (These days, you can even play PM2 in your browser.)

Before starting to play, this time I noticed an interesting mix of files in the root directory — thingslike contract.gnd, icons.vga, anim.bin, goal.voc, and others—and wondered how hard it would be to decode them with modern tooling. Surely the .gnd and .vga files were graphic assets, the .voc files audio, and so on. A little reverse‑engineering later, here’s what I found about the image formats…