Many people who play Minecraft limit themselves to using its building blocks more or less for their intended purpose, whether it's by building simple houses or creating sprawling, unforgettable underwater wonderlands. Yet YouTuber SethBling has repeatedly found them well-suited to higher callings. For his latest project, he used command blocks to cobble together a functional interpreter for the BASIC programming language that works within the confines of vanilla Minecraft's existing ruleset.
It's not exactly fast, and it slows down with continued use. SethBling partly attributes the sluggishness to Minecraft's 20 Hz refresh clock, which limits the command blocks to operating 20 times per second. But it certainly works, as he demonstrates by running a program that prints out prime numbers on a gigantic whiteboard with the help of an onscreen keyboard. Later, he runs a script that allows a Minecraft "turtle" to mine a tunnel without his supervision (and without mods or add-ons, for that matter). He never shows it, but the turtle reportedly can place blocks as well.
Minecraft players have been getting "computers" to work in the game for a while, but such efforts usually focus on the OpenComputer mod and the Lua programming language. Last November, in fact, we showed how one player had managed to control lights in the real world with an in-game switch. One major exception, though, was Laurens Weyn's massive "Commodore 32" computer from 2014, which uses command blocks much like SethBlind's creation.
In SethBling's case, programming the many, many command blocks involved took him around two weeks, and he wrote the code for each block by hand. If you're interested in trying it out for yourself, though, you won't have to put in that kind of time as the whole package is available for download via SethBling's website.