Microsoft’s big surprise at E3 today was backwards compatibility for some Xbox 360 games running on the Xbox One. The software company is planning to offer more than 100 titles at launch later this year, and Xbox One preview members can test a selection today. Behind the scenes, Microsoft has built a full Xbox 360 emulator for its Xbox One console. "Xbox One Backward Compatibility is an Xbox 360 emulator that runs on Xbox One and is used to play Xbox 360 games," says a Microsoft spokesperson in a statement to The Verge.

Microsoft promises there will be hundreds of Xbox 360 games for Xbox One

When you launch an Xbox 360 game on the Xbox One the software will startup a virtual version of the Xbox 360 console to play the game. If you have a disc that’s on the supported list then it will simply download the game and ask you to keep the disc in the tray for validation purposes. If you have digital copies of Xbox 360 games then they’ll just automatically show up. While there’s only a small selection for preview right now, Microsoft is promising to add a lot more. "We’ll have over 100 titles this holiday, with hundreds more added in the months to come," says a Microsoft spokesperson.

There are currently more than 1,000 Xbox 360 games available, so it’s possible if Microsoft sticks to its "hundreds" plan that the company will cover most popular titles. Games won’t show up automatically though, and publishers have to agree to having titles listed and emulated. "We have to do packaging and validation work on each title to make it available through Xbox One backward compatibility," explains a Microsoft spokesperson. This won’t be the first time Microsoft has emulated Xbox games. Microsoft built an emulator for the Xbox 360 back in 2005 to allow it to run some original Xbox games. Originally it was a small list that started with titles like Halo, but the company expanded it over the course of two years to support more than 450 games.