LOS ANGELES—While Microsoft's pre-E3 press conference focused largely on newer video games, the event also filled in a pretty major gap for hardware-upgrading holdouts: backward compatibility. Starting later this year, the company's newest console, the Xbox One, will support a limited number of older Xbox 360 games—and Xbox One preview program users will get a shot even sooner than that.

Gamers will have two ways of playing old games that are part of the backward-compatible initiative. If users already purchased the games digitally through Xbox Live, they can simply log in and re-download the game on Xbox One without paying any additional cost. If they own the game as a disc, they'll have to download the game to their Xbox One hard drive, and the system will then check for the disc before launching the game.

Technical details on how this works are still unknown. The hardware of the Xbox 360 is very different from the hardware of the Xbox One, and pure emulation of the kind used in console emulators such as MESS and arcade emulators like MAME is technically improbable; Xbox 360 is simply too fast and too new. The limited compatibility and need to download even those games that are owned on disc suggests to us that some mix of recompilation and emulation is in use.

We also don't know if there will be differences in framerate or resolution. At least one Xbox One feature will be made available to 360 games: using Kinect voice commands, such as taking a screenshot while playing old games.

The list of games in the first wave can be found here. Much like the Xbox 360's limited support for the first Xbox's games, more 360 games will be added to the backward compatibility list over time—and there's no guarantee that a favorite 360 game will ever be brought forward to work on Xbox One. Nonetheless, Microsoft promises over 100 titles to start, with hundreds more coming in the future.

This announcement follows the January 2015 launch of Sony's PlayStation Now service, which offers a far more limited form of backward compatibility—namely, players who already own a game either digitally or on disc must still pay to access a PlayStation 3 game on PlayStation 4, and that session is powered by PlayStation Now's cloud service as opposed to working locally on a user's PS4 system.

This is a breaking report, and we will actively update it with new information as it becomes available.