This is how Xbox Series S backwards compatibility really works

UPDATE: For the authoritative take on Microsoft's new flagship console, Digital Foundry's full Xbox Series X review is now live.

While Microsoft has been very forthcoming about how backwards compatibility is going to work with Xbox Series X, it hasn't been quite so clear in how the smaller, cheaper Series S console works in running games from prior Xbox generations. Last week, I had the chance to talk with the developers of the hardware and the news sounds very promising. Enhanced backwards compatibility features aren't just the preserve of the more expensive console - Series S has an interesting range of features too.

Kicking off with games running on the vintage 2001 Xbox - the 'OG' machine - we've confirmed that Xbox Series S will run these games at an enhanced resolution. There's a 3x boost to resolution on both axes, meaning that titles targeting 480p on the original machine will hit a maximum of 1440p on Series S, presumably with a range of performance benefits. The good news continues with the enhanced Xbox 360 titles that were released for Xbox One X. These games will also be enhanced for Series S, this time running with a 2x2 resolution multiplier, bringing titles that ran at native 720p up to 1440p. While this doesn't match the max 4K we saw on Xbox One X, there is the potential for improved performance elsewhere thanks to the much faster Zen 2 CPU architecture. In scenarios where Xbox One X was limited by its Jaguar CPU cores, Series S has much more headroom.

It's already been confirmed that the ways in which Series S and Series X handle Xbox One titles varies. Only the Series X will benefit from Xbox One X enhancements to existing games - which typically boils down to resolution boosts, higher quality textures and other graphics-driven effects. Xbox Series S brings its additional horsepower to bear in improving the experience of Xbox One S titles instead. This is more limiting in some respects (a game hard-coded to run at 900p will not run any higher on Series S, for example) but the new console benefits from increased resolutions in games that use dynamic resolution scaling, as well as improvements to texture filtering quality. Obviously, running games from solid state storage reduces loading times significantly, while the Auto HDR feature we've seen running on Series X also features on Series S - all games should present nicely on HDR screens, whether they natively support high dynamic range or not. It's a feature I personally can't wait to test. Finally, it goes without saying that CPU-limited titles should also deliver more stable performance at target frame-rates.

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However, there is one more feature that Microsoft has only mentioned in passing - and now we know more details, we're excited by it: the idea that select Xbox One titles will run at double the frame-rate on the new consoles, including Series S.

"We designed the Series S to enhance the Xbox One S games in a way that the Xbox One X can't do," system architect Andrew Goossen tells us. "We made it easy for existing Xbox One S games to be updated to run with double the frame-rate when played on Series S as well. When games are updated, existing games can query to determine whether they're running on the new console. And in terms of the performance, the Series S provides well over double the effective CPU and GPU performance over the Xbox One, making it pretty straightforward for the games to do this. And in fact, the Series S GPU runs the Xbox One S games with better performance than the Xbox One X."

The way Xbox One X handled non-enhanced Xbox One S titles was interesting - effectively users saw the enhanced GPU only running at half-rate, its compute units split between vertex and pixel processing. The new consoles are designed to run legacy Xbox One titles with the full power of both CPU and the new RDNA 2 GPUs.

"There's no real perf tuning necessary when you do this, and so often it's just as easy as changing three lines of code, and then the game works." Goossen adds. "Even when it's not that easy, the fixes are still pretty minor. We had one triple-A title where doubling the frame-rate really worked perfectly, except that the crowd animation was twice as fast as normal. And so, those sorts of fixes are typically very, very easy for developers to go fix. We're working with game developers and publishers to update [their titles]. It'll basically be select games that run at a doubled frame-rate on the Series S."

Some games will be able to see this functionality enabled by the developer themselves, while others may be collaborations between the game maker and Microsoft's compatibility team.

"In some cases, for more service-based games or games that are still have active communities, it'll actually be easier for the developer to do it," says Xbox director of program management, Jason Ronald. "And then in other cases, we may be able to do things at our level, on behalf of the title, similar to how we've done it in the past. These are all things that we're actively working through as we get closer to launch and we'll have more to share about specific enhancements in specific titles closer to launch."

So, as we understand it, existing Xbox titles can be enhanced for both Series S and Series X in different ways - the compatibility team can step in with its own specific type of magic, opening the door to running 30fps games at 60fps and 60fps titles at 120fps. Alternatively, developers now have the tools to see where their existing Xbox One games are running - and if it's a Series S or Series X machine, aspects like doubling the frame-rate become possible (and perhaps other features could be enabled too) - all without having to 'port' their games to the next-gen platform. The extent to which take-up will be there is of crucial importance, of course, but the potential is certainly very exciting and we're really looking forward to testing this out on both of the new Xbox machines.