When it was first revealed that the PS4 and Xbox One were both embracing x86 architecture, the implications were clear to in-the-know PC gamers: Sticking to the same architecture with subsequent consoles (like the One X and PS4 Pro) should make backward compatibility a breeze. We're seeing this again with the substantial compatibility commitments for the Series X and the PS5.
In the case of the latter, there's some specialized work going into the processor that sounds like it'll act as a fallback in case old PS4 games can't take advantage of the PS5's vastly improved hardware. Speaking with Digital Foundry's Richard Leadbetter, Sony's Lead System Architect Mark Cerny touches on why the PS5's backward compatibility solution for PS4 titles includes "legacy modes" at all when the PS4, PS4 Pro, and PS5 all run on x86 architecture.
Legacy modes will offer the PS5 more options when it comes to running older games in a satisfactory manner. The PS4 and PS4 Pro both use Jaguar chips (as do all Xbox One models), while the PS5 will use a version of AMD's new Zen 2 microarchitecture chips (as will Series X). All those AMD chips use the same x86 instruction set, but the timings of how those instructions are executed can be "substantially different," says Cerny, hence the legacy modes:
We worked [with] AMD to customize our particular Zen 2 cores; they have modes in which they can more closely approximate Jaguar timing. We're keeping that in our back pocket, so to speak, as we proceed with the backwards compatibility work.
The reasoning behind this is a bit technical, but it boils down to this: Just because the PS5 has more power doesn't mean the result of running PS4 titles as-is will always be higher frame rates and smoother gameplay. Especially for titles that have been designed to eke performance out of the PS4 and PS4 Pro's particular architectures, a difference in timing could lead to incompatibility or undesirable differences in performance and speed. Presumably, as Leadbetter explains further in Digital Foundry's post-interview analysis video, if the PS5's power isn't a good fit for a title, legacy modes should ensure nearly identical performance that a PS4 or PS4 Pro would offer.
This could be why Cerny's initial messaging about the PS5's compatibility with PS4 titles had to be clarified later by Sony. Cerny mentioned that the top 100 PS4 games had been specifically targeted for backward compatibility; Sony then added it believes that the "overwhelming majority of the 4,000+ PS4 titles" will work.
"We're expecting backward compatible titles will run at a boosted frequency on PS5 so that they can benefit from higher or more stable frame rates and potentially higher resolutions," Sony's statement reads. As Leadbetter's talk with Cerny confirms, having certain PS4 titles run better on PS5 may require some additional work to get things running just right. If Sony or a game's developers can't do that work, the legacy modes still exist. If those are as close to the original Jaguar timings as Cerny claims, then in theory only the handful of titles that don't receive PS5 compatibility work and that don't agree with the legacy mode timings should be incompatible or compromised.
For more on the PS5's technical specifications, watch Leadbetter's video or head over to his full writeup at Eurogamer.