The bolded *is* certain if BC's being done in hardware. Zen 2+RDNA2 is *not* 1:1 to Jaguar+GCN. It's a more significant difference than the differences between base PS4 and Pro, obviously, where Boost Mode *already* has issues with some games, and since the difference is on an architectural level there's no way to turn this inherent "boost" off.
That's what Cerny was referring to when he said games have to deal with a faster machine. It's not just clock speeds, you can't suddenly turn the entire structure of a PS5 into a PS4 unless you have the actual chip in there (which is why Cerney talked about the PS3 BC before that slide as a comparison).
I agree that there are changes besides the clockspeed. However, regarding the architectural differences Mr. Cerny had this to say:
Mark Cerny said:
Achieving this unification of functionality took years of efforts by AMD, as any roadmap advancement creates a potential divergence in logic.
That is obviously claiming that very substantial work went into ensuring legacy instruction support no matter what new features were also added. It would be odd if the result of this incredible effort was that...almost no games work.
And again, the exact same points apply to PS4 Pro as well: enhanced microarchitecture and higher clocks than its older sibling. And Sony's response was to make the clock boost optional, even though only a very small portion of titles are affected. I therefore am skeptical that in a situation where the clocks are enormously more different, and the end result is nearly complete failure, that they'd make the clock boost mandatory.
"we are expecting almost all of them to be playable at launch" is also a direct quote from Cerny. He didn't say "in boost mode", he just outright made that statement. Pretty odd if he meant "in boost mode";
When he said that, two sentences earlier he'd just mentioned the clock boost as the confounding factor that necessitates game-by-game testing. How would it be odd to mean playable with boosting?
Worth noting that even if you limited the PS5's CPU to the same clockspeeds as the PS4, the performance would still be significantly faster.
For the legacy modes, it is very likely they
do limit the PS5 GPU to PS4 or PS4 Pro speed. That's shown by the GitHub leak, and it matches to their Pro approach. Yes, there's still a discrepancy in performance. But as I showed above, years of work was applied to making this functional anyway.
