Assuming we have an 8 Core A78 set-up, do we expect 1 core to be reserve for OS/networking and background tasks? Would retaining 4 A57s for OS and BC make sense? What's the best guess based on the nvidia hack?
It will not retain legacy cores, it would not constitute a considerable benefit and would mean squeezing the most out of it would incur addtional complexity and much higher power consumption. We KNOW it doesn't have "legacy cores", anyway, thanks to said leak.
It's almost certainly an 8 core, single cluster, of A78C. Not multithreaded, so it can't do the "Half a core to OS, half a core to game" thing.
We can't say for certain how much the OS and Applets will use, I would suspect 1 core, since 1 Drake core is so much more powerful than 1 TX1 core. That leaves 7 cores for games, which is comparable to other consoles even if it lacks multithreading.
For BC mode, now, I'm NOT a professional programmer, I can't write assembly, I don't study these things, but what I would ASSUME would be 3 cores used for the "game", like how a virtual machine can be assigned specific cores and memory limits, would with the other cores doing misc. tasks to help with the translation/virtualisation. 7 cores of A78C should not have ANY problems virtualising an environment where software compiled for just 3 A57 cores can run.
While it isn't quite as simple as "it's just ARM at the end of the day"; it's also... just ARM at the end of the day, and newer ARM cores are broadly compatible, almost entirely, with older ARM cores.