Code:
PS4 PS4 Pro
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CPU: 8cores @ 1,6Ghz 8cores @ 2,13Ghz
GPU: 18CU @ 800Mhz 36CU @ 911Mhz
So what happens when you run a PS4 game on the PS4 Pro when the game do not have a PS4 Pro patch.
It runs the CPU @ 1,6Ghz the GPU @ 800Mhz, AND it blocks/restricts access to half of the CU's and only use 18CU's just like a real PS4.
In essence it "emulates*" the PS4 hardware.
My take/understanding on the BC modes based on what Sony has said and what we have seen and heard of PS4 games running on PS5.
The PS5 has 3 BC Modes (not all games can just run at higher CPU and GPU clocks because it can break game logic and such).
PS4 BC Mode
PS4 Pro BC Mode
PS5 Boost Mode
When you run a PS4 game on PS5 one of these modes will be chosen, (how I do not know for certain, but I'd suspect a whitelist of some sort "because**")
PS4 BC Mode:
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If the game chooses this BC Mode the CPU and the GPU will again (just like on PS4 Pro) run the CPU @ 1,6Ghz the GPU @ 800Mhz, AND it blocks/restricts access to half of the CU's and only use 18CU's just like a real PS4.
PS4 Pro BC Mode:
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If the game chooses this BC Mode the CPU and the GPU will now run the CPU @ 2,13Ghz the GPU @ 911Mhz, and will use ALL 36 CU's just like a PS4 Pro patched game would on an actual PS4 Pro.
PS5 Boost Mode:
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If the game chooses this BC Mode the CPU and the GPU will now probably have access to PS5's full CPU and GPU speeds and all CU's.
Whether or not it has full access to Zen2 and/or RNDA2 IPC improvements and uplifts, we can only speculate until confirmed by developers.
*
Whether or not you want to call it "emulation", it restricts the hardware by mimicking / behaving like the original hardware.
**
That "because" was for the fact that not all games seems to need a patch to run on the PS5 (in PS5 Boost Mode) and they just automatically run at the highest resolution and framerate the game was trying to target on PS4 and/or PS4 Pro, where as other games seem to need a patch or they have issues when running on the PS5.