[one_even_prime]

Apple chips share the same physical memory between the GPU and the CPU. Still, they don't have USM/UVM (Unified Shared Memory/Unified Virtual Memory), that is, the GPU and the CPU can't access the same data concurrently and easily. Programs must map/unmap pages to control which device accesses it, and that's a very expensive operation.

[tubs]

They don't need to be unmapped just for the other one to use it. source: I wrote GPU drivers for over 10 years.