Metal and Apple Silicon
Metal is Apple's graphics-and-compute API, and on Apple silicon it sits on top of a hardware fact that has no real CUDA equivalent: the CPU and GPU are not two devices connected by a bus, they are two sets of cores reading the same physical memory. That single fact changes what "offload to the GPU" even means on this hardware, and it's the reason this page exists separately from the rest of the portability folder rather than being folded into a general graphics-API page alongside Vulkan and DirectX Compute.
Unified Memory
Every allocation covered so far draws a hard line between host and device memory: a pointer is valid on one side or the other, and moving data across the line is an explicit cudaMemcpy the programmer writes and pays for. Unified Memory erases that line for the source code — one pointer, usable from both host and device — while the underlying hardware and driver still have to physically move bytes between two separate memory systems whenever the data is touched from the "wrong" side. Understanding when that migration happens, and how to steer it, is the difference between Unified Memory being a convenience and Unified Memory being a performance trap.