A peripheral is a piece of digital logic sitting on the same bus as your RAM. It has no API, no calling convention and no way to be invoked. The only interface it exposes is a small block of addresses: write a word to one of them and some flip-flops change state; read from another and you get the current state of some wires. That is the whole model.
The Register File and Occupancy covers how a kernel's register usage feeds directly into the occupancy calculation. This page is the other side of that same fact: what registers actually hold, what forces a value out of the register file and into memory instead, and how to tell when that's happened to your kernel.
Occupancy is defined in the glossary as the ratio of resident warps to the maximum an SM supports, and Latency, Throughput, and Latency Hiding explains why that ratio matters — resident warps are what supply the concurrent memory requests Little's Law demands. This page is about the other half: occupancy is not a tunable dial, it's the output of a fixed calculation against fixed hardware limits, and the register file is usually the tightest of those limits.
A Cortex-M shows you sixteen 32-bit registers at any moment, and thirteen of them are genuinely interchangeable scratch space. The other three are the processor's control surface — a stack pointer that is secretly one of two, a link register that sometimes holds a return address and sometimes holds a magic number, and a program counter that is one bit weirder than it looks. Alongside those sit half a dozen special registers that are not in the main file at all: you cannot load or store them, and the only way to touch them is a pair of dedicated instructions.
Overview