CPU register
The CPU's fastest storage — 16–32 named positions directly in the CPU core. Word-sized (usually 64 bits). Everything the CPU processes passes through registers.
x86_64: 16 general-purpose (RAX-R15) + 16 SSE/AVX (XMM/YMM/ZMM) + special (RIP, RFLAGS). ARM64: 31 GP registers + SP + 32 SIMD. RISC-V: 32 GP. Modern CPUs have internally 100+ "physical registers" that are mapped dynamically (register renaming). Compiler register allocation (Chaitin's algorithm) decides which variables end up in registers. Spilling = when a variable has to go to the stack due to register pressure.