Microcode is loaded into some on-die volatile RAM every boot by the CPU... there is a CPU instruction to do it. AFAIK: First the CPU loads an on die ROM of the

Microcode is loaded into some on-die volatile RAM every boot by the CPU... there is a CPU instruction to do it. AFAIK: First the CPU loads an on die ROM of the microcode it was released with on it's own. Second the BIOS/UEFI may load an update into the CPU before boot if it has a newer one. Third linux and some other OS will load any newer microcode into the CPU at the bootloader stage before the kernel loads. This is also commonly the case for other modern hardware, not just CPUs... although I expect it doesn't need to happen before the kernel loads and is probably bundled into their respective drivers.