Microcode
termnetworkingprogramming
A layer inside a processor that implements its published instruction set in terms of simpler internal operations.
It is the reason a processor's behaviour can change without the silicon changing. An instruction a program issues is not necessarily executed directly by hardware; on many processors it is decoded into a sequence of internal steps the chip's designers can revise after manufacture. That became widely visible when speculative-execution vulnerabilities were mitigated partly by microcode updates - patches that alter what a processor does, shipped through firmware, for a defect in something most people think of as fixed at the foundry.
Also known as: ucode, micro-code