Also known as: firmware
Firmware is low-level software loaded onto a device’s chip to control its hardware directly, sitting between the pure hardware and ordinary application programs.1
Overview
On a microcontroller there is no operating system, so the firmware is the whole program: it runs bare metal, owning the chip from the moment it powers on. Firmware is stored in non-volatile flash memory so it survives a power cycle, and it is changed by reflashing the chip rather than by installing software the usual way.
Where it fits
Almost every embedded device runs firmware of some kind — from a thermostat to a Wi-Fi radio. Updating it means writing a new image to flash, which is why “firmware update” and “reflashing” describe the same act. The tiny radios whose signals fill the airwaves GopherTrunk listens to are all driven by firmware of this sort.