The firmware is the general concept of the BIOS. Popularly, the firmware of IBM PC compatible computers or other electronic systems such as arcade machines, calculators and routers are called BIOS.

In electronic systems and computing, firmware is "the combination of a hardware device, e.g. an integrated circuit, and computer instructions and data that reside as read only software on that device". As a result, firmware usually cannot be modified during normal operation of the device. Typical examples of devices containing firmware are embedded systems (such as traffic lights, consumer appliances, and digital watches), computers, computer peripherals, mobile phones, and digital cameras. The firmware contained in these devices provides the control program for the device.

Firmware is held in non-volatile memory devices such as ROM, EPROM, or flash memory. Changing the firmware of a device may rarely or never be done during its economic lifetime; some firmware memory devices are permanently installed and cannot be changed after manufacture. Common reasons for updating firmware include fixing bugs or adding features to the device. This may require ROM integrated circuits to be physically replaced, or flash memory to be reprogrammed through a special procedure. Firmware such as the ROM BIOS of a personal computer may contain only elementary basic functions of a device and may only provide services to higher-level software. Firmware such as the program of an embedded system may be the only program that will run on the system and provide all of its functions.