Firmware has evolved to mean the programmable content of a hardware device, which can consist of machine language instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device.

A common feature of firmware is that it can be updated post-manufacturing, either electronically, or by replacing a storage medium such as a socketed memory chip.

Firmware can — but is not required to — expose an externally accessible interface. For example, in some modem implementations the firmware is not directly accessible, but is part of a combination of hardware and firmware that responds to commands from the host system.

Firmware has traditionally been stored in ROM, however cost and performance requirements have driven component vendors to adopt various replacements, including non-volatile media such as EEPROM and Flash, or SRAM solutions, such as the firmware loaded by an operating system device driver, as described below.