Originally, firmware had read-only memory (ROM) and programmable read-only memory (PROM). It was designed to be permanent. Eventually PROM chips could be updated and were called erasable programmable read-only memory (EPROM). But EPROM was expensive, time consuming to update and challenging to use. Firmware eventually evolved from ROM to flash memory firmware; thus, it became easier to update and user friendly.

There are levels of firmware:

1. Low Level Firmware: This is found in ROM, OTP/PROM and PLA structures. Low level firmware is often read-only memory and cannot be changed or updated. It is sometimes referred to as hardware.
2. High Level Firmware: This is used in flash memory for updates that is often considered as software.
3. Subsystems: These have their own fixed microcode embedded in flash chips, CPUs and LCD units. A subsystem is usually considered part of the hardware device as well as high level firmware.

BIOS, modems and video cards are usually easy to update. But firmware in storage devices usually gets overlooked; there are no standardized systems for updating firmware. Fortunately, storage devices do not need to be updated often.