Read-Only Memory (ROM), is a type of electronic storage that comes built in to a device during manufacturing. You’ll find ROM chips in computers and many other types of electronic products; VCRs, game consoles, and car radios all use ROM to complete their functions smoothly. ROM chips come built into an external unit – like flash drives and other auxiliary memory devices – or installed into the device’s hardware on a removable chip. Non-volatile memory like ROM remains viable even without a power supply.

What Does ROM Do?

ROM stores “firmware,” which is permanent software that’s hard-wired with integrated circuits. The manufacturer fits a specifically designed ROM chip into a device during the building process.

Unlike the non-volatile memory of a hard drive, it’s difficult and time-consuming to rewrite a ROM chip’s memory. In most cases, it’s impossible to alter integrated circuits, which may mean you can’t update more basic forms of ROM. You may be able to reprogram some modern types of ROM, but only at slow speeds and with special equipment. These hurdles make traditional ROM poorly-suited for functions that require frequent feature updates or are prone to bugs or security issues.

Conventional ROM is pre-programmed to suit its intended hardware. It often controls functions that are both integral to the utility of the device and unlikely to need modification or personalization. One kind of data usually stored in a ROM chip is the Basic Input-Output System (BIOS). A device’s BIOS controls the way the device responds to cues from the user at the most fundamental level, such as a computer’s “booting-up” process that occurs when the system powers on.

Types of ROM

Here’s an overview of the different types of ROM, ranging from the most basic to more versatile.

ROM: Classic or “mask-programmed” ROM chips contain integrated circuits. A ROM chip sends a current through a specific input-output pathway determined by the location of fuses among the rows and columns on the chip. The current can only travel along a fuse-enabled pathway and thus can only return via the output the manufacturer chooses. Rewiring is functionally impossible, and so there’s no way to modify these types of ROM chips. While producing a template for an original ROM chip is laborious, chips made according to an existing template can be much more affordable.

PROM: Programmable ROM, or PROM, is essentially a blank version of ROM that you can purchase and program once with the help of a special tool called a programmer. A blank PROM chip allows current to run through all possible pathways; the programmer chooses a pathway for the current by sending a high voltage through the unwanted fuses to “burn” them out. Static electricity can create the same effect by accident, so PROMs are more vulnerable to damage than conventional ROMs.

EPROM: Erasable Programmable ROM chips allow you to write and rewrite them many times. These chips feature a quartz window through which a specialized EPROM programmer emits a specific frequency of ultraviolet light. This light burns out all the tiny charges in the EPROM to reopen its circuits. This exposure effectively renders the chip blank again, after which you can reprogram it according to the same process as a PROM. EPROM chips will eventually wear out, but they frequently have lifetimes of over 1000 erasures.

 –EEPROM: To modify an Electrically Erasable Programmable ROM chip, apply localized electrical fields to erase and rewrite the data. EEPROMs have several advantages over other types of ROM. Unlike the earlier forms, you can rewrite EEPROM without dedicated equipment, without removing it from the hardware, and in specifically designated increments. You don’t have to erase and rewrite everything to make a single edit.