Think back to the last time your hard drive crashed (we hope it was a long time ago). Chances are that you were acutely stressed and panicked because you couldn’t properly load your operating system, which is stored on your hard drive. But you might have been able to run the basic programs you needed to boot up your computer, reinstall your operating system, recover your files, and keep your cool. All of those critical functions were made possible by ROM storage.

In this article, we’ll answer all your questions about ROM: what ROM is, why it’s important, the types of ROM, and what makes ROM different than RAM.

What is ROM?

ROM stands for read-only memory. It’s a type of storage that can permanently store data on computers, devices, and chips.

There are two main characteristics of ROM. First, ROM is non-volatile. This means the data remains stored in ROM even when it’s not powered. The second characteristic is that data stored in ROM cannot be electronically modified once the ROM device is manufactured and the initial data is programmed into the device. And while there are ROM devices today that can be reprogrammed, this can only be done with special equipment, at slow speeds, and usually only a certain number of times.

Why is ROM important?

ROM is a permanent and unchangeable form of data storage. While this may sound quite limiting, ROM has distinct advantages for certain computer science and hardware applications.

When you turn on your computer or phone, the device needs a few essential instructions just to start up correctly. These instructions are stored on ROM chips that contain essential information for the basic input/output system (BIOS), the boot-up process, reading and writing to peripheral devices, and software for basic utilities. All of this is stored in ROM because it’s such a secure and reliable form of storage.

Another application of ROM is for electronic devices that perform specific functions. These devices contain firmware or pre-written operating instructions and configurations stored in ROM. Examples of devices that contain firmware include digital watches, pocket calculators, cameras, keyboards, computer monitors, and modems.

What are the different types of ROM?

Since the widespread use of ROM in the 1960s, there have been several improvements to the basic ROM chip. Today, there are four main types of ROM: Mask ROM, PROM, EPROM, and EEPROM.

Mask ROM

The earliest and most basic form of ROM contains a series of integrated circuits. A mask ROM chip sends current via specific input-output pathways determined by fuses on the chip. Current can only travel via fuse-enabled pathways that are wired by the manufacturer. It’s practically impossible to rewire mask ROM chips, which means there’s no way to modify or rewrite data on the chip.

Mask ROM chips are designed to be mass-produced. While designing and producing the first template takes a lot of time and money, creating identical ROM chips from an existing template is much cheaper.

PROM

A programmable ROM (PROM) chip starts as a blank version of a classic ROM chip — in other words, every possible pathway is open. A programmer then chooses the input-output pathways for the PROM chip based on the data to be stored. Any unwanted fuses are then burnt out with high voltage. Note that it’s only possible to destroy fuses on a PROM once, which means that PROM chips can only be programmed once.

Since PROM chips don’t need to be programmed at a manufacturing facility, they’re also known as field-programmable ROM (FPROM). But while PROM chips are more convenient and flexible than ROM chips, they’re also more vulnerable to damage. That’s because a static electricity discharge can accidentally burn out fuses on the PROM chip and corrupt the data.

EPROM

Erasable programmable ROM (EPROM) chips offer a limited way to rewrite data on the chip. EPROM chips are initially programmed in the same way as PROM chips. But these chips are also built with a quartz window through which a special tool can emit ultraviolet (UV) light. The UV light exposure essentially reopens every circuit, and the chip can be reprogrammed.

EPROM chips offer a lot more versatility than PROM or mask ROM chips, but it comes at a cost. First, UV exposure slowly degrades the chip, and most EPROM chips have lifetimes of about 1,000 erasures. Second, the risk of accidental UV exposure from sunlight and fluorescent light makes EPROM chips vulnerable to accidental erasure if they’re not properly covered.

EEPROM

The most advanced form of ROM is electrically erasable programmable ROM (EEPROM). Data on EEPROM chips can be erased and rewritten by applying local electric fields to the chip. This makes it possible to selectively edit the data rather than needing to erase and rewrite everything on the chip, as with EPROM. And unlike earlier types of ROM, you can rewrite data on an EEPROM chip without special equipment.

EEPROM chips also have limited rewrite cycles, though modern lifetimes are now over a million erasures. You can find EEPROM chips in computers as well as smart cards and keyless entry systems. And descending from EEPROM is flash memory, which you can find in smartphones, cameras, and USB drives everywhere.

ROM vs. RAM

If you’re learning about hardware components, then you’ve likely come across the term RAM as well as ROM. While both have to do with memory storage, random-access memory (RAM) works differently than ROM. RAM is a form of volatile storage, which means that RAM chips must be powered to store data. Once the power is cut off, the data disappears. This is why RAM is described as “short-term memory.”

Compared to RAM, data stored in ROM can take longer to access. That’s why ROM data is often copied to RAM, where it can be quickly accessed when it’s needed. When you shut down your computer or device, everything in RAM is erased while the ROM data remains safe.

Understanding ROM is important for developers

While most people are used to working with rewritable forms of solid-state storage, the truth is that ROM is still critically supporting us behind the scenes to make sure our devices still work when things go wrong. And if you’re ready to begin a career in computer science, IoT, or even game development, ROM will play a central role in how you develop new software and products.

So, what else should you know about ROM? We recommend taking an online programming course to keep your skills sharp. And if you’re not sure what to learn next, our developer career paths will help guide you in building the right skills you need to succeed in your new career.