What Is ROM and How Does It Work?

Dusan Vasic

Updated: May 6, 2023

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When most people think of computer memory, they think of random access memory (RAM) or permanent storage units, such as hard disks or SSDs.

However, RAM stores data temporarily while a device is running. Storage disks are similar to ROM in that they don’t lose their data after the device they’re installed in is turned off. Still, neither of those explanations answers the question, “What is ROM?” Luckily, this article will cover everything you need to know about this memory type, its subvariants, and how it all works.

Read-Only Memory (ROM) Definition

If we want to define ROM, we should say it is a type of computer memory that can’t be modified by the user. It is used to store a device’s BIOS (basic input/output system) and other important system files, i.e., a computer’s firmware. ROM is non-volatile, meaning it does not need power to retain its data.

Almost all computers contain a certain amount of ROM. It is used in video games, optical storage, compact discs (CD) such as a CD-ROM, as well as devices such as calculators or laser printers, and many others.

Many people confuse ROM and RAM and often talk about them interchangeably, but they have several key differences. For one, ROM is non-volatile, meaning it does not need power to retain its contents. RAM is volatile, meaning it requires power to keep its data stored. As soon as its home device is turned off, all data stored in the RAM is lost. Additionally, RAM has more extensive memory capabilities.

How Does ROM Work?

A small, long-lasting battery inside the computer powers the ROM, which comprises two main parts: the OR logic gates and the decoder. In ROM, input is received by the decoder in a binary form, while the output is its decimal equivalent. The decimal result of the decoder serves as the input for the OR gates in ROM.

There is a grid of columns and rows in ROM chips that can be turned on or off. If they are turned on, the value is 1, and the lines are linked by a diode. However, the lines are not connected if the value is 0. One storage element in the memory chip is represented by each element of the array.

The diodes allow a one-direction flow with a particular threshold called forward break over. This determines the amount of current needed before the diode passes the flow on. The forward break-over voltage is typically 0.6 V in silicon-based hardware. 

ROM chips sometimes send a charge that goes above the forward break over to the column with a selected row that’s grounded to a particular cell. In cases when a diode is at the cell, the charge converts to the binary system, and the cell will be “on” at a value of 1.

Types of ROM

1. PROM stands for programmable read-only memory. This type of ROM is a form of digital memory whose bits – basic units of data whose value can be either 1 or 0 – are locked by an anti-fuse or a fuse.

A fuse begins with low resistance, and its job is to break the electrically conductive path when the current goes over a particular value. An anti-fuse works in the opposite direction  – it begins at high resistance and creates a conductive path in situations when the voltage goes over a specific value.

2. MROM is another type of read-only memory whose contents were programmed by the circuit manufacturer instead of the user. It got its name “masked read-only memory” because its chips are masked in the process of photolithography. When we talk about this type of ROM, we refer to a specific kind of ROM chip and not the entire memory category.

The current is sent from the ROM chip through input-output pathways. The locations of the fuses determine these pathways. The only way for the current to travel is through a fuse-enabled path, and the only way to go back is via the output chosen by the manufacturer. You cannot modify or rewire these ROM chips. 

3. EPROM or erasable programmable read-only memory is sometimes referred to as ROM that can be reprogrammed. You’ll need to shine an intense UV light through a window inside the chip to perform erasure. Ordinary lighting is not strong enough for erasure, but bright sunlight could do the trick. In most cases, you’ll be able to perform about 1,000 erasures before the chips wear out.

4. EEPROM is another modifiable type of ROM. However, it’s much more convenient than others, as you can rewrite it without special equipment or removing it from the machine. If you want to make one edit, there’s no need to change everything.

ROM Advantages and Disadvantages 

ROM makes communication between hardware components possible. Also, it doesn’t need any refreshing due to its static nature. The ROM’s contents can always be verified, and it’s less expensive than RAM. Moreover, it’s more reliable than RAM because it’s non-volatile and way easier to test.

In fact, the read-only nature of this memory is simultaneously its biggest advantage and disadvantage. ROM memory can only display data and can’t be changed, which means that, if you want to overwrite, modify, or delete something, that won’t be possible. However, this also means your data won’t go missing, which turns it one of the most significant advantages of this type of memory.

ROM Applications 

There’s a variety of ROM applications in all sorts of electronic devices. The most common applications include:

• Appliances such as washing machines, induction stoves, microwaves, smart TVs, and TV remotes
• Office equipment such as scanners, monitors, plotters, and fax machines
• Storing firmware
• Updating firmware

Conclusion

ROM is an essential part of most electronic devices we use daily, ranging from washing machines to scanners and fax machines. It’s a type of memory that stores data permanently and can be read but not modified or deleted. 

The types of ROM include PROM, MROM, EPROM, and EEPROM, each with its own specific characteristics. ROM has many advantages, such as the fact that it’s non-volatile and more reliable than RAM.