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What Is Read-Only Memory (ROM)? Types and Examples

05th Jun, 2024
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    What Is Read-Only Memory (ROM)? Types and Examples

    Computers are now broadly considered to be a necessity for everyone in today's digitally transformed landscape. With years of continuous evolution, computers have brought us to this new age where literally everything is at our fingertips. Therefore, knowing about them is essential for everyone. 

    In this article, we will discuss what is a read-only memory (ROM), different types of ROM, what is the use of ROM, the need for ROM in a computer, and the difference between ROM and RAM. Learn more about memory types while working with cross-platform document-oriented databases that store large scale of data in our Data Science and Machine Learning course.  

    What is Read-Only Memory (ROM)?

    ROM refers to a computer memory chip that retains permanent or semi-permanent information. Simply put, it is a non-volatile storage device or computer memory accessory that keeps the contents or data saved even after your computer has been turned off. Along with the primary memory device, RAM (Random Access Memory), ROM (Read Only Memory)—also popularly known as the secondary memory device—is another essential component of a computer.

    ROMs are configured with programs by the chip makers during manufacturing. Its data cannot be altered later by programming, rewriting, or erasing. However, there are different types of ROM that enable you to modify their contents. We will also explain the types of ROM in the following section of the article. 

    The Block Diagram of ROM

    block diagram of ROM
    Source: eeeguide.com 

    As shown in the above diagram, there are "n" input lines and "m" output lines in the ROM block. The bit combinations of input variables are referred to as addresses. Any bit combination that exits through an output line is known as a word, where the number of output lines, m, is the same as the number of bits in a word.

    The addresses of the n variables are referenced by a binary number. With 'n' input variables, there are, therefore, 2 possible addresses. A ROM contains 2 distinct addresses, which equates to 2 different words. Each output word has a unique address. The words displayed on the output lines at any given moment are determined by the address value applied to their input lines. 

    The Internal Structure of ROM

    internal structure of ROM
    Source: eeeguide.com 

    Two fundamental parts comprise ROM's internal structure: a decoder and OR gates. The circuit, a decoder, decrypts the encoded structure into a decimal structure, such as binary coded decimal (BCD). As a result, the output in its decimal structure is the binary equivalent of the input. The outputs of the decoder will be the outputs of every OR gate in the ROM. 

    Let's use a 64 x 4 ROM as an example, as shown in the image above. The above read-only memory has 64 words with a 4-bit size. It would produce four output lines as a result, and each of the 64 words specified on the output lines is chosen from the six input lines because there are only six inputs in this ROM.  

    Because 2^6 equals 64, there are 6 inputs in this 64 x 4 ROM, allowing for the specification of 64 minimum terms or addresses using 6 variables. There is a distinct word that has been chosen for each address input. For instance, the word number 0 will be chosen and applied to the output lines if the input address is 000000. The word number 63 is chosen and applied to the output lines if the input address is 111111. 

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    What Does Read Only Memory Do?

    The ROM is powered by a small, long-lasting battery inside the computer. The decoder and OR logic gates are its two fundamental parts. The decoder takes binary input from ROM and outputs the decimal equivalent. The decimal output of the decoder serves as the input for the OR gates in ROM. 

    ROM functions similarly to an array of disks. The system can be turned on and off using a grid made up of rows and columns. Each element of the array is paired up with a specific memory element on the ROM chip. A diode is utilized to link the correlating components. The address input is utilized to locate the specific memory location whenever a request is submitted. The value that is taken from the ROM chip must coincide with what is in the selected element of the array. To kick-start your journey in Data Science, enroll in KnowledgeHut Data Science and Machine Learning course.  

    Types of Read Only Memory and Their Applications

    There are five types of read only memory in computers known as: 

    1. MROM 
    2. PROM 
    3. EPROM 
    4. EEPROM 
    5. Flash Memory 

    1. Masked Read Only Memory (MROM)

    MROM is the earliest kind of read-only memory (ROM). It is no longer in use because it has become outdated. This type of hardware memory device is where the manufacturer stores software and information during the manufacturing process. As a result, this type of pre-programmed chip cannot be altered, reprogrammed, or erased later. 

    Integrated circuits are used to make the MROM chips. The arrangement of fuses among the chip's rows and columns determines the input-output pathway that chips use to transmit current. The manufacturer's chosen output is the only channel through which the current can return because it must travel along a fuse-enabled path. As a result, this memory cannot be modified or have information written over it. 

    Applications of MROM 

    • Network Operating Systems 
    • Server Operating Systems 

    2. Programmable Read Only Memory (PROM)

    The PROM is a blank Read Only Memory. It is developed as a blank memory that is later programmed. This type of memory is kept empty while manufacturing, and one can program it using an exclusive device called a programmer. You can buy it and then program it once. 

    In this chip, the current passes through every possible pathway. The programmer can select a specific path for the current and send a high voltage through all the unwanted fuses in order to burn those fuses. It enables the user to program it or add data and software per his requirements. Therefore, it is also called the user-programmable ROM. 

    A device known as a PROM programmer or PROM burner is used in order to write information on a PROM. The process of programming a PROM chip is mostly known as "PROM Burning." Once the chip has been programmed, its content cannot be altered. Therefore, it is also known as a one-time programmable device. 

    Applications of PROM 

    • Mobile Phones 
    • Video game consoles 
    • TV Remote 
    • Early Computer Bios 
    • Radio-Frequency Identification 

    3. Erasable and Programmable Read Only Memory (EPROM)

    As implied by the name, an EPROM is a specific type of read-only memory chip that can erase the programmed data. High voltage can be used to program the programmable read-only memory to write data, and the data is retained until it is exposed to ultraviolet light for up to 10 minutes or longer. As implied by the name, an EPROM is a specific type of read-only memory chip, which has the ability to erase the programmed data. High voltage can be used to program the programmable read-only memory to write data, and the data is retained until it is exposed to ultraviolet light for up to 10 minutes or longer. 

    Typically, an EPROM eraser can accomplish this goal and allow the memory to be reprogrammed. On the memory package, a quartz transparent frame is set aside for this use for easy exposure. 

    Applications of EPROM 

    • Modem 
    • Video Card 
    • Compact Flash 
    • Smart Memory 
    • Memory Stick 
    • Computer BIOS Chip 

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    4. Electrically Erasable and Programmable Read Only Memory (EEPROM)

    Similar to the previously mentioned EPROM, EEPROM is a type of read-only memory that can be programmed and erased without a transparent window by exposing the memory to an electrical charge. 

    It has 10,000 or so reprogramming and erase cycles where both erasing and programming take between 4 and 10 milliseconds. The EEPROM allows users to program and easily erase any selected location. It is erased one byte at a time rather than the entire chip. As a result, reprogramming may be flexible but tedious.  

    Applications of EEPROM 

    • Computer Bios Chip 
    • Microcontrollers for Smart Cards 

    5. Flash Memory

    A contemporary variety of EEPROM is flash memory. Its modern designs have the feature of very high endurance, and flash memory can be erased and rewritten more quickly than conventional EEPROM (exceeding 1,000,000 cycles). 

    In 2007, individual integrated circuits (ICs) with capacities of up to 32 GB were made possible by modern NAND flash memory, which, combined with its physical durability, allows it to replace magnetic media in some applications, such as USB flash drives.

    Advantages of ROM

    ROM contains instructions that enable communication between different hardware components. Besides being essential for the storage and operation of the BIOS, ROM can also be used to manage data, hold utilities, and read peripherals.  

    Here is a list of advantages offered by ROM -  

    • It is a non-volatile memory and less expensive than RAM. 
    • Higher reliability compared to RAM. 
    • Has a much simpler structure compared to RAM. 
    • Since it is static, there is no need for a refreshing time. 
    • Easy to test. 

    The Difference Between RAM and ROM

    Let us have a closer look into the differences between ROM and RAM in the table below -

    Temporary StoragePermanent Storage
    Store Data in MBsStore Data in GBs and TBs
    Used in Normal OperationsUsed for the Startup Process of computers
    Writing Data is FasterWriting Data is Slower

    What is the Use of ROM?

    ROM contains the necessary instructions for various hardware components to communicate with one another. It can be used for basic data management, storing software for fundamental tasks and utilities, reading and writing to peripheral devices, and storing and operating the BIOS.

    Computers use ROM to store the commands necessary to start the system, so starting the system is the main purpose of ROM. ROM also serves as an updater for computer firmware. A smart mobile phone's ROM is similar to that of a computer system's ROM. This information is already stored in the ROM of the mobile phone. A mobile phone ROM's first function is to display the company name to the user if the phone is on or off. Smartphone users may have noticed that every time they switch on or off their phones, the name of the company they are using is displayed. In addition to instructing the RAM to load the operating system, the ROM of a mobile phone also acts as a device for storing files.


    A ROM is an integrated circuit that has been pre-programmed with specific data during the manufacturing process. A single integrated circuit (IC) housing the decoder and OR gates make up ROM. A few Read-Only Memory examples are: MROM (masked read-only memory), PROM (programmable read only memory), EPROM (erasable programmable read only memory), EEPROM (electrically erasable programmable read only memory), and Flash Memory.  

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    We hope that you have fully understood what ROM and the types of ROMs is. Please leave feedback for us if you still don't understand. 

    Read-Only Memory FAQs

    1How is data stored in ROM?

    In order to store data in ROM, the compilers convert code into variables that processors understand. Based on the definition, the compiler allocates memory for the variable and references the address accordingly 

    2What is ROM capacity?

    A ROM chip has a storage capacity of several megabytes (MB) of data usually, ranging between 4 and 8 MB. And unlike RAM, it does not require a continuous power supply to store data. 

    3What is the difference between RAM and ROM?

    RAM is a high-speed volatile memory that stores data when power is being continuously supplied. Usually, RAMs are larger in size and higher in capacity with data stored being easily accessible. They can store only a few GBs (gigabytes) of data and are costlier compared to ROM. 

    ROM is a non-volatile memory that stores data even after the power is turned off. They are small in size, have low capacity, and are slower compared to RAM. ROM is capable of storing multiple megabytes of data and is cheaper than RAM.

    4What are the examples of ROM?

    Here are 5 examples of ROM: 

    1. MROM  
    2. PROM  
    3. EPROM  
    4. EEPROM  
    5. Flash Memory

    Monica Gupta


    I am Monica Gupta with 19+ years of experience in the field of Training and Development. I have done over 500 Corporate Trainings. I am currently working as a freelancer for several years. My core area of work is Java, C++, Angular, PHP, Python, VBA.

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