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read-only memory (ROM)

Contributor(s): Kate Brush

Read-only memory, or ROM, is a type of computer storage containing non-volatile, permanent data that, normally, can only be read, not written to. ROM contains the programming that allows a computer to start up or regenerate each time it is turned on. ROM also performs large input/output (I/O) tasks and protects programs or software instructions. Once data is written on a ROM chip, it cannot be removed.

Almost every computer incorporates a small amount of ROM that contains the start-up firmware. This boot firmware is called the basic input/output system (BIOS). This software consists of code that instructs the boot-up processes for the computer -- such as loading the operating system (OS) into the random access memory (RAM) or running hardware diagnostics. Consequently, ROM is most often used for firmware updates.

However, ROM is also utilized in video game consoles, allowing one system to run various games. Additionally, ROM is used in optical storage, including different kinds of compact discs (CD) -- such as CD-ROM and CD-RW. ROM is also used frequently in calculators and peripheral devices like laser printers, whose fonts are commonly stored in ROM.

Types of ROM

ROM may sometimes be called maskROM (MROM). MROM is a form of read-only memory that is static and programmed into an included circuit by the manufacturer. Solid-state ROM, the oldest type of ROM, is an example of maskROM. With the original ROM, since it was truly read-only, it had to be removed and physically replaced in order to change any of its contents.

However, new types of ROM have emerged that are still non-volatile, but can be reprogrammed; these types are categorized as programmable read-only memory (PROM). PROM can be used to update firmware, such as BIOS, through the utilization of installation software.

Types of PROM include:

  • Ultraviolet-erasable ROM (UV-ROM) - ROM whose contents can be erased using ultraviolet light, and then reprogrammed.
  • Erasable programmable ROM (EPROM) - A type of ROM that is programmed using high voltages and exposure to ultraviolet light for about 20 minutes.
  • Electrically-erasable programmable ROM (EEPROM) - Often used in older computer chips and to control BIOS, EEPROM can be erased and reprogrammed several times while enabling the erase and writing of only one location at a time. Flash memory is an updated version of EEPROM that allows numerous memory locations to be changed at the same time.

How does ROM work?

ROM is sustained by a small, long-life battery in the computer. It contains two basic components: the decoder and the OR logic gates. In ROM, the decoder receives input in binary form; the output will be the decimal equivalent. The OR gates in ROM use the decoder's decimal output as their input.

ROM performs like a disk array. It contains a grid of rows and columns that are used to turn the system on and off. Every element of the array correlates with a specific memory element on the ROM chip. A diode is used to connect the corresponding elements.

When a request is received, the address input is used to find the specific memory location. The value that is read from the ROM chip should match the contents of the chosen array element.

ROM vs. RAM

Unlike a computer's RAM, the data in ROM is not lost when the computer power is turned off. While the ROM chip is commonly used in the startup operations for the computer, the RAM chip is often used in the recurrent tasks of the computer once the OS has been configured.

Another difference between ROM and RAM is the amount of space they contain. ROM chips can only store several megabytes (MB) of data, usually amounting to between 4 and 8 MB per ROM chip. RAM chips can store multiple gigabytes (GB); this storage often ranges from 1 to 265 GB per RAM chip. 1 GB is considered to be the equivalent of 1000 MBs. Therefore, RAM displays more extensive memory capabilities.

It is almost impossible to operate a computer using only ROM. RAM is necessary to run useful and changeable programs. Therefore, computers must incorporate both forms of memory.

Advantages of ROM

ROM provides the necessary instructions for communication between various hardware components. As mentioned before, it is essential for the storage and operation of the BIOS, but it can also be used for basic data management, to hold software for basic processes of utilities and to read and write to peripheral devices.

Other advantages of ROM include:

  • Its static nature means it does not require refreshing.
  • It is easy to test.
  • ROM is more reliable than RAM since it is non-volatile in nature and cannot be altered or accidentally changed.
  • The contents of the ROM can always be known and verified.
  • Less expensive than RAM.

 

This was last updated in August 2019

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Is ROM integral to the motherboard? May it be read, in a no boot up condition?
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