In the world of computers and electronics, there are many terms that can be confusing. A term that comes up often is ‘ROM’. But what is ROM, and how does it function within the framework of computer systems? Let’s unpack this data storage concept.
What is ROM?
ROM is an acronym for Read-Only Memory. It refers to a type of computer memory that stores data permanently.
A ROM memory chip contains fixed instructions that you cannot change. It is also non-volatile, meaning its contents are retained even if the device loses power. This feature makes ROM ideal for storing critical system settings, firmware, and other essential ROM data that should not be lost.
RAM vs ROM
RAM stands for Random Access Memory and is volatile, meaning that RAM is erased when the computer loses power. ROM chips, on the other hand, are non-volatile, meaning they retain their data even if you turn off the computer.
ROM vs hard drive
Hard drives store data magnetically and you can write to them multiple times. However, unlike a hard drive, ROM stores data permanently, and you cannot rewrite ROM contents without special equipment or procedures.
How ROM works
Like RAM, a ROM chip works by storing data in memory cells, organized in an array. Each memory cell contains a fixed arrangement of transistors that represent binary data, usually zeros and ones.
During the manufacturing process, methods such as photolithography or electrical programming ensure that the data is permanently physically encoded in these memory cells.
Read ROM memory
There are two basic components involved in addressing and reading memory cells in ROM.
Memory cells
ROM consists of memory cells, which are the basic units for storing data. These cells are organized in an array and can contain one bit of information, usually in the form of a 0 or 1.
Word lines and bit lines
Addressing and reading memory cells in the ROM array involves word lines and bit lines.
To access a specific memory, the corresponding word line is activated, selecting a particular row of memory cells. During the read operation, the selected memory cells on the activated word line transfer their stored data to the corresponding bit lines for further processing or output.
6 different types of ROM
There are several types of ROM, each with its own unique features and applications. The most common are:
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Read-only memory (ROM): This is the default ROM that contains permanently stored data. It is typically used for critical system functions and cannot be rewritten.
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Programmable read-only memory (PROM): PROM allows users to write data to the memory chip using special equipment. Once programmed, the data is fixed and cannot be changed.
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Erasable programmable read-only memory (EPROM): EPROM chips can be erased and reprogrammed multiple times using high voltages or exposure to ultraviolet (UV) light.
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Electrically erasable programmable read-only memory (EEPROM): EEPROM chips can be electrically rewritten without the need for UV light, making them easier to reprogram.
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Flash memory: This type of EEPROM uses in-circuit wiring to erase by applying an electric field. Flash memory runs faster than traditional EEPROMs because it writes data 512 bytes at a time instead of just one byte at a time.
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Mask ROM: Also known as “hard-wired ROM”, Mask ROM is programmed during the manufacturing process (such as for storing firmware and system code) and cannot be changed afterwards.
Example applications of ROM
ROM finds applications in various hardware components, including computer systems, gaming consoles and embedded devices. Here are some common uses.
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Operating systems: ROM often stores essential operating system (OS) components so that they remain intact even after power is turned off and on.
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Firmware: Devices such as BIOS (Basic Input/Output System) use ROM to store firmware, which initializes hardware components during the boot process.
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System settings: Critical system settings and configurations are often stored in ROM.
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Game consoles: Game cartridges use ROM chips to permanently store game data.
How PROM works
PROM chips (Figure 2) have a grid of columns and rows, just like regular ROMs. The difference is that each intersection of a column and row in a PROM chip has a fuse connecting them.
PROM functions by allowing users to write data to the memory chip after production, usually using specialized programming equipment.
Programming PROM chips
PROM cells contain fusible bonds that are initially intact and represent a default state (usually all 1’s). During programming, electrical pulses or currents are applied to specific locations on the chip, selectively burning the fusible links.
This changes the status of the corresponding memory cells to 0s. Once programmed, the data is recorded and the user cannot change it.
PROM pros and cons
Blank PROMs are inexpensive and ideal for prototyping the data for a ROM before committing to the costly ROM manufacturing process. However, PROMs are more vulnerable than ROMs. A shock of static electricity can easily cause fuses in the PROM to blow, causing essential bits to change from 1 to 0.
How EPROM works
EPROM works through a process of selective erasure and reprogramming. EPROM cells consist of floating gate transistors that can hold or release electrons, causing binary data to appear as charged or discharged state.
Programming EPROM chips
During programming, high voltages are applied to specific memory cells, injecting electrons into the floating gate and changing the conductivity of the transistor, thereby storing data.
To erase the data, the EPROM chip is exposed to ultraviolet (UV) light, which removes the charge from the floating gates and returns the cells to their default state. Once the chip is cleared, new data can be programmed into the EPROM cells using the same high-voltage program.
EPROM capability
EPROM’s ability to be erased and reprogrammed multiple times makes it suitable for applications that require occasional updates or revisions, such as storing firmware and BIOS in electronic devices.
How EEPROMs and Flash Memory Work
EEPROM and flash memory work on similar principles, using floating-gate transistors to store data.
Both EEPROM and flash memory provide non-volatile storage solutions, making them suitable for applications that require frequent data updates or modifications, such as storing system settings, firmware, and user data on various electronic devices.
Programming EEPROM chips
EEPROM stores data by charging or discharging the floating gates of individual memory cells via electrical programming.
Unlike EPROM, EEPROM does not require exposure to UV light to erase; instead, a high-voltage signal is applied to selectively remove the stored charge from the floating gates, allowing multiple write-erase cycles.
Flash memory programming
Similarly, flash memory stores data by capturing or releasing electrons in floating gates, but it operates on a larger scale, organizing memory cells into blocks and sectors.
Flash memory uses a mechanism called tunneling to move electrons in and out of the floating gates during programming and erasing, respectively. Flash memory is designed for erasing and programming blocks, allowing for more efficient storage and retrieval.
We created this article using AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.
Original article: What is ROM? How read-only memory works on computers
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