- ROM Definition: Read Only Memory (ROM) is defined as a type of non-volatile memory that allows multiple reads but only one or a few writes, retaining data even when power is off.
- Hard-Wired Memory: ROM is a hard-wired memory device with a row-column grid connected by diodes, meaning it doesn’t need frequent refresh cycles like RAM.
- Types of ROM: There are various types of ROM, including Masked ROM (MROM), Programmable ROM (PROM), Erasable Programmable ROM (EPROM), and Electrically Erasable Programmable ROM (EEPROM).
- Applications: ROM is used in many electronic devices like calculators, printers, and gaming consoles to store boot-up programs and firmware.
- EEPROM and Flash ROM: EEPROM and Flash ROM are types of ROM that can be electrically erased and reprogrammed, with Flash ROM offering faster performance and access to multiple memory locations.
Read Only Memory (ROM) is non-volatile memory: it retains stored data when power is removed. The term ROM covers several technologies with different programming and erase capabilities. Mask ROM is fixed during manufacture. PROM is programmed once. EPROM, EEPROM and flash can be erased and programmed again. Each device has specified limits for data retention and, where applicable, program and erase cycles.
A row-and-column matrix fitted with selected diodes is one simple way to illustrate ROM, but it is not the architecture of every ROM device. Modern devices commonly use transistor-based memory cells. Non-volatile cells do not need refresh to retain data. Dynamic RAM does need periodic refresh, whereas static RAM does not. ROM-family devices store firmware, boot code, lookup tables and fixed configuration data in products such as appliances, printers and embedded controllers.
The main ROM technologies are:
Masked Read Only Memory (MROM)
MROM stores data through patterns defined during chip fabrication. Photolithography masks help create the wiring and device features in the integrated circuits; the finished memory is not programmed by the equipment maker or end user. Its fixed contents make mask ROM suitable for mature, high-volume designs where the code will not change. The initial manufacturing setup cost is offset across a large production run, and the cells can be compact compared with reprogrammable semiconductor memory.
Programmable Read Only Memory (PROM)
PROMs are supplied in an unprogrammed state and are written once with a suitable programmer. Depending on the device, programming permanently changes fuse or antifuse links. The programmer applies controlled pulses, sometimes at a higher voltage than the normal read supply. Because this physical change cannot be reversed, a PROM cannot be erased and programmed with new data.
Erasable and Programmable Read Only Memory (EPROM)
EPROM uses an array of floating-gate MOS transistors. Electrical programming pulses place charge on selected floating gates. A windowed package lets ultraviolet (UV) light reach the chip and erase the array, normally as a whole, before it is programmed again. Plastic packages without a window are one-time programmable. EPROM was widely used for prototypes and firmware storage, but its removal and UV-erase process make it less convenient than electrically erasable memory.
Electrically Erasable and Programmable Read Only Memory (EEPROM)
EEPROMs are erased and programmed electrically. A circuit can therefore update the memory in place when its hardware and control software support writing. Reads are much faster than erase-and-write operations. Many EEPROM devices support byte writes, while others also provide page operations. Write endurance, retention, timing, supply limits and erase behaviour must be checked in the device data sheet.
Two related terms are:Electrically Alterable Read Only Memory (EAROM)
EAROM is a historical name for electrically alterable non-volatile memory. It allowed selected data to be changed without replacing the whole device. Its slow write operation could require a separate programming supply. It was suited to settings and calibration values that changed infrequently. Modern EEPROM has largely replaced this technology.
Flash Read Only Memory (FROM)
Flash memory is an electrically erasable non-volatile memory derived from EEPROM technology. It commonly programs data in pages or words and erases larger sectors or blocks rather than individual bytes. This organisation supports higher-density storage and efficient bulk updates. NOR and NAND flash have different access patterns and uses, so programming speed, erase size, endurance and retention depend on the selected device.
