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'''Read-only memory''' is any of several types of computer memory designed to hold constant data and cannot be modified during the ordinary course of operation.

== Solid state ROM ==
These forms of ROM are integrated circuits with no moving parts:
;[[wikipedia:Mask ROM|Mask ROM]]: Program is encoded on the "mask", or the shape of the integrated circuit, when the chip is fabricated. During the NES's commercial era, retail NES games were mass-produced as mask ROM because they were very cheap in volume.
;[[wikipedia:Programmable read-only memory|Programmable read-only memory]] (PROM), One-time programmable (OTP): Early PROMs were an array of fuses that could be changed from 1 to 0. Later PROMs were UVEPROMs without a window.
;[[wikipedia:EPROM|Ultraviolet erasable programmable read-only memory]] (UVEPROM): Variant of PROM using field-effect transistors that turn back to 1 upon exposure to strong ultraviolet light through a window over the chip. In general, one would erase one of them by putting it in a box that shines UV on them for about 20 minutes. Developer carts during the NES's commercial era used this sort of EPROM.
;[[wikipedia:EEPROM|Electrically erasable programmable read-only memory]] (EEPROM): Variant of PROM using floating-gate transistors that can be set to 0 using electricity or set to 1 using more electricity. Early solid-state disk devices were EEPROM based.
;[[wikipedia:Flash memory|Flash memory]]: EEPROM that can be erased a page at a time, where a page can be several thousand bytes or more. Modern developer and retail carts use flash memory.

An NES Game Pak has a PRG ROM connected to the [[CPU]] and either a second CHR ROM or a CHR RAM (or, rarely, both) connected to the [[PPU]].

Solid-state ROMs are measured by the number of words times the length of each word.
On the NES and Super NES, each word is 8 bits; on the Sega Genesis it is 16 bits.
The size in words of the vast majority of ROMs is a power of 2: 16384, 32768, etc.
[http://forums.nesdev.org/viewtopic.php?p=72781#p72781 Some Super NES carts support multiple PRG ROM chips] allowing a non-power-of-two as a sum of different ROM sizes, but on the NES, no Nintendo board supports more than one PRG ROM or more than one CHR ROM, except for some very early mapper 0 boards ([[NROM#Variants|RTROM and STROM]]) that took two 8 KiB PRG ROMs and one 8 KiB CHR ROM.

== Optical disc ==
;[[wikipedia:CD-ROM|CD-ROM]]: An optical disc that holds data to be copied into a RAM and used there. There are three kinds of CD: CD-ROM acts like Mask ROM, CD-R acts like PROM, and CD-RW with [[wikipedia:packet writing|packet writing]] acts like flash memory. Access to CD is much slower than access to solid-state ROM.

== ROM images ==
The term '''ROM''' can also refer to a '''ROM image''', a computer file that represents the entire contents of a ROM or a set of ROMs.
The majority of NES ROM images are distributed in the [[iNES]] or [[NES 2.0]] format.