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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:

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.
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.
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.
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.
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. 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 (RTROM and STROM) that took two 8 KiB PRG ROMs and one 8 KiB CHR ROM.

Optical disc

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 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.