ROM
From PHWiki
ROM originally refers to the ROM (Read Only Memory) chip where a game's program data is stored on a game cart on a cart based game system.
In the emulation and flash card world, ROM (short for "ROM image", "ROM dump", or "ROM file") often refers to the data dump of the contents of the ROM. Sometimes "ROM" in the sense of "ROM image" is spelled in lowercase letters as "rom" to distinguish it from the read-only memory itself.
Even more inaccurate, but commonly understood, optical (CD-ROM/DVD-ROM) and magnetic media (hard drive) based dumps or images of such data is also sometimes referred to as being ROM files as well.
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