ROM is an acronym of Read Only Memory. ROMs are computer memory chips with permanent read-only data. ROMs are used for storing an operating system (KERNAL), a BIOS, an OS or a character set. The C64 is supplied with BASIC in a bootable ROM.
A variation of the ROM is the EPROM (Erasable Programmable Read Only Memory). EPROMs can be used - using adapter PCBs - to replace C64 chips such as KERNAL, BASIC, or CHARSET ROMs, and are used frequently in the context of, e.g., hardware-based fast loaders. EPROMs are rewriteable via EPROM burners, such as Tiny Eprommer. EPROMs are erased using UV light, having a small lid covering the erasure window.
Another variation is the EEPROM (Electrically Erasable Programmable Read Only Memory), which instead are erased using a pin on the chip itself. This is one of the most common types of programmable ROM.
Yet another variation is the PROM (Programmable Read Only Memory), which is programmable only once by applying high voltage. This is rarely seen.
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