Internal memory, also called main or primary memory, refers to memory that can be accessed quickly while the computer is running. There are two main types of internal memory: RAM and ROM. RAM (random access memory) is a form of memory that can be read and changed in any order, typically used to store working data and machine code. It exists as integrated circuit chips with MOS (metal oxide semiconductor) memory cells. ROM (read only memory) is a type of electronic storage that comes built into a device, and cannot be changed. ROM is further divided into MROM, PROM, EPROM, and EEPROM.