ROM (read only memory) is a flash memory chip that contains a small amount of non-volatile memory. Non-volatile means that its contents cannot be changed and it retains its memory after the computer is turned off.

ROM contains the BIOS which is the firmware for the motherboard. The BIOS contains the bootstrap – the program which takes the computer through steps that lead up to the loading of the operating system (OS). It happens between turning on the power and the computer beeping to say it is starting to load the OS. This process is known as POST (power on self test) on a PC.

The boot sequence is the computer's initial start-up process. After the boot, the OS controls the CPU and supplies the programs to run.

Types of ROM include:

  • PROM (programmable read-only memory) - manufactured as blank ROM. PROM chips can be bought cheaply and programmed directly by a programmer. They are not rewritable so they can only be programmed once.
  • EEPROM (electrically erasable programmable read-only memory) - popular in PCs and smartphones as the firmware can be easily updated by the manufacturer. This is similar to a rewritable CD in that the chip can be reprogrammed.
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