ROM – Read only memory : what is it? Definition and Types

ROM, also called ROM (Read Only Memory) or non-volatile memory, is memory that stores instructions permanently even in the event of a power outage, unlike random access memory or RAM (Random Access Memory), which stores files temporarily.

ROM is used to store immutable information on the computer, such as the instructions needed to start up (BIOS, POST, boot loader or CMOS setup), UEFI firmware or firmware related to peripherals (printer, etc.).

ROM is an integral part of the motherboard, and it is permanently powered by a small battery.

The access time to ROM is approximately 15 times longer than for RAM (150 nanoseconds against approximately 10 seconds), the latter requiring high execution speed.

To speed up startup, certain instructions from the ROM are copied to RAM during startup (this is called shadowing or “phantom memory”).

The different types of ROMs

Originally, ROM was read-only and could not be changed by the user. By extension, today we call certain erasable, programmable or reprogrammable memories ROM.

• ROM: first generation read-only memory.
• PROM (Programmable Read Only Memory): the system “blows” fuses to erase them and therefore it can only be reprogrammed once.
• EPROM (Erasable Programmable Read Only Memory): can be erased and rewritten in the presence of high intensity ultraviolet rays.
• EEPROM (Electrically Erasable Read Only Memory): can be erased by a simple electric current, that is to say that it can be erased without removing it from the computer. It is the most advanced in read/write capacity but also the most expensive. Removable Flash memory storage devices, such as USB sticks or SD cards are actually EEPROM type devices.

ROM memory on a phone or Smartphone

On a mobile phone or a smartphone, the ROM is none other than the hard disk storage which integrates the operating system, the applications and the files (music, photos, downloads…).

On Android, it is possible to install a custom ROM (Custom ROM), free of pre-installed applications. This makes it lighter and faster. This also makes it possible to take advantage of the latest application features even when the manufacturer no longer offers updates.