Read-only memory

Read-only memory or ROM is used as a storage medium in computers. Because it
cannot (easily) be written to, its main uses lie in the distribution of
software that is very closely related to a certain hardware, and not likely
to need upgrading. For example a graphics card may implement some basic
functionality through software contained on a ROM.

There is a trend to put less and less software into static ROMs, and more on
disk storage, making changes easier. Home computers in the early 1980s came
with their complete operating system in ROM. There was no reasonable
alternative because disk drives were generally optional. Upgrading to a
newer version meant using either a soldering iron or a set of DIP sockets
and replacing the old ROM chip with a new one. By the 2000s operating system
are not generally on ROM anymore. Computers may still rely on some software
in ROMs, but even that is more likely to reside on a Flash-ROM (see below).
Mobile phones and personal digital assistants are likely to have software in
ROM (or at least flash memory). Video game consoles use ROM based software
include the Super Famicom, Super Nintendo Entertainment System (North
American and European version of the Super Famicom), the N64, and the Game
Boy. Such ROMs are sealed into plastic cases suitable for handling and
repeated insertion, known as cartridges or "carts" (or "Game Pak" if you are
Nintendo). By extension ROM may also refer to a data file that contains an
image of the software normally distributed in a ROM, such as a copy of a
video game cartridge (often a violation of copyright or sui generis mask
work rights unless your jurisdiction has a fair use protection).

One reason why some data still sits in ROMs is speed -- disks are an order
of magnitude slower. Even more important, though, is that you cannot read
software that is needed to drive a disk from the disk itself. Hence the 
BIOS or a bootloader for a computer is often on ROM.

Classic ROM chips are written to during production and cannot change content
afterwards.

   * PROMs (Programmable Read-Only Memory) can be written to (programmed)
     via a special device, a PROM programmer. The writing often takes the
     form of destroying internal links with the result that a PROM can only
     be programmed once.
   * EPROMs (Erasable Programmable Read-Only Memory) can be erased by
     exposure to ultraviolet light then rewritten via an EPROM programmer.
     Repeated exposure to ultraviolet light will eventually destroy the
     EPROM but it generally takes many exposures before the EPROM becomes
     unusable.
   * Flash memory or EEPROMs (Electrically Erasable Read-Only Memory) can be
     electrically erased then written to (flashed) without taking them out
     of the computer. Flashing is much slower than writing to RAM (Random
     Access Memory) (or reading from any ROM).

RAM can be read faster than most ROMs, therefore ROM content that is used
often is sometimes copied to RAM (shadowed).

By applying write protection memory turns (temporarily) into read-only
memory.