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Read-only
memory
Read-only memory (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 hardware,
and not likely to need upgrading. For example, a networking or
graphics card may implement some basic functionality through
software contained on a ROM. ROMs typically take the shape of IC
packages, i.e. black "computer chips", not immediately
distinguishable from other chips like RAMs
but for the text printed on the chips.
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, often including a BASIC
programming language interpreter.
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 systems for desktop computers are not generally on ROM
anymore. Computers may still rely on some software in ROMs, like
their BIOS,
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 that use ROM based software include the Super
Famicom, Super
Nintendo Entertainment System North American and European
versions of the Super Famicom), the Nintendo
64, 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).
Many home computers also used ROM cartridges for distributing
games and other types of software.
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 – see bootstrap.
Hence the BIOS or a bootloader for a computer is often stored in
ROM. Another application for ROMs is in storing software for embedded
systems operating in physically demanding environments
(exposed to, say, vibration, or high G-forces), where rotating
media like disks are less appropriate.
Classic
mask-programmed ROM chips are written to during production and
cannot change content afterwards. But there are other types of
non-volatile solid-state memory:
-
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, read/write memory may be turned (temporarily)
into read-only memory.
ROM
as in ROM image may 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).
This
content from Wikipedia
is licensed under the GNU
Free Documentation License.
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