Read-only storage
Rom is also the name of a toy and comic book character Rom (Spaceknight).
ROM is also an abbreviation referring to the Royal
Ontario Museum.
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 firmware (software that is very closely related to hardware, and not likely to need frequent upgrading).
One common usage of ROMs are in data files for video game cartridges emulation (often a violation of copyright or sui generis mask work rights unless your jurisdiction has a
fair use protection).
Modern semiconductor ROMs typically take the shape of IC packages, i.e. "computer chips", not immediately distinguishable
from other chips like RAMs but for the text printed on the
chips. Many microcontrollers consist of a ROM part in addition to a
CPU core, some on-board peripherals, and RAM. Home computers of the early
1980s came with their complete operating system in ROM, often including a BASIC programming language interpreter. There was no reasonable alternative because floppy 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.
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 Magnavox Odyssey², and the Game Gear. Such ROMs are sealed into plastic cases suitable for handling and repeated
insertion, known as cartridges or "carts" (called
"Game Paks" by Nintendo). Many home computers also used ROM cartridges for
distributing games and other types of software.
There is a trend to put less and less software into static ROMs, and more on disk storage, making changes easier. 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).
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. Also, a networking or graphics card may implement some basic functionality
through software contained on a ROM chip. Another application for ROMs is in storing software for embedded systems operating in physically demanding environments (exposed to, say, vibration, or high
acceleration), where rotating media like disks are more vulnerable to damage.
Types of ROMs
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.
- EAROMs (Electrically Alterable Read-Only
Memory) can be modified a bit at a time, but writing is intended to be an infrequent operation; most of the time the memory is
used as a ROM. EAROM may be used to store critical system setup information in a non-volatile way. For many applications, EAROM has been supplanted by CMOS RAM backed-up by a lithium battery.
- Flash memory or EEPROMs (Electrically Erasable Read-Only Memory) allow
the entire ROM (or selected banks of the ROM) to be electrically erased (flashed back to zero) then written to
without taking them out of the computer (camera, MP3 player, etc.). Flashing is much slower than writing to RAM (Random Access
Memory) (or reading from any ROM).
- By applying write protection, read/write memory may be turned
(temporarily) into read-only memory.
- A CD-ROM isn't a form of electrical ROM in the traditional sense, but rather a kind
of compact disk. A CD-ROM is Read Only, hence the name, while CD-R is
Write-Once-Read-Many (analagous to an EPROM), and CD-RW is Read/Write (analagous to a Flash EEPROM).
- A form of pre-semiconductor ROM technology is found in the core
rope, which was used in the Apollo spacecraft
computers. This ROM was programmed by hand by weaving word line wires inside or outside of ferrite transformer cores.
Speed of ROMs
Reading speed
Although this has varied over time, today, large RAMs can be read faster than most large ROMs. Therefore ROM content that is
used often is sometimes copied to RAM (shadowed).
Writing speed
For those types of ROMs that can be written to or electrically altered, the writing speed is always much slower than the
reading speed and it may require unusual voltages or the movement of jumper plugs to apply write-enable signals to the
EAROM/Flash ROM.
EEPROM/EAROM lifetime
Because they are written by forcing electrons through an layer of electrical
insulation onto a floating transistor gate, re-writeable ROMs will only
withstand a limited number of write cycles before the insulation is permanently damaged. In very old EAROMs, this damage could
occur in as few as 1,000 write cycles. In modern Flash EEPROMs, the life may be in excess of 10,000 or even 100,000 cycles, but
it is by no means infinite. It is partially for this reason (as well as their limited, more-expensive capacity) that Flash ROMs
are unlikely to completely supplant magnetic disk drives in the near
future.
The reading life of a ROM is not limited by the number of cycles performed. The reading life of EAROMs and EEPROMs may
be limited by charge leakage from the floating gates of the storage transistors, especially at high temperatures or in high
radiation environments. Masked ROMs or fused EPROMs do not
suffer from this effect; once a 1 or 0, always a 1 or 0 (although fuse re-growth was once a
problem in early fused PROMs).
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