Read Only Memory Chips Are Used to

Computer memory types

Volatile

DRAM, e.grand. DDR SDRAM SRAM Upcoming Z-RAM TTRAM Historical Delay line memory Selectron tube Williams tube

Non-volatile

ROM PROM EPROM EEPROM Flash memory Upcoming FeRAM MRAM CBRAM PRAM SONOS RRAM Racetrack memory NRAM Historical Drum retentiveness Magnetic core memory Plated wire memory Chimera retentiveness Twistor retentivity

Read-but memory, usually known by its acronym ROM, is a class of storage media used in computers and other electronic devices. In its strictest sense, ROM refers to semiconductor-fabricated memory that contains information permanently stored in it, with no allowance for future modification. This is the the oldest type of solid land ROM and is known as mask ROM.

More modern types of ROM—such as PROM (Programmable Read-Just Memory), EPROM (Erasable Programmable Read-Only Retentivity), and wink EEPROM (Electrically Erasable Programmable Read-Only Memory)—may exist reprogrammed, with or without erasure of earlier data. They are nevertheless described as "read-only memory" considering the reprogramming procedure is generally infrequent, comparatively deadening, and often does not permit random access writing to individual memory locations. Despite the simplicity of mask ROM, economies of scale and field-programmability frequently make reprogrammable technologies more flexible and inexpensive, then that mask ROM is rarely used in new products.

Contents

• 1 History
• 2 Types of ROMs
  • two.1 Semiconductor based
  • 2.2 Other technologies
  • 2.3 Historical examples
• 3 Speed of ROMs
  • 3.i Reading speed
  • 3.2 Writing speed
• 4 Endurance and data retention
• 5 ROM images
• 6 Applications
  • vi.1 Utilise of ROM for program storage
  • vi.2 Apply of ROM for data storage
• 7 Run across also
• eight Notes
• 9 References
• 10 Credits

ROM media are used mainly to distribute firmware—that is, software closely tied to specific hardware and unlikely to require frequent updates.

History

The simplest type of solid land ROM is as old as semiconductor engineering science itself. Combinational logic gates can exist joined manually to map n-chip address input onto arbitrary values of m-flake data output (a await-up tabular array). With the invention of the integrated circuit came mask ROM. Mask ROM consists of a grid of give-and-take lines (the address input) and bit lines (the data output), selectively joined together with transistor switches, and can represent an arbitrary look-upward tabular array with a regular physical layout and predictable propagation delay.

In mask ROM, the information is physically encoded in the excursion, so it can merely be programmed during fabrication. This leads to a number of serious disadvantages:

1. It is simply economical to buy mask ROM in large quantities, since users must contract with a foundry to produce a custom design.
2. The turnaround time betwixt completing the design for a mask ROM and receiving the finished product is long, for the same reason.
3. Mask ROM is impractical for R&D work since designers ofttimes need to modify the contents of retention as they refine a blueprint.
4. If a production is shipped with faulty mask ROM, the but way to prepare it is to retrieve the production and physically replace the ROM.

Subsequent developments have addressed these shortcomings. PROM, invented in 1956, allowed users to program its contents exactly once by physically altering its structure with the application of high-voltage pulses. This addresses issues 1 and ii above, since a company can simply guild a large batch of fresh PROM chips and programme them with the desired contents at its designers' convenience. The 1971 invention of EPROM essentially solved trouble 3, since EPROM (different PROM) can be repeatedly reset to its unprogrammed state by exposure to strong ultraviolet light. EEPROM, invented in 1983, went a long way to solving trouble 4, since an EEPROM can be programmed in-identify if the containing device provides a means to receive the program contents from an external source (east.g. a personal calculator via a series cable). Flash memory, invented at Toshiba in the mid-1980s, and commercialized in the early 1990s, is a grade of EEPROM that makes very efficient use of bit expanse and can be erased and reprogrammed thousands of times without damage.

All of these technologies improved the flexibility of ROM, but at a significant price-per-fleck, so that in large quantities mask ROM would remain an economical choice for many years. (Decreasing toll of reprogrammable devices had almost eliminated the marketplace for mask ROM past the year 2000.) Furthermore, despite the fact that newer technologies were increasingly less "read-merely," almost were envisioned only as replacements for the traditional use of mask ROM.

The well-nigh recent development is NAND flash, too invented past Toshiba. Its designers explicitly broke from past practise, stating that "the aim of NAND Flash is to supercede hard disks,"^[1] rather than the traditional use of ROM every bit a form of non-volatile primary storage. Every bit of 2007, NAND has partially achieved this goal past offering throughput comparable to hard disks, higher tolerance of concrete daze, extreme miniaturization (in the form of USB wink drives and tiny microSD memory cards, for example), and much lower power consumption.

Types of ROMs

The get-go EPROM, an Intel 1702, with the die and wire bonds clearly visible through the erase window.

Semiconductor based

Archetype mask-programmed ROM chips are integrated circuits that physically encode the data to be stored, and thus it is impossible to change their contents after fabrication. Other types of non-volatile solid-state memory permit some degree of modification:

• Programmable read-simply memory (PROM), or one-time programmable ROM (OTP), tin be written to or programmed via a special device called a PROM developer. Typically, this device uses high voltages to permanently destroy or create internal links (fuses or antifuses) within the chip. Consequently, a PROM can only be programmed once.
• Erasable programmable read-but memory (EPROM) can be erased past exposure to stiff ultraviolet light (typically for 10 minutes or longer), then rewritten with a process that over again requires awarding of higher than usual voltage. Repeated exposure to UV lite will eventually article of clothing out an EPROM, only the endurance of most EPROM fries exceeds 1000 cycles of erasing and reprogramming. EPROM chip packages tin often be identified by the prominent quartz "window" which allows UV light to enter. Afterward programming, the window is typically covered with a label to preclude adventitious erasure. Some EPROM chips are factory-erased before they are packaged, and include no window; these are effectively PROM.
• Electrically erasable programmable read-only memory (EEPROM) is based on a similar semiconductor structure to EPROM, only allows its entire contents (or selected banks) to be electrically erased, then rewritten electrically, so that they need not exist removed from the figurer (or camera, MP3 player, etc.). Writing or flashing an EEPROM is much slower (milliseconds per fleck) than reading from a ROM or writing to a RAM (nanoseconds in both cases).

• • Electrically alterable read-only memory (EAROM) is a type of EEPROM that can be modified ane bit at a time. Writing is a very irksome procedure and again requires higher voltage (unremarkably around 12 V) than is used for read access. EAROMs are intended for applications that require infrequent and simply partial rewriting. EAROM may be used equally not-volatile storage for critical system setup information; in many applications, EAROM has been supplanted past CMOS RAM supplied by mains power and backed-upwardly with a lithium bombardment.
  • Flash memory (or merely wink) is a modernistic type of EEPROM invented in 1984. Flash retentivity can exist erased and rewritten faster than ordinary EEPROM, and newer designs characteristic very high endurance (exceeding 1,000,000 cycles). Modern NAND flash makes efficient utilize of silicon flake area, resulting in individual ICs with a capacity as high as 16 GB (every bit of 2007); this characteristic, forth with its endurance and physical durability, has allowed NAND flash to replace magnetic in some applications (such as USB wink drives). Flash memory is sometimes called flash ROM or wink EEPROM when used as a replacement for older ROM types, just non in applications that take advantage of its ability to exist modified quickly and frequently.

By applying write protection, some types of reprogrammable ROMs may temporarily become read-only retention.

Other technologies

At that place are other types of not-volatile memory that are not based on solid-state IC technology, including:

• Optical storage media, such CD-ROM which is read-but (analogous to masked ROM). CD-R is Write Once Read Many (analogous to PROM), while CD-RW supports erase-rewrite cycles (coordinating to EEPROM); both are designed for backwards-compatibility with CD-ROM.

Historical examples

Transformer matrix ROM (TROS), from the IBM Arrangement 360/20.

• Diode matrix ROM, used in small amounts in many computers in the 1960s as well as electronic desk calculators and keyboard encoders for terminals. This ROM was programmed by installing discrete semiconductor diodes at selected locations between a matrix of give-and-take line traces and bit line traces on a printed excursion board.
• Resistor, capacitor, or transformer matrix ROM, used in many computers until the 1970s. Similar diode matrix ROM, it was programmed by placing components at selected locations between a matrix of word lines and scrap lines. ENIAC's Function Tables were resistor matrix ROM, programmed by manually setting rotary switches. Diverse models of the IBM System/360 and circuitous peripherial devices stored their microcode in either capacitor (called BCROS for Balanced Capacitor Read Only Storage on the 360/50 & 360/65 or CCROS for Card Capacitor Read Only Storage on the 360/30) or transformer (chosen TROS for Transformer Read Only Due southtorage on the 360/20, 360/xl and others) matrix ROM.
• Core rope, a class of transformer matrix ROM technology used where size and/or weight were critical. This was used in NASA/MIT's Apollo Spacecraft Computers, December'due south PDP-viii computers, and other places. This blazon of ROM was programmed past manus by weaving "give-and-take line wires" within or outside of ferrite transformer cores.
• The perforated metallic character mask ("stencil") in Charactron cathode ray tubes, which was used equally ROM to shape a wide electron beam to form a selected grapheme shape on the screen either for brandish or a scanned electron axle to grade a selected character shape every bit an overlay on a video signal.
• Various mechanical devices used in early computing equipment. A machined metallic plate served as ROM in the dot matrix printers on the IBM 026 and IBM 029 key punches.

Speed of ROMs

Reading speed

Although the relative speed of RAM vs. ROM has varied over time, equally of 2007 big RAM chips tin can be read faster than most ROMs. For this reason (and to make for uniform access), ROM content is sometimes copied to RAM or "adumbral" earlier its kickoff utilize, and subsequently read from RAM.

Writing speed

For those types of ROM that tin be electrically modified, writing speed is ever much slower than reading speed, and it may crave unusually loftier voltage, the movement of jumper plugs to apply write-enable signals, and special lock/unlock command codes. Modernistic NAND Flash achieves the highest write speeds of any rewritable ROM technology, with speeds as high as xv MiB/s (or seventy ns/bit), by allowing (indeed requiring) large blocks of retentiveness cells to be written simultaneously.

Endurance and data retention

Because they are written by forcing electrons through a layer of electrical insulation onto a floating transistor gate, rewritable ROMs can withstand simply a limited number of write and erase cycles before the insulation is permanently damaged. In the earliest EAROMs, this might occur later as few as i,000 write cycles, while in modern Flash EEPROM the endurance may exceed 1,000,000, but it is by no means infinite. This limited endurance, as well equally the college cost per scrap, means that flash-based storage is unlikely to completely supercede magnetic disk drives in the nearly future.

The time span over which a ROM remains accurately readable is not limited past write cycling. The data retentiveness of EPROM, EAROM, EEPROM, and Flash may be limited past accuse leaking from the floating gates of the memory cell transistors. Leakage is exacerbated at high temperatures or in high-radiation environments. Masked ROM and fuse/antifuse PROM practice not endure from this result, as their data retention depends on concrete rather than electric permanence of the integrated circuit (although fuse re-growth was once a trouble in some systems).

ROM images

The contents of ROM chips in video game console cartridges tin can be extracted with special software or hardware devices. The resultant memory dump files are known as ROM images, and can exist used to produce duplicate cartridges, or in console emulators. The term originated when most console games were distributed on cartridges containing ROM chips, just achieved such widespread usage that it is all the same practical to images of newer games distributed on CD-ROMs or other optical media.

ROM images of commercial games commonly contain copyrighted software. The unauthorized copying and distribution of copyrighted software is commonly a violation of copyright laws (in some jurisdictions duplication of ROM cartridges for backup purposes may be considered fair utilize). Even so, in that location is a thriving community engaged in the illegal distribution and trading of such software. In such circles, the term "ROM images" is sometimes shortened simply to "ROMs" or sometimes inverse to "romz" to highlight the connexion with "warez."

Applications

Employ of ROM for program storage

Every stored-program computer requires some form of non-volatile storage to store the initial program that runs when the computer is powered on or otherwise begins execution (a process known equally bootstrapping, often abbreviated to "booting" or "booting up"). Also, every non-little computer requires some class of mutable memory to tape changes in its state as it executes.

Forms of read-only retentiveness were employed as non-volatile storage for programs in nigh early stored-program computers, such every bit ENIAC after 1948 (until so it was not a stored-program computer as every program had to exist manually wired into the machine, which could take days to weeks). Read-just memory was simpler to implement since information technology required only a mechanism to read stored values, and not to change them in-place, and thus could be implemented with very rough electromechanical devices (see historical examples above). With the advent of integrated circuits in the 1960s, both ROM and its mutable analogue static RAM were implemented as arrays of transistors in silicon chips; still, a ROM memory cell could be implemented using fewer transistors than an SRAM retentiveness prison cell, since the latter requires a latch (comprising 5-twenty transistors) to retain its contents, while a ROM prison cell might consist of the absence (logical 0) or presence (logical ane) of a unmarried transistor connecting a bit line to a give-and-take line.^[2] Consequently, ROM could be implemented at a lower price-per-bit than RAM for many years.

Most home computers of the 1980s stored a BASIC interpreter or operating system in ROM as other forms of not-volatile storage such as magnetic disk drives were too expensive. For example, the Commodore 64 included 64 KiB of RAM and twenty KiB of ROM contained a Basic interpreter and the "KERNAL" (sic) of its operating organization. Later domicile or office computers such equally the IBM PC XT oftentimes included magnetic disk drives, and larger amounts of RAM, assuasive them to load their operating systems from disk into RAM, with only a minimal hardware initialization cadre and bootloader remaining in ROM (known as the BIOS in IBM-compatible computers). This system immune for a more complex and easily upgradeable operating system.

In modern PCs, "ROM" (or Wink) is used to store the basic bootstrapping firmware for the chief processor, too every bit the diverse firmware needed to internally control self contained devices such as graphic cards, hard disks, DVD drives, and TFT screens, in the system. Today, many of these "read-but" memories – especially the BIOS – are often replaced with Flash memory (see below), to permit in-place reprogramming should the need for a firmware upgrade arise. However, simple and mature sub-systems (such as the keyboard or some communication controllers in the ICs on the primary lath, for example) may employ mask ROM or OTP (one time programmable).

ROM and successor technologies such as Wink are prevalent in embedded systems. This governs everything from industrial robots to appliances and consumer electronics (MP3 players, set-top boxes, etc) all of which are designed for specific functions, but nevertheless based on general-purpose microprocessors in nigh cases. With software usually tightly coupled to hardware, programme changes are rarely needed in such devices (which typically lack devices such equally difficult disks for reasons of cost, size, and/or power consumption). As of 2008, most products employ Wink rather than mask ROM, and many provide some means for connexion to a PC for firmware updates; a digital audio player'south might exist updated to support a new file format for instance. Some hobbyists take taken advantage of this flexibility to reprogram consumer products for new purposes; for case, the iPodLinux and OpenWRT projects have enabled users to run full-featured Linux distributions on their MP3 players and wireless routers, respectively.

ROM is also useful for binary storage of cryptographic data, as information technology makes them hard to replace, which may be desirable in order to raise data security.

Use of ROM for data storage

Since ROM (at least in hard-wired mask form) cannot be modified, it is really only suitable for storing data which is not expected to need modification for the life of the device. To that cease, ROM has been used in many computers to store look-up tables for the evaluation of mathematical and logical functions (for example, a floating-point unit might tabulate the sine office to facilitate faster computation). This was especially effective when CPUs were slow and ROM was cheap compared to RAM.

Notably, the display adapters of early personal computers stored tables of bitmapped font characters in ROM. This ordinarily meant that the text display font could not exist changed interactively. This was the example for both the CGA and MDA adapters bachelor with the IBM PC XT.

The use of ROM to store such minor amounts of information has disappeared almost completely in modern general-purpose computers. All the same, Flash ROM has taken on a new function as a medium for mass storage or secondary storage of files.

See also

• Computer
• Computer science
• Electronics
• Integrated excursion
• Random admission memory
• Semiconductor

Notes

1. ↑ Inoue, A., and D. Wong. Apr 2003. NAND Flash Applications Design Guide, page half dozen. Retrieved January 23, 2009.
2. ↑ Millman, Jacob, and Arvin Grabel. 1988. Microelectronics, 2d ed. Chapters on "Combinatorial Digital Circuits" and "Sequential Digital Circuits." New York: McGraw-Loma. ISBN 007100596X.

References

ISBN links back up NWE through referral fees

• Jacob, Bruce, Spencer Ng, and David Wang. 2007. Memory Systems: Cache, DRAM, Disk. San Francisco, CA: Morgan Kaufmann. ISBN 978-0123797513
• Millman, Jacob, and Arvin Grabel. 1988. Microminiaturization, 2d ed. New York: McGraw-Hill. ISBN 007100596X
• White, Ron. 2008. How Computers Work, 9th ed. Indianapolis, IN: Que Pub. ISBN 978-0789736130
• Young, Roger. 2002. How Computers Work: Processor and Primary Memory. Bloomington, IN: 1st Books. ISBN 1403325820

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