Read-only Memory (Rom) Chips Have Information Stored in Them by the Manufacturer.

Read-only memory (ROM) is a class of storage medium used in computers and other electronic devices. Information stored in ROM can but exist modified slowly, with difficulty, or not at all, so it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to need frequent updates).

Strictly, read-only memory refers to memory that is hard-wired, such equally diode matrix and the later mask ROM. Although discrete circuits can exist altered (in principle), integrated circuits (ICs) cannot and are useless if the information is bad. The fact that such memory can never exist changed is a big drawback; more recently, ROM commonly refers to memory that is read-only in normal operation, while reserving the fact of some possible style to change it.

Other types of not-volatile retention such as erasable programmable read only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM or Wink ROM) are sometimes referred to, in an abbreviated way, equally "read-just memory" (ROM); although these types of retentiveness can be erased and re-programmed multiple times, writing to this retentivity takes longer and may require unlike procedures than reading the memory. When used in this less precise way, "ROM" indicates anot-volatile retention which serves functions typically provided by mask ROM, such as storage of programme lawmaking and nonvolatile information.

History

Many game consoles employ interchangeable ROM cartridges, allowing for one system to play multiple games.

Read-only memory was used for Jacquard looms.

The simplest blazon of solid-state ROM is equally quondam as the semiconductor technology itself. Combinational logic gates tin can exist joined manually to map n-bit address input onto arbitrary values of m-bit data output (a look-upward table). With the invention of the integrated circuit camemask ROM. Mask ROM consists of a grid of word lines (the address input) and chip lines (the data output), selectively joined together with transistor switches, and can represent an arbitrary wait-up table with a regular physical layout and predictable propagation delay.

In mask ROM, the information is physically encoded in the excursion, and then it tin can only exist programmed during fabrication. This leads to a number of serious disadvantages:

1. It is only economical to buy mask ROM in large quantities, since users must contract with a foundry to produce a custom blueprint.
2. The turnaround time between completing the pattern for a mask ROM and receiving the finished production is long, for the aforementioned reason.
3. Mask ROM is impractical for R&D work since designers frequently demand to alter the contents of memory every bit they refine a design.
4. If a product is shipped with faulty mask ROM, the simply way to gear up it is to call up the production and physically supercede the ROM in every unit shipped.

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 awarding of high-voltage pulses. This addressed problems 1 and 2 to a higher place, since a visitor can only order a big batch of fresh PROM fries and program them with the desired contents at its designers' convenience. The 1971 invention of EPROM essentially solved problem 3, since EPROM (unlike PROM) tin can be repeatedly reset to its unprogrammed state by exposure to strong ultraviolet light. EEPROM, invented in 1983, went a long mode to solving problem 4, since an EEPROM can be programmed in-place if the containing device provides a means to receive the program contents from an external source (for example, a personal reckoner via aserial cablevision). Wink memory, invented at Toshiba in the mid-1980s, and commercialized in the early 1990s, is a form of EEPROM that makes very efficient use of chip area and can be erased and reprogrammed thousands of times without damage.

All of these technologies improved the flexibility of ROM, but at a significant cost-per-chip, then that in large quantities mask ROM would remain an economic choice for many years. (Decreasing price of reprogrammable devices had almost eliminated the marketplace for mask ROM by the yr 2000.) Rewriteable technologies were envisioned equally replacements for mask ROM.

The most recent development is NAND flash, too invented at Toshiba. Its designers explicitly broke from by practice, stating plainly that "the aim of NAND Flash is to replace hard disks,"rather than the traditional utilize of ROM equally a course of non-volatile master storage. Equally of 2007, NAND has partially achieved this goal by offering throughput comparable to hard disks, higher tolerance of physical shock, extreme miniaturization (in the form of USB flash drives and tiny microSD retentivity cards, for example), and much lower power consumption.

Use for storing programs

Every stored-program computer may utilize a form of non-volatile storage (that is, storage that retains its data when power is removed) to shop 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"). Likewise, every non-fiddling computer needs some class of mutable retention to record changes in its state equally it executes.

Forms of read-merely memory were employed as non-volatile storage for programs in nearly early stored-program computers, such as ENIAC after 1948. (Until then it was non a stored-program computer as every program had to be manually wired into the auto, which could have days to weeks.) Read-only retentivity was simpler to implement since it needed but a mechanism to read stored values, and not to change them in-place, and thus could be implemented with very crude electromechanical devices (see historical examples below). With the advent of integrated circuits in the 1960s, both ROM and its mutable counterpart static RAM were implemented equally arrays of transistors in silicon chips; however, a ROM memory cell could be implemented using fewer transistors than an SRAM retentiveness cell, since the latter needs a latch (comprising 5-twenty transistors) to retain its contents, while a ROM cell might consist of the absence (logical 0) or presence (logical 1) of one transistor connecting a bit line to a word line. Consequently, ROM could be implemented at a lower cost-per-scrap than RAM for many years.

Near domicile computers of the 1980s stored a Bones interpreter or operating organisation in ROM every bit other forms of non-volatile storage such as magnetic disk drives were too costly. For instance, the Commodore 64 included 64 KB of RAM and xx KB of ROM independent a BASIC interpreter and the "KERNAL" of its operating system. Subsequently home or office computers such as the IBM PC XT frequently included magnetic deejay drives, and larger amounts of RAM, allowing them to load their operating systems from disk into RAM, with simply a minimal hardware initialization core and bootloader remaining in ROM (known every bit the BIOS in IBM-uniform computers). This organization allowed for a more circuitous and hands upgradeable operating system.

In modern PCs, "ROM" (or flash) is used to store the bones bootstrapping firmware for the principal processor, as well equally the various firmware needed to internally command self-contained devices such as graphic cards, hard disks, DVD drives, TFT screens, etc., in the organisation. Today, many of these "read-merely" memories – especially the BIOS – are often replaced with Flash memory (see below), to permit in-identify reprogramming should the need for a firmware upgrade arise. However, simple and mature sub-systems (such as the keyboard or some advice controllers in the integrated circuits on the principal board, for example) may employ mask ROM or OTP (ane-time programmable).

ROM and successor technologies such as flash are prevalent in embedded systems. These are in everything from industrial robots to abode appliances and consumer electronics(MP3 players, set-top boxes, etc.) all of which are designed for specific functions, just are based on full general-purpose microprocessors. With software normally tightly coupled to hardware, program changes are rarely needed in such devices (which typically lack hard disks for reasons of toll, size, or power consumption). Every bit of 2008, near products use Flash rather than mask ROM, and many provide some ways for connecting to a PC for firmware updates; for case, a digital audio player might be updated to support a new file format. Some hobbyists accept taken advantage of this flexibility to reprogram consumer products for new purposes; for case, the iPodLinux and OpenWrt projects accept enabled users to run full-featured Linux distributions on their MP3 players and wireless routers, respectively.

ROM is likewise useful for binary storage of cryptographic data, as it makes them hard to replace, which may exist desirable in lodge to enhance data security.

Employ for storing data

Since ROM (at least in difficult-wired mask form) cannot be modified, information technology is actually only suitable for storing information which is not expected to need modification for the life of the device. To that terminate, ROM has been used in many computers to store expect-upwards tables for the evaluation of mathematical and logical functions (for example, a floating-betoken unit might tabulate the sine role in order to facilitate faster ciphering). This was especially effective when CPUs were irksome and ROM was cheap compared to RAM.

Notably, the display adapters of early personal computers stored tables of bitmapped font characters in ROM. This unremarkably meant that the text display font could non be changed interactively. This was the case for both the CGA and MDA adapters bachelor with the IBM PC XT.

The use of ROM to shop such small amounts of data has disappeared nigh completely in modern full general-purpose computers. However, Flash ROM has taken over a new role as a medium for mass storage or secondary storage of files.

Types

The first 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 information to exist stored, and thus it is impossible to alter their contents after fabrication. Other types of non-volatile solid-country memory permit some degree of modification:

• Programmable read-but memory (PROM), or one-time programmable ROM (OTP), can be written to or programmed via a special device called a PROM programmer. Typically, this device uses loftier voltages to permanently destroy or create internal links (fuses or antifuses) within the flake. Consequently, a PROM tin can only be programmed one time.
• Erasable programmable read-simply retention (EPROM) can be erased by exposure to strong ultraviolet low-cal (typically for 10 minutes or longer), and then rewritten with a process that again needs higher than usual voltage applied. Repeated exposure to UV light will somewhen habiliment out an EPROM, but the endurance of most EPROM chips exceeds 1000 cycles of erasing and reprogramming. EPROM chip packages can oft be identified by the prominent quartz "window" which allows UV light to enter. Afterwards programming, the window is typically covered with a characterization to forbid adventitious erasure. Some EPROM chips are mill-erased before they are packaged, and include no window; these are effectively PROM.
• Electrically erasable programmable read-simply memory (EEPROM) is based on a like semiconductor construction to EPROM, simply allows its entire contents (or selected banks) to be electrically erased, then rewritten electrically, so that they need not be removed from the calculator (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 one bit at a time. Writing is a very slow process and once more needs higher voltage (usually 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 as non-volatile storage for critical system setup information; in many applications, EAROM has been supplanted by CMOS RAM supplied by mains power and backed-upward with a lithium battery.
  • Flash memory (or simply flash) is a modernistic type of EEPROM invented in 1984. Flash memory can exist erased and rewritten faster than ordinary EEPROM, and newer designs feature very high endurance (exceeding 1,000,000 cycles). Modernistic NAND flash makes efficient use of silicon fleck surface area, resulting in private ICs with a chapters as high every bit 32 GB as of 2007; this feature, along with its endurance and physical durability, has allowed NAND flash to supercede magnetic in some applications (such as USB flash drives). Flash memory is sometimes called flash ROM or flash EEPROM when used as a replacement for older ROM types, only not in applications that take advantage of its power to be modified apace and frequently.

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

Other technologies

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

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

Historical examples

Transformer matrix ROM (TROS), from the IBM System 360/twenty

• Diode matrix ROM, used in modest amounts in many computers in the 1960s also as electronic desk-bound calculators and keyboard encoders for terminals. This ROM was programmed by installing discrete semiconductor diodes at selected locations between a matrix of discussion line traces and bit line traces on a printed circuit lath.
• 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 bit lines. ENIAC's Part Tables were resistor matrix ROM, programmed past manually setting rotary switches. Various models of the IBM Organization/360and complex peripheral devices stored their microcode in either capacitor (chosen BCROS for counterbalanced capacitor read-only storage on the 360/50 and 360/65, or CCROS for charged capacitor read-but storage on the 360/30) or transformer (calledTROS for transformer read-only storage on the 360/20, 360/forty and others) matrix ROM.
• Cadre rope, a form of transformer matrix ROM technology used where size and weight were critical. This was used inNASA/MIT'south Apollo Spacecraft Computers, DEC's PDP-eight computers, and other places. This type of ROM was programmed by hand by weaving "word line wires" within or outside of ferrite transformer cores.
• Dimond Ring stores, in which wires are threaded through a sequence of large ferrite rings that role only every bit sensing devices. These were used in TXE telephone exchanges.
• The perforated metal character mask ("stencil") in Charactron cathode ray tubes, which was used as ROM to shape a wide electron beam to form a selected character shape on the screen either for display or a scanned electron beam to class a selected character shape as an overlay on a video signal.

Speed

Reading

Although the relative speed of RAM vs. ROM has varied over fourth dimension, as of 2007 large RAM fries can be read faster than almost ROMs. For this reason (and to let uniform access), ROM content is sometimes copied to RAM or adumbral before its showtime utilize, and afterward read from RAM.

Writing

For those types of ROM that tin exist electrically modified, writing speed is always much slower than reading speed, and information technology may need unusually high voltage, the movement of jumper plugs to apply write-enable signals, and special lock/unlock command codes. Mod NAND Wink achieves the highest write speeds of any rewritable ROM technology, with speeds as high every bit 15 MB/southward (or lxx ns/bit), past assuasive (needing) big blocks of memory 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, rewriteable ROMs can withstand only a limited number of write and erase cycles earlier the insulation is permanently damaged. In the primeval EAROMs, this might occur afterwards every bit few as 1,000 write cycles, while in modern Wink EEPROM theendurance may exceed i,000,000, but information technology is by no ways infinite. This limited endurance, also as the higher cost per bit, ways that Flash-based storage is unlikely to completely supersede magnetic disk drives in the most time to come.

The timespan over which a ROM remains accurately readable is not express by write cycling. The data memory of EPROM, EAROM, EEPROM, and Flash may be limited past charge leaking from the floating gates of the memory cell transistors. Leakage is accelerated by loftier temperatures or radiation. Masked ROM and fuse/antifuse PROM practice not endure from this result, as their information retention depends on physical rather than electrical permanence of the integrated excursion (although fuse re-growth was once a problem in some systems).

Content images

The contents of ROM chips in video game panel cartridges tin be extracted with special software or hardware devices. The resultant memory dump files are known as ROM images, and can be used to produce duplicate cartridges, or in console emulators. The term originated when nigh console games were distributed on cartridges containing ROM fries, just achieved such widespread usage that information technology is still applied 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 usually a violation of copyright laws (in some jurisdictions, duplication of ROM cartridges for backup purposes may be considered fair apply). Nevertheless, there is a thriving community engaged in the illegal distribution and trading of such software and abandonware. In such circles, the term "ROM images" is sometimes shortened simply to "ROMs" or sometimes changed to "romz" to highlight the connection with "warez".

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Source: https://courses.lumenlearning.com/zeliite115/chapter/reading-read-only-memory/

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