Read-only Memory (Rom) Is Temporary and Volatile. Ram Is More Permanent and Non-volatile.

Electronic memory that cannot be changed

Many game consoles utilize interchangeable ROM cartridges, allowing for 1 system to play multiple games. Shown hither is the inside of a Pokemon Silver Game Male child cartridge. The ROM is the IC on the right labeled "MX23C1603-12A".

Read-only retentiveness (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified afterwards the manufacture of the memory device. Read-simply memory is useful for storing software that is rarely inverse during the life of the organization, also known as firmware. Software applications (like video games) for programmable devices tin be distributed as plug-in cartridges containing ROM.

Read-only retentiveness strictly refers to retentivity that is hard-wired, such every bit diode matrix or a mask ROM integrated circuit (IC), which cannot be electronically^[a] changed after manufacture. Although discrete circuits tin be altered in principle, through the improver of bodge wires and/or the removal or replacement of components, ICs cannot. Correction of errors, or updates to the software, require new devices to be manufactured and to replace the installed device.

Floating-gate ROM semiconductor retentivity in the form of erasable programmable read-simply memory (EPROM), electrically erasable programmable read-only memory (EEPROM) and flash memory tin can exist erased and re-programmed. But usually, this tin can only exist washed at relatively ho-hum speeds, may require special equipment to reach, and is typically only possible a sure number of times.^[1]

The term "ROM" is sometimes used to mean a ROM device containing specific software, or a file with software to be stored in EEPROM or Flash Memory. For example, users modifying or replacing the Android operating system depict files containing a modified or replacement operating system as "custom ROMs" after the type of storage the file used to be written to.

History [edit]

Discrete-component ROM [edit]

IBM used capacitor read-merely storage (CROS) and transformer read-but storage (TROS) to store microcode for the smaller Arrangement/360 models, the 360/85, and the initial two Organization/370 models (370/155 and 370/165). On some models there was also a writeable command store (WCS) for additional diagnostics and emulation support. The Apollo Guidance Calculator used core rope retentiveness, programmed past threading wires through magnetic cores.

Solid-country ROM [edit]

The simplest blazon of solid-country ROM is as erstwhile every bit the semiconductor technology itself. Combinational logic gates tin can be joined manually to map n-bit accost input onto arbitrary values of m-bit data output (a look-up table). With the invention of the integrated circuit came mask ROM. Mask ROM consists of a grid of word lines (the address input) and bit lines (the information output), selectively joined together with transistor switches, and can represent an capricious look-upwardly table with a regular physical layout and predictable propagation filibuster.

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

1. It is only economic to purchase mask ROM in large quantities, since users must contract with a foundry to produce a custom design.
2. The turnaround time between completing the design for a mask ROM and receiving the finished production is long, for the same reason.
3. Mask ROM is impractical for R&D work since designers ofttimes need to modify the contents of memory equally they refine a design.
4. If a product is shipped with faulty mask ROM, the only way to fix it is to recall the product and physically replace the ROM in every unit of measurement shipped.

Subsequent developments take addressed these shortcomings. Programmable read-only memory (PROM), invented by Wen Tsing Grub in 1956,^{[two] [3]} allowed users to programme its contents exactly one time by physically altering its structure with the application of loftier-voltage pulses. This addressed problems one and ii above, since a company can merely order a large batch of fresh PROM chips and program them with the desired contents at its designers' convenience.

The advent of the metallic–oxide–semiconductor field-event transistor (MOSFET), invented at Bong Labs in 1959,^[4] enabled the practical use of metallic–oxide–semiconductor (MOS) transistors as memory cell storage elements in semiconductor memory, a role previously served by magnetic cores in computer memory.^[5] In 1967, Dawon Kahng and Simon Sze of Bell Labs proposed that the floating gate of a MOS semiconductor device could exist used for the cell of a reprogrammable ROM, which led to Dov Frohman of Intel inventing erasable programmable read-only retentivity (EPROM) in 1971.^[vi] The 1971 invention of EPROM essentially solved problem three, since EPROM (unlike PROM) can be repeatedly reset to its unprogrammed state by exposure to stiff ultraviolet light.

Electrically erasable programmable read-only memory (EEPROM), developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at the Electrotechnical Laboratory in 1972,^[7] went a long way to solving problem 4, since an EEPROM tin be programmed in-identify if the containing device provides a means to receive the programme contents from an external source (for example, a personal computer via a serial cable). Wink retentivity, invented by Fujio Masuoka at Toshiba in the early 1980s and commercialized in the late 1980s, is a class of EEPROM that makes very efficient utilise of chip area and can be erased and reprogrammed thousands of times without damage. It permits erasure and programming of just a specific function of the device, instead of the unabridged device. This can be done at high speed, hence the name "wink".^{[8] [ix]}

All of these technologies improved the flexibility of ROM, but at a pregnant cost-per-flake, so that in large quantities mask ROM would remain an economic choice for many years. (Decreasing cost of reprogrammable devices had near eliminated the market for mask ROM by the year 2000.) Rewriteable technologies were envisioned as replacements for mask ROM.

The nearly contempo development is NAND wink, likewise invented at Toshiba. Its designers explicitly broke from past do, stating apparently that "the aim of NAND flash is to replace hard disks,"^[x] rather than the traditional utilize of ROM as a class of non-volatile principal storage. Equally of 2021^[update], NAND has most completely achieved this goal by offering throughput college than hard disks, lower latency, college tolerance of physical shock, extreme miniaturization (in the form of USB flash drives and tiny microSD retention cards, for example), and much lower power consumption.

Use for storing programs [edit]

Many stored-program computers utilize a course of non-volatile storage (that is, storage that retains its data when power is removed) to store the initial programme that runs when the calculator is powered on or otherwise begins execution (a process known^[b] as bootstrapping, ofttimes abbreviated to "booting" or "booting up"). Also, every not-trivial computer needs some grade of mutable memory to record changes in its land equally it executes.

Forms of read-only retention were employed equally non-volatile storage for programs in most early stored-program computers, such as ENIAC after 1948. (Until so it was not a stored-programme computer equally every program had to be manually wired into the machine, which could take days to weeks.) Read-only memory was simpler to implement since it needed only a mechanism to read stored values, and non to change them in-place, and thus could be implemented with very crude electromechanical devices (run across historical examples beneath). 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; however, a ROM memory cell could be implemented using fewer transistors than an SRAM memory cell, since the latter needs a latch (comprising 5-xx transistors) to retain its contents, while a ROM cell might consist of the absence (logical 0) or presence (logical one) of one transistor connecting a bit line to a word line.^[11] Consequently, ROM could exist implemented at a lower toll-per-bit than RAM for many years.

Most dwelling computers of the 1980s stored a BASIC interpreter or operating system in ROM equally other forms of non-volatile storage such as magnetic disk drives were also costly. For example, the Commodore 64 included 64 KB of RAM and 20 KB of ROM containing a BASIC interpreter and the KERNAL operating system. Later abode or role computers such as the IBM PC XT often included magnetic disk 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-compatible computers). This arrangement allowed for a more complex and easily upgradeable operating arrangement.

In modern PCs, "ROM" is used to store the basic bootstrapping firmware for the processor, as well as the various firmware needed to internally control self-contained devices such as graphic cards, hd drives, solid state drives, optical disc drives, TFT screens, etc., in the arrangement. Today, many of these "read-but" memories – especially the BIOS/UEFI – are often replaced with EEPROM or Flash retention (see below), to permit in-place reprogramming should the demand for a firmware upgrade arise. However, simple and mature sub-systems (such as the keyboard or some communication controllers in the integrated circuits on the main board, for instance) may utilize mask ROM or OTP (one-time programmable).

ROM and successor technologies such as flash are prevalent in embedded systems. These are in everything from industrial robots to home appliances and consumer electronics (MP3 players, set up-peak boxes, etc.) all of which are designed for specific functions, just are based on general-purpose microprocessors. With software usually tightly coupled to hardware, program changes are rarely needed in such devices (which typically lack hard disks for reasons of toll, size, or ability consumption). Equally of 2008, most products utilize Wink rather than mask ROM, and many provide some means for connecting to a PC for firmware updates; for example, a digital audio actor might be updated to back up a new file format. Some hobbyists take taken advantage of this flexibility to reprogram consumer products for new purposes; for example, 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 difficult to supplant, which may be desirable in order to enhance information security.

Use for storing data [edit]

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

Notably, the display adapters of early on personal computers stored tables of bitmapped font characters in ROM. This usually meant that the text display font could not 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 store such small amounts of data has disappeared almost completely in modern general-purpose computers. Even so, NAND Wink has taken over a new role as a medium for mass storage or secondary storage of files.

Types [edit]

Manufacturing plant programmed [edit]

Mask ROM is a read-simply memory whose contents are programmed past the integrated circuit manufacturer (rather than past the user). The desired retention contents are furnished by the customer to the device manufacturer. The desired data is converted into a custom mask layer for the final metallization of interconnections on the memory chip (hence the name).

It is mutual practice to utilise rewritable non-volatile memory – such every bit UV-EPROM or EEPROM – for the development stage of a project, and to switch to mask ROM when the code has been finalized. For example, Atmel microcontrollers come in both EEPROM and mask ROM formats.

The main reward of mask ROM is its cost. Per bit, mask ROM is more compact than any other kind of semiconductor memory. Since the cost of an integrated excursion strongly depends on its size, mask ROM is significantly cheaper than whatsoever other kind of semiconductor memory.

However, the i-time masking cost is loftier and there is a long plough-effectually time from design to product stage. Pattern errors are costly: if an fault in the information or code is institute, the mask ROM is useless and must exist replaced in order to change the code or data. ^[12]

Every bit of 2003, four companies produce most such mask ROM fries: Samsung Electronics, NEC Corporation, Oki Electric Manufacture, and Macronix.^{[13] [ needs update ]}

Some integrated circuits contain just mask ROM. Other integrated circuits comprise mask ROM as well as a variety of other devices. In particular, many microprocessors accept mask ROM to store their microcode. Some microcontrollers have mask ROM to store the bootloader or all of their firmware.

Archetype mask-programmed ROM chips are integrated circuits that physically encode the information to be stored, and thus it is impossible to alter their contents after fabrication.

Field programmable [edit]

• Programmable read-simply memory (PROM), or erstwhile programmable ROM (OTP), can be written to or programmed via a special device called a PROM developer. Typically, this device uses loftier voltages to permanently destroy or create internal links (fuses or antifuses) within the chip. Consequently, a PROM can just be programmed one time.
• Erasable programmable read-only memory (EPROM) can be erased by exposure to strong ultraviolet light (typically for 10 minutes or longer), and so rewritten with a procedure that again needs higher than usual voltage practical. Repeated exposure to UV lite volition eventually wear out an EPROM, just 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. After programming, the window is typically covered with a label to prevent accidental erasure. Some EPROM chips are mill-erased before they are packaged, and include no window; these are effectively PROM.
• Electrically erasable programmable read-only retention (EEPROM) is based on a similar semiconductor structure to EPROM, but allows its unabridged contents (or selected banks) to be electrically erased, then rewritten electrically, then that they need not be removed from the computer (whether full general-purpose or an embedded computer in a camera, MP3 player, etc.). Writing or flashing an EEPROM is much slower (milliseconds per scrap) than reading from a ROM or writing to a RAM (nanoseconds in both cases).
  • Electrically alterable read-merely memory (EAROM) is a type of EEPROM that tin be modified i flake at a time. Writing is a very slow procedure and again needs higher voltage (usually effectually 12 V) than is used for read access. EAROMs are intended for applications that crave infrequent and but fractional rewriting. EAROM may exist used every bit non-volatile storage for critical arrangement setup information; in many applications, EAROM has been supplanted by CMOS RAM supplied by mains power and backed-up with a lithium battery.
  • Flash memory (or simply wink) is a modern type of EEPROM invented in 1984. Wink retentivity tin be erased and rewritten faster than ordinary EEPROM, and newer designs feature very high endurance (exceeding ane,000,000 cycles). Modern NAND flash makes efficient use of silicon chip area, resulting in private ICs with a chapters as high equally 32 GB as of 2007^[update]; this feature, along with its endurance and physical durability, has allowed NAND flash to replace magnetic in some applications (such as USB wink drives). NOR flash retentivity is sometimes called flash ROM or flash EEPROM when used as a replacement for older ROM types, but not in applications that take reward of its ability to be modified quickly and frequently.

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

Other technologies [edit]

There are other types of non-volatile retentiveness which are not based on solid-state IC applied science, including:

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

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

• Diode matrix ROM, used in minor amounts in many computers in the 1960s every bit well equally electronic desk calculators and keyboard encoders for terminals. This ROM was programmed by installing discrete semiconductor diodes at selected locations between a matrix of word line traces and scrap line traces on a printed circuit board.
• Resistor, capacitor, or transformer matrix ROM, used in many computers until the 1970s. Similar diode matrix ROM, information technology was programmed by placing components at selected locations betwixt a matrix of word lines and scrap lines. ENIAC's Function Tables were resistor matrix ROM, programmed past manually setting rotary switches. Various models of the IBM System/360 and complex peripheral devices stored their microcode in either capacitor (called BCROS for balanced capacitor read-merely storage on the 360/fifty and 360/65, or CCROS for carte capacitor read-only storage on the 360/30) or transformer (called TROS for transformer read-merely storage on the 360/xx, 360/40 and others) matrix ROM.
• Core rope, a form of transformer matrix ROM technology used where size and weight were critical. This was used in NASA/MIT'south Apollo Spacecraft Computers, DEC's PDP-8 computers, the Hewlett-Packard 9100A calculator, and other places. This type of ROM was programmed by hand by weaving "word line wires" inside or outside of ferrite transformer cores.
• Diamond Ring stores, in which wires are threaded through a sequence of large ferrite rings that part only as sensing devices. These were used in TXE phone exchanges.

Speed [edit]

Although the relative speed of RAM vs. ROM has varied over time, as of 2007^[update] large RAM chips can be read faster than near ROMs. For this reason (and to allow uniform access), ROM content is sometimes copied to RAM or adumbral earlier its commencement use, and afterward read from RAM.

Writing [edit]

For those types of ROM that can be electrically modified, writing speed has traditionally been much slower than reading speed, and it may need unusually high voltage, the movement of jumper plugs to employ write-enable signals, and special lock/unlock command codes. Mod NAND Flash achieves the highest write speeds of any rewritable ROM technology, with speeds equally loftier equally 10 GB/s. This has been enabled past the increased investment in both consumer and enterprise solid state drives and flash memory products for higher end mobile devices. On a technical level the gains have been achieved past increasing parallelism both in controller blueprint and of storage, the use of large DRAM read/write caches and the implementation of retentiveness cells which tin store more than than one fleck (DLC, TLC and MLC). The latter approach is more failure decumbent but this has been largely mitigated by overprovisioning (the inclusion of spare capacity in a product which is visible only to the bulldoze controller) and by increasingly sophisticated read/write algorithms in drive firmware.

Endurance and data memory [edit]

Because they are written by forcing electrons through a layer of electrical insulation onto a floating transistor gate, rewriteable ROMs can withstand just a limited number of write and erase cycles before the insulation is permanently damaged. In the primeval EPROMs, this might occur after every bit few every bit one,000 write cycles, while in modern Wink EEPROM the endurance may exceed 1,000,000. The limited endurance, as well as the higher price per bit, means that Flash-based storage is unlikely to completely supplant magnetic disk drives in the near future.^{[ citation needed ]}

The timespan over which a ROM remains accurately readable is not express by write cycling. The data retention of EPROM, EAROM, EEPROM, and Flash may be time-limited by charge leaking from the floating gates of the memory cell transistors. Early generation EEPROM'due south, in the mid 1980'south mostly cited 5 or 6 year data retention. A review of EEPROM'due south offered in the year 2022 shows manufacturers citing 100 year data retention. Adverse environments will reduce the retention time (leakage is accelerated past loftier temperatures or radiation). Masked ROM and fuse/antifuse PROM do non suffer from this effect, as their data retention depends on physical rather than electric permanence of the integrated circuit, although fuse re-growth was one time a trouble in some systems.^[14]

Content images [edit]

The contents of ROM chips can be extracted with special hardware devices and relevant controlling software. This practice is common for, every bit a primary case, reading the contents of older video game console cartridges. Another case is making backups of firmware/OS ROMs from older computers or other devices - for archival purposes, equally in many cases, the original chips are PROMs and thus at chance of exceeding their usable data lifetime.

The resultant retentiveness dump files are known as ROM images or abbreviated ROMs, and tin be used to produce duplicate ROMs - for example to produce new cartridges or every bit digital files for playing in console emulators. The term ROM image originated when most console games were distributed on cartridges containing ROM fries, but accomplished such widespread usage that it is still applied to images of newer games distributed on CD-ROMs or other optical media.

ROM images of commercial games, firmware, etc. usually contain copyrighted software. The unauthorized copying and distribution of copyrighted software is a violation of copyright laws in many jurisdictions, although duplication for backup purposes may exist considered fair use depending on location. In any case, there is a thriving customs engaged in the distribution and trading of such software and abandonware for preservation/sharing purposes.

Timeline [edit]

Date of introduction	Chip proper noun	Capacity (bits)	ROM type	MOSFET	Manufacturer(due south)	Process	Expanse	Ref
1956	?	?	PROM	?	Arma	?	?	[2] [3]
1965	?	256-scrap	ROM	Bipolar TTL	Sylvania	?	?	[15]
1965	?	1 kb	ROM	MOS	General Microelectronics	?	?
1969	3301	i kb	ROM	Bipolar	Intel	?	?	[15]
1970	?	512-scrap	PROM	Bipolar TTL	Radiation	?	?	[6]
1971	1702	two kb	EPROM	Static MOS (silicon gate)	Intel	?	xv mm²	[6] [16]
1974	?	four kb	ROM	MOS	AMD, Full general Musical instrument	?	?	[15]
1974	?	?	EAROM	MNOS	General Instrument	?	?	[6]
1975	2708	eight kb	EPROM	NMOS (FGMOS)	Intel	?	?	[17] [18]
1976	?	two kb	EEPROM	MOS	Toshiba	?	?	[xix]
1977	µCOM-43 (PMOS)	16 kb	PROM	PMOS	NEC	?	?	[20]
1977	2716	16 kb	EPROM	TTL	Intel	?	?	[21] [22]
1978	EA8316F	16 kb	ROM	NMOS	Electronic Arrays	?	436 mm²	[15] [23]
1978	µCOM-43 (CMOS)	16 kb	PROM	CMOS	NEC	?	?	[20]
1978	2732	32 kb	EPROM	NMOS (HMOS)	Intel	?	?	[17] [24]
1978	2364	64 kb	ROM	NMOS	Intel	?	?	[25]
1980	?	16 kb	EEPROM	NMOS	Motorola	4,000 nm	?	[17] [26]
1981	2764	64 kb	EPROM	NMOS (HMOS II)	Intel	three,500 nm	?	[17] [26] [27]
1982	?	32 kb	EEPROM	MOS	Motorola	?	?	[26]
1982	27128	128 kb	EPROM	NMOS (HMOS II)	Intel	?	?	[17] [26] [28]
1983	?	64 kb	EPROM	CMOS	Signetics	3,000 nm	?	[26]
1983	27256	256 kb	EPROM	NMOS (HMOS)	Intel	?	?	[17] [29]
1983	?	256 kb	EPROM	CMOS	Fujitsu	?	?	[30]
January 1984	MBM 2764	64 kb	EEPROM	NMOS	Fujitsu	?	528 mm²	[31]
1984	?	512 kb	EPROM	NMOS	AMD	i,700 nm	?	[26]
1984	27512	512 kb	EPROM	NMOS (HMOS)	Intel	?	?	[17] [32]
1984	?	one Mb	EPROM	CMOS	NEC	one,200 nm	?	[26]
1987	?	4 Mb	EPROM	CMOS	Toshiba	800 nm	?	[26]
1990	?	xvi Mb	EPROM	CMOS	NEC	600 nm	?	[26]
1993	?	8 Mb	MROM	CMOS	Hyundai	?	?	[33]
1995	?	1 Mb	EEPROM	CMOS	Hitachi	?	?	[34]
1995	?	16 Mb	MROM	CMOS	AKM, Hitachi	?	?	[34]

See also [edit]

• Flash memory
• Random access retentivity
• Read-mostly memory (RMM)
• Write-only memory

Notes [edit]

1. ^ Some discrete component ROM could be mechanically altered, e.g., by adding and removing transformers. IC ROMs, however, cannot be mechanically inverse.
2. ^ Other terms are used as well, east.g., "Initial Program Load" (IPL).

References [edit]

This audio file was created from a revision of this article dated 12 April 2005 (2005-04-12), and does not reflect subsequent edits.

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23. ^ "1982 Itemize" (PDF). NEC Electronics. Retrieved 20 June 2019.
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Source: https://en.wikipedia.org/wiki/Read-only_memory

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