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Wednesday, July 22, 2015

History of hard disk drives

History of hard disk drives

In 1953 IBM recognized the immediate application for what it termed a "Random Access File" having high capacity and rapid random access at a relatively low cost.[1] After considering technologies such as wire matrices, rod arrays, drums, drum arrays, etc.,[1] the engineers at IBM's San Jose California laboratory invented the hard disk drive.[2] The disk drive created a new level in the computer data hierarchy, then termed Random Access Storage but today known as secondary storage, less expensive and slower than main memory (then typically drums) but faster and more expensive than tape drives.[3]
The commercial usage of hard disk drives began in 1956 with the shipment of an IBM 305 RAMAC system including IBM Model 350 disk storage.[4] US Patent 3,503,060issued March 24, 1970, and arising from the IBM RAMAC program is generally considered to be the fundamental patent for disk drives.[5]
Each generation of disk drives replaced larger, more sensitive and more cumbersome devices. The earliest drives were usable only in the protected environment of a data center. Later generations progressively reached factories, offices and homes, eventually reaching ubiquity.
Disk media diameter was nominally 8 or 14 inches (200 or 360 mm) and were typically mounted in standalone boxes (resembling washing machines) or large equipment rack enclosures. Individual drives often required high-current AC power due to the large motors required to spin the large disks. Hard disk drives were not commonly used with microcomputers until after 1980, when Seagate Technology introduced the ST-506, the first 5.25 inches (133 mm) drive.
The capacity of hard drives has grown exponentially over time. When hard drives became available for personal computers, they offered 5-megabyte capacity. During the mid-1990s the typical hard disk drive for a PC had a capacity of about 1 gigabyte.[6] As of December 2014, desktop hard disk drives typically had a capacity of 500 to 4000 gigabytes, while the largest-capacity drives were 8 terabytes.

Contents

1950s–1970s

Main article: Early IBM disk storage
A partially disassembled IBM 350 (RAMAC)
Removable disk packs
A removable 14 inch disk pack for a disk drive. Protective cover is removed. Read/write heads stayed in the drive, only the media platters were removable.
The IBM 350 Disk File, invented by Reynold Johnson, was introduced in 1956 with the IBM 305 RAMAC computer. This drive had fifty 24 inches (0.61 m) platters, with a total capacity of five million 6-bit characters (3.75 megabytes).[7] A single head assembly having two heads was used for access to all the platters, yielding an average access time of just under 1 second.
The IBM 1301 Disk Storage Unit,[8] announced in 1961, introduced the usage of heads having self-acting air bearings (self-flying heads) with one head per each surface of the disks.
Also in 1961, Bryant Computer Products introduced its 4000 series disk drives. These massive units stood 52 inches (1.3 m) tall, 70 inches (1.8 m) wide, and had up to 26 platters, each 39 inches (0.99 m) in diameter, rotating at up to 1,200 rpm. Access times were from 50 to 205 milliseconds (ms). The drive's total capacity, depending on the number of platters installed, was up to 205,377,600 bytes (205 MB).[9][10]
The first disk drive to use removable media was the IBM 1311 drive. It was introduced in 1962 using the IBM 1316 disk pack to store two million characters. It was followed by the IBM 2311 (1964) 5 megabyte and IBM 2314 (1965) 29 megabyte disk pack HDDs.
Memorex in 1968 shipped the first HDD, the Memorex 630, plug compatible to an IBM model 2311 marking the beginning of independent competition (Plug Compatible Manufacturers or PCMs) for HDDs attached to IBM systems. It was followed in 1969 by the Memorex 660, an IBM 2314 compatible, which was OEM'ed to DEC and resold as the RP02.
In 1973, IBM introduced the IBM 3340 "Winchester" disk drive, the first significant commercial use of low mass and low load heads with lubricated platters. This technology and its derivatives remained the standard through 2011. Project head Kenneth Haughton named it after the Winchester 30-30 rifle because it was planned to have two 30 MB spindles; however, the actual product shipped with two spindles for data modules of either 35 MB or 70 MB.[11] The name 'Winchester' and some derivatives are still common in some non-English speaking countries to generally refer to any hard disks (e.g. Hungary, Russia).
Also in 1973, Control Data Corporation introduced the first of its series of SMD disk drives using conventional disk pack technology. The SMD family became the predominant disk drive in the minicomputer market into the 1980s.
During the 1970s, captive production, dominated by IBM's production for its own use, remained the largest revenue channel for HDDs, though the relative importance of the OEM channel grew. Led by Control Data, Diablo Systems, CalComp and Memorex, the OEMsegment reached $631 million in 1979, but still well below the $2.8 billion associated with captive production.[12]

1980s, the PC era

As the 1980s began, hard disk drives were a rare and very expensive optional feature on personal computers (PCs); however by the late '80s, hard disk drives were standard on all but the cheapest PC.
Most hard disk drives in the early 1980s were sold to PC end users by systems integrators such as the Corvus Disk System or the systems manufacturer such as the Apple ProFile. The IBM PC XT in 1983 included an internal standard 10MB hard disk drive, and soon thereafter internal hard disk drives proliferated on personal computers.

Timeline

  • 1956 – IBM 350, first commercial disk drive, 5 million characters (6-bit).
  • 1961 – IBM 1301 Disk Storage Unit introduced with one head per surface and aerodynamic flying heads, 28 million characters (6-bit) per module.
  • 1961 – Bryant Computer Products division of Ex-Cell-O, 1 meter platters, 1200 RPM, up to 205MB.
  • 1962 – IBM 1311 introduced removable disk packs containing 6 disks, storing 2 million characters per pack
  • 1964 – IBM 2311 with 7.25 megabytes per disk pack
  • 1964 – IBM 2310 removable cartridge disk drive with 1.02 MB on one disk
  • 1965 – IBM 2314 with 11 disks and 29 MB per disk pack
  • 1968 – Memorex is first to ship an IBM-plug-compatible disk drive
  • 1970 – IBM 3330 Merlin, introduced error correction, 100 MB per disk pack
  • 1973 – IBM 3340 Winchester introduced removable sealed disk packs that included head and arm assembly, 35 or 70 MB per pack
  • 1973 – CDC SMD announced and shipped, 40 MB disk pack
  • 1976 – 1976 IBM 3350 "Madrid" – 317.5 Megabytes, eight 14" disks, Re-introduction of disk drive with fixed disk media
  • 1979 – IBM 3370 introduced thin film heads, 571 MB, non-removable
  • 1979 – 1979 IBM 62PC "Piccolo" – 64.5 Megabytes, six 8" disks, First 8-inch HDD
  • 1980 – The IBM 3380 was the world's first gigabyte-capacity disk drive. Two 1.26 GB, head disk assemblies (essentially two HDDs) were packaged in a cabinet the size of a refrigerator,[13] weighed 249 kg, and had a price tag of 40,000 USD which is 114,491 USD in present day terms.[14]
  • 1980 – ST-506 first 51⁄4 inch drive released with capacity of 5 megabytes, cost 1500 USD
  • 1983 – RO351/RO352 first 31⁄2 inch drive released with capacity of 10 megabytes [15]
  • 1986 – Standardization of SCSI
  • 1988 – PrairieTek 220 – 20 Megabytes, two 2.5" disks, First 2.5 inch HDD.
  • 1989 – Jimmy Zhu and H. Neal Bertram from UCSD proposed exchange decoupled granular microstructure for thin film disk storage media, still used today.
  • 1990 – 1990 IBM 0681 "Redwing" – 857 Megabytes, twelve 5.25" disks. First HDD with PRML Technology (Digital Read Channel with 'partial response maximum likelihood' algorithm)
  • 1991 – IBM 0663 "Corsair" – 1,004 Megabytes, eight 3.5" disks; first HDD using magnetoresistive heads
  • 1991 – Integral Peripherals 1820 "Mustang" – 21.4 Megabytes, one 1.8" disk, first 1.8 inch HDD[16]
  • 1992 – HP Kittyhawk first 1.3-inch hard-disk drive –
  • 1993 – IBM 3390 model 9, the last Single Large Expensive Disk drive announced by IBM
  • 1994 – IBM introduces Laser Textured Landing Zones (LZT)
  • 1997 – IBM Deskstar 16GP "Titan" – 16,800 Megabytes, five 3.5" disks; first (Giant Magnetoresistance) heads
  • 1997 – Seagate introduces the first hard drive with fluid bearings[17]
  • 1998 – UltraDMA/33 and ATAPI standardized
  • 1999 – IBM releases the Microdrive in 170 MB and 340 MB capacities
  • 2002 – 137 GB addressing space barrier broken
  • 2003 – Serial ATA introduced
  • 2003 – IBM sells disk drive division to Hitachi
  • 2004 – MK2001MTN first 0.85 inch drive released by Toshiba with capacity of 2 gigabytes[16]
  • 2005 – First 500 GB hard drive shipping (Hitachi GST)
  • 2005 – Serial ATA 3Gbit/s standardized
  • 2005 – Seagate introduces Tunnel MagnetoResistive Read Sensor (TMR) and Thermal Spacing Control
  • 2005 – Introduction of faster SAS (Serial Attached SCSI)
  • 2005 – First perpendicular magnetic recording (PMR) HDD shipped: Toshiba 1.8-inch 40/80 GB[18]
  • 2006 – First 750 GB hard drive (Seagate)
  • 2006 – First 200 GB 2.5" hard drive utilizing perpendicular recording (Toshiba)
  • 2006 – Fujitsu develops heat-assisted magnetic recording (HAMR) that could one day achieve one terabit per square inch densities.[19]
  • 2007 – First 1 terabyte[20] hard drive[21] (Hitachi GST)
  • 2008 – First 1.5 terabyte[20] hard drive[22] (Seagate)
  • 2009 – First 2.0 terabyte hard drive[23] (Western Digital)
  • 2010 – First 3.0 terabyte hard drive[24][25] (Seagate, Western Digital)
  • 2010 – First Hard Drive Manufactured by using the Advanced Format of 4,096 bytes a block ("4K") instead of 512 bytes a block[26]
  • 2011 – First 4.0 terabyte hard drive[27] (Seagate)
  • 2011 - Floods hit many hard drive factories. Predictions of a worldwide shortage of hard disk drives cause prices to double.[28][29][30]
  • 2012 – Western Digital announces the first 2.5-inch, 5mm thick drive, and the first 2.5-inch, 7mm thick drive with two platters.[31] (Western Digital)
  • 2012 – HGST announces helium-filled hard disk drives, promising cooler operation and the ability to increase the maximum number of platters from five to seven in the 3.5" form factor.[32] (Hitachi GST)
  • 2012 – TDK demonstrates 2 TB on a single 3.5-inch platter [33]
  • 2012 – Toshiba re-enters the 3.5" desktop hard disk drive market with capacities up to 3 TB.[34] This is made possible by the U.S. Federal Trade Commission demanding that Western Digital and Hitachi GST give assets and intellectual property rights to Toshiba.[35] Prior to this, Toshiba had only manufactured 2.5" laptop HDDs for many years.
  • 2013 – Seagate announces that it will ship hard disk drives with capacities up to 5 TB using shingled magnetic recording (SMR), a method where tracks are written to partially overlap each other. The read head, being smaller, can still read the overlapped tracks.[36]
  • 2013 – HGST announces a helium-filled 6 TB hard disk drive for enterprise applications.[37]
  • 2013 – Western Digital demonstrates heat assisted magnetic recording (HAMR) technology.[38][39][40][41]
  • 2014 – Seagate introduces 6 TB hard drives that do not use helium, in turn increasing their power consumption and lowering their overall cost.[42]
  • 2014 – Seagate ships worlds first 8 TB hard drives.[43]
  • 2014 - Western Digital's HGST subsidiary sampling 10 TB hard drives filled with helium.[44]

Manufacturing history

A Western Digital 3.5 inch 250 GBSATA HDD; this specific model features both SATA and Molex power inputs
Seagate hard disk drives being manufactured in a factory in Wuxi,China
See also List of defunct hard disk manufacturers
Diagram of HDD manufacturer consolidation
As of December 2011, virtually all of the world's HDDs were manufactured by three large companies: Seagate,[45] Western Digital, andToshiba. Hitachi (HGST) was acquired by Western Digital in 2012.[46]
The market has continued to consolidate since the 1980s as dozens of manufacturers exited or were acquired. The first notable casualty in the PC era was Computer Memories Inc. or CMI; after an incident with faulty 20MB AT disks in 1985,[47] CMI's reputation never recovered, and they exited the HDD business in 1987. Another notable failure was MiniScribe, which went bankrupt in 1990 after it was found that they had engaged in accounting fraud and inflated sales numbers for several years. Many other companies (like Kalok,Microscience, LaPine, Areal, Priam, and PrairieTek) also did not survive the shakeout, and had disappeared by 1993; Micropolis was able to hold on until 1997, and JTS, a relative latecomer, lasted only a few years and was gone by 1999, after attempting to manufacture in India. JTS originated a 3″ form factor for use in laptop computers. Quantum and Integral also invested in the 3″ form factor; but the form factor failed to catch on. Rodime was an important manufacturer during the 1980s, but stopped making disks in the early 1990s to concentrate on technology licensing; they hold a number of patents related to 3.5-inch form factor HDDs.
The following is the genealogy of the remaining participants:
In 2011, based on market research firm IDC, the biggest hard drive makers were Seagate Technology and Western Digital Corp., but the largest national producer was China, followed by Thailand which makes about a quarter of the world's hard drives. The concentration of hard disk drive producers in only a few countries made the supply vulnerable to disruptions like the 2011 Thailand floods.[53]



HARD DISK DRIVE GUIDE
A Brief History of the Hard Disk Drive

The hard disk drive has short and fascinating history.  In 24 years it evolved from a monstrosity with fifty two-foot diameter disks holding five MBytes (5,000,000 bytes) of data to today's drives measuring 3 /12 inches wide and an inch high (and smaller) holding 400 GBytes (400,000,000,000 bytes/characters).  Here, then, is the short history of this marvelous device.
Before the disk drive there were drums... In 1950 Engineering Research Associates of Minneapolis built the first commercial magnetic drum storage unit for the U.S. Navy, the ERA 110.  It could store one million bits of data and retrieve a word in 5 thousandths of a second.
In 1956 IBM invented the first computer disk storage system, the 305 RAMAC (Random Access Method of Accounting and Control).  This system could store five MBytes.  It had fifty, 24-inch diameter disks!
By 1961 IBM had invented the first disk drive with air bearing heads and in 1963 they introduced the removable disk pack drive.
In 1970 the eight inch floppy disk drive was introduced by IBM.  My first floppy drives were made by Shugart who was one of the "dirty dozen" who left IBM to start their own companies.  In 1981 two Shugart 8 inch floppy drives with enclosure and power supply cost me about $350.00.  They were for my second computer.  My first computer had no drives at all.
In 1973 IBM shipped the model 3340 Winchester sealed hard disk drive, the predecessor of all current hard disk drives.  The 3340 had two spindles each with a capacity of 30 MBytes, and the term "30/30 Winchester" was thus coined.
Seagate ST4053 40 MByte
5 1/4 inch, full-height "clunker"
with ST506 interface and voice coil
circa 1987. My cost was $435.00.
In 1980, Seagate Technology introduced the first hard disk drive for microcomputers, the ST506.  It was a full height (twice as high as most current 5 1/4" drives) 5 1/4" drive, with a stepper motor, and held 5 Mbytes.  My first hard disk drive was an ST506.  I cannot remember exactly how much it cost, but it plus its enclosure, etc. was well over a thousand dollars.  It took me three years to fill the drive.  Also, in 1980 Phillips introduced the first optical laser drive.  In the early 80's, the first 5 1/4" hard disks with voice coil actuators (more on this later) started shipping in volume, but stepper motor drives continued in production into the early 1990's.   In 1981, Sony shipped the first 3 1/2" floppy drives.
In 1983 Rodime made the first 3.5 inch rigid disk drive.  The first CD-ROM drives were shipped in 1984, and "Grolier's Electronic Encyclopedia," followed in 1985.  The 3 1/2" IDE drive started its existence as a drive on a plug-in expansion board, or "hard card."  The hard card included the drive on the controller which, in turn, evolved into Integrated Device Electronics (IDE) hard disk drive, where the controller became incorporated into the printed circuit on the bottom of the hard disk drive.   Quantum made the first hard card in 1985.
In 1986 the first 3 /12" hard disks with voice coil actuators were introduced by Conner in volume, but half (1.6") and full height 5 1/4" drives persisted for several years.  In 1988 Conner introduced the first one inch high 3 1/2" hard disk drives.  In the same year PrairieTek shipped the first 2 1/2" hard disks.
In 1997 Seagate introduced the first 7,200 RPM, Ultra ATA hard disk drive for desktop computers and in February of this year they introduced the first 15,000 RPM hard disk drive, the Cheetah X15.  Milestones for IDE DMA, ATA/33, and ATA/66 drives follow:
  • 1994 DMA, Mode 2 at 16.6 MB/s
  • 1997 Ultra ATA/33 at 33.3 MB/s
  • 1999 Ultra ATA/66 at 66.6 MB/s
6/20/00  IBM triples the capacity of the world's smallest hard disk drive.  This drive holds one gigabyte on a disk which is the size of an American quarter.  The world's first gigabyte-capacity disk drive, the IBM 3380, introduced in 1980, was the size of a refrigerator, weighed 550 pounds (about 250 kg), and had a price tag of $40,000.
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ROM

ROM

Short for Read-Only Memory, ROM is a storage medium that is used with computers and other electronic devices. As the name indicates, data stored in ROM may only be read; it is either modified with extreme difficulty or not at all. ROM is mostly used for firmware updates. A simple example of ROM is the cartridge used with video game consoles; which allows one system to run multiple games. Another example of ROM is EEPROM, which is a programmable ROM used for the computer BIOS, as shown in the picture.
AMIBIOS BIOS
Note: Unlike Random Access Memory (RAM), ROM is non-volatile which means it keeps its contents regardless of whether or not it has power.

Related pages

History

Many game consoles use interchangeable ROM cartridges, allowing for one system to play multiple games.
Read-only memory was used for Jacquard looms.[2]
The simplest type of solid state ROM is as old as semiconductor technology itself. Combinational logic gates can be joined manually to map n-bit address input onto arbitrary values of m-bit data output (a look-up table). With the invention of the integrated circuitcame mask ROM. Mask ROM consists of a grid of word lines (the address input) and bit lines (the data output), selectively joined together with transistor switches, and can represent an arbitrary look-up table with a regular physical layout and predictablepropagation delay.
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 economical to buy 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 product is long, for the same reason.
  3. Mask ROM is impractical for R&D work since designers frequently need to modify the contents of memory as 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 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 application of high-voltage pulses. This addressed problems 1 and 2 above, since a company can simply order a large batch of fresh PROM chips and program them with the desired contents at its designers' convenience. The 1971 invention of EPROM essentially solved problem 3, since EPROM (unlike 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 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 computer via a serial cable). Flash 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, so that in large quantities mask ROM would remain an economical choice for many years. (Decreasing cost of reprogrammable devices had almost eliminated the market for mask ROM by the year 2000.) Rewriteable technologies were envisioned as replacements for mask ROM.
The most recent development is NAND flash, also invented at Toshiba. Its designers explicitly broke from past practice, stating plainly that "the aim of NAND Flash is to replacehard disks,"[3] rather than the traditional use of ROM as a form of non-volatile primary storage. As 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 memory cards, for example), and much lower power consumption.

Use for storing programs

Every stored-program computer may use a form of non-volatile storage (that is, storage that retains its data when power is removed) to store the initial program that runs when the computer is powered on or otherwise begins execution (a process known as bootstrapping, often abbreviated to "booting" or "booting up"). Likewise, every non-trivial computer needs some form of mutable memory to record changes in its state as it executes.
Forms of read-only memory were employed as non-volatile storage for programs in most early stored-program computers, such as ENIAC after 1948. (Until then it was not a stored-program computer as 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 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 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 5-20 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.[4] Consequently, ROM could be implemented at a lower cost-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 non-volatile storage such as magnetic disk drives were too costly. For example, the Commodore 64 included 64 KB of RAM and 20 KB of ROM contained a BASIC interpreter and the "KERNAL" of its operating system. Later home or office 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 only a minimal hardware initialization core and bootloader remaining in ROM (known as the BIOS in IBM-compatible computers). This arrangement allowed for a more complex and easily upgradeable operating system.
In modern PCs, "ROM" (or flash) is used to store the basic bootstrapping firmware for the main processor, as well as the various firmware needed to internally control self-contained devices such as graphic cards, hard disks, DVD drives, TFT screens, etc., in the system. Today, many of these "read-only" 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 integrated circuits on the main board, for example) may employ 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-top boxes, etc.) all of which are designed for specific functions, but 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 cost, size, or power consumption). As of 2008, most products use Flash rather than mask ROM, and many provide some means for connecting to a PC for firmware updates; for example, a digital audio player might be updated to support a newfile format. Some hobbyists have 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 it makes them difficult to replace, which may be desirable in order to enhance information security.

Use for storing data

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 end, 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 function in order 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 usually meant that the text display font could not be changed interactively. This was the case for both the CGA and MDA adapters available 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. 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 andwire bonds clearly visible through the erase window.

Semiconductor based[edit]

Classic 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-only 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 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-only memory (EPROM) can be erased by exposure to strong ultraviolet light (typically for 10 minutes or longer), then rewritten with a process that again needs higher than usual voltage applied. Repeated exposure to UV light will eventually wear out an EPROM, but the endurance of most EPROM chips exceeds 1000 cycles of erasing and reprogramming. EPROM chip packages can 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 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, but allows its entire contents (or selected banks) to be electrically erased, then rewritten electrically, so that they need not be removed from the computer (or camera, MP3 player, etc.). Writing or flashing an EEPROM is much slower (milliseconds per bit) 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 again needs higher voltage (usually around 12 V) than is used for read access. EAROMs are intended for applications that require infrequent and only 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-up with a lithium battery.
    • Flash memory (or simply flash) is a modern type of EEPROM invented in 1984. Flash memory can be erased and rewritten faster than ordinary EEPROM, and newer designs feature very high endurance (exceeding 1,000,000 cycles). Modern NAND flash makes efficient use of silicon chip area, resulting in individual ICs with a capacity as high as 32 GB as of 2007; this feature, along with its endurance and physical durability, has allowed NAND flash to replace 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, but not in applications that take advantage of its ability to be modified quickly and frequently.
By applying write protection, some types of reprogrammable ROMs may temporarily become read-only memory.

Other technologies

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

Historical examples

Transformer matrix ROM (TROS), from the IBM System 360/20
  • Diode matrix ROM, used in small amounts in many computers in the 1960s as well as electronic desk calculators andkeyboard encoders for terminals. This ROM was programmed by installing discrete semiconductor diodes at selected locations between a matrix of word line traces and bit line traces on a printed circuit board.
  • Resistor, capacitor, or transformer matrix ROM, used in many computers until the 1970s. Like diode matrix ROM, it was programmed by placing components at selected locations between a matrix of word lines and bit lines. ENIAC's Function Tables were resistor matrix ROM, programmed by manually setting rotary switches. Various models of the IBMSystem/360 and complex peripheral devices stored their microcode in either capacitor (called BCROS for balanced capacitor read-only storage on the 360/50 and 360/65, or CCROS for charged capacitor read-only storage on the 360/30) or transformer (called TROS for transformer read-only storage on the 360/20, 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 inNASA/MIT's Apollo Spacecraft Computers, DEC's PDP-8 computers, and other places. This type of ROM was programmed by hand by weaving "word line wires" inside or outside of ferrite transformer cores.
  • Dimond Ring stores, in which wires are threaded through a sequence of large ferrite rings that function only as 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 form a selected character shape as an overlay on a video signal.

Speed

Reading

Although the relative speed of RAM vs. ROM has varied over time, as of 2007 large RAM chips can be read faster than most ROMs. For this reason (and to allow uniform access), ROM content is sometimes copied to RAM or shadowed before its first use, and subsequently read from RAM.

Writing

For those types of ROM that can be electrically modified, writing speed is always much slower than reading speed, and it may need unusually high voltage, the movement of jumper plugs to apply write-enable signals, and special lock/unlock command codes. Modern NAND Flash achieves the highest write speeds of any rewritable ROM technology, with speeds as high as 15 MB/s (or 70 ns/bit), by allowing (needing) large 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 before the insulation is permanently damaged. In the earliest EAROMs, this might occur after as few as 1,000 write cycles, while in modern Flash EEPROM theendurance may exceed 1,000,000, but it is by no means infinite. This limited endurance, as well as the higher cost per bit, means that Flash-based storage is unlikely to completely supplant magnetic disk drives in the near future.
The timespan over which a ROM remains accurately readable is not limited by write cycling. The data retention of EPROM, EAROM, EEPROM, and Flash may be limited by charge leaking from the floating gates of the memory cell transistors. Leakage is accelerated by high temperatures or radiation. Masked ROM and fuse/antifuse PROM do not suffer from this effect, as their data retention depends on physical rather than electrical permanence of the integrated circuit (although fuse re-growth was once a problem in some systems).

Content images

Main article: ROM image
The contents of ROM chips in video game console cartridges can 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 most console games were distributed on cartridges containing ROM chips, but achieved 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 usually 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 use). 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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