Read-only memory: Difference between revisions

'''Read-only memory''' ('''ROM''') is a type of [[non-volatile memory]] used in [[computers]] and other [[electronic devices]]. Data stored in ROM cannot be electronically modified after the manufacture of the [[memory device]]. Read-only memory is useful for storing [[software]] that is rarely changed during the life of the system, also known as [[firmware]]. Software applications, (likesuch as [[video gamesgame]])s, for programmable devices can be distributed as [[ROM cartridge|plug-in cartridges containing ROM]].

Strictly speaking, ''read-only memory'' refers to memory that is hard-wired memory, such as [[diode matrix]] or a [[Read-only memory#Solid-state ROM|mask ROM]] [[integrated circuit]] (IC), whichthat cannot be electronically{{efn|Some discrete component ROM could be mechanically altered, e.g., by adding and removing transformers. IC ROMs, however, cannot be mechanically changed.}} changed after manufacture. Although discrete circuits can be altered in principle, through the addition of [[Jump wire|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.

The term "ROM" is sometimes used to refer to a ROM device containing specific software or a file with software to be stored in a writable ROM device. For example, users modifying or replacing the [[Android operating system]] describe files containing a modified or replacement operating system as "[[custom ROM]]s" after the type of storage the file used to be written to, and they may distinguish between ROM (where software and data is stored, usually [[Flash memory]]) and RAM.

Forms of read-only memory were employed as non-volatile storage for programs in most early stored-program computers, such as [[ENIAC]] [[ENIAC#ProgrammingImprovements |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|historical examples]] below). With the advent of [[integrated circuit]]s in the 1960s, both ROM and its mutable counterpart [[static RAM]] were implemented as arrays of [[transistor]]s 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 (electronics)|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.<ref>See chapters on "Combinatorial Digital Circuits" and "Sequential Digital Circuits" in Millman & Grable, ''Microelectronics,'' 2nd ed.</ref> Consequently, ROM could be implemented at a lower cost-per-[[bit]] than RAM for many years.

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]], [[hard disk drive]]s, [[solid -state drive]]s, [[optical disc drive]]s, [[TFT screen]]s, etc., in the system. Today, many of these "read-only" memories – especially the [[BIOS]]/[[UEFI]] – are often replaced with [[EEPROM]] or [[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 [[Programmable read-only memory|OTP]] (one-time programmable).

=== Use for storing data ===

* In a ROM with transistors in a NOR configuration, using a photomask to define only specific areas of a grid with transistors, to fill with metal thus connecting to the grid only part of all the transistors in the ROM chip<ref name="umich"/> thus making a grid where transistors that are connected cause a different electrical response when addressed, than spaces in the grid where the transistors are not connected, a connected transistor may represent a 1 and an unconnected one a 0, or viceversa. This is the least expensive, and fastest way of making mask ROM<ref name="umich"/> as it only needs one mask with data, and has the lowest density of all mask ROM types as it is done at the metallization layer,<ref name="umich"/> whose features can be relatively large in respect to other parts of the ROM. This is known as contact-programmed ROM. In ROM with a NAND configuration, this is known as metal-layer programming and the mask defines where to fill the areas surrounding transistors with metal which short-circuits the transistors instead, a transistor that is not short circuited may represent a 0, and one that is may represent a 1, or viceversa.<ref name="citeseerx">

* Using two masks to define two types of ion implantation regions for transistors, to change their electrical properties when addressed in a grid and define two types of transistors.<ref name="umich"/> The type of transistor defines if it represents a 1 or a 0 bit. One mask defines where to deposit one type of ion implantation (the "1" transistors), and another defines where to deposit the other (the "0" transistors). This is known as voltage threshold ROM (VTROM) as the different ion implantation types define different voltage thresholds in the transistors, and it's the voltage threshold on a transistor that defines a 0, or a 1. Can be used with NAND and NOR configurations. This technique offers a high level of resistance against optical reading of the contents as ion-implantation regions are difficult to distinguish optically,<ref name="citeseerx"/> which may be attempted with [[decapping]] of the ROM and a microscope.

* Using two levels of thickness for a gate oxide in transistors,<ref name="umich"/> and using a mask to define where to deposit one thickness of oxide, and another mask to deposit the other. Depending on the thickness a transistor can have different electrical properties and thus represent either a 1 or a 0.

* Using several masks to define the presence or absence of the transistors themselves, on a grid. Addressing a non-existent transistor may be interpreted as a 0, and if a transistor is present it may be interpreted as a 1, or viceversa. This is known as active-layer programming.<ref name="citeseerx"/>

Mask ROM transistors can be arranged in either NOR or NAND configurations and can achieve one of the smallest cell sizes possible as each bit is represented by only one transistor. NAND offers higher storage density than NOR. OR configurations are also possible, but compared to NOR it only connects transistors to V_cc instead of V_ss.<ref name="citeseerx"/> Mask ROMs used to be the most inexpensive, and are the simplest semiconductor memory devices, with only one metal layer and one polysilicon layer, making it the type of semiconductor memory with the highest manufacturing yield<ref name="umich"/> (the highest number of working devices per manufacturing run). ROM can be made using one of several semiconductor device fabrication technologies such as [[CMOS]], [[NMOS logic|nMOS]], [[PMOS logic|pMOS]], and [[bipolar transistorstransistor]]s.<ref>{{cite book | url=https://books.google.com/books?id=9VHMBQAAQBAJ&dq=prom+fuse&pg=PA760 | title=The Electronics Handbook | isbn=978-1-4200-3666-4 | last1=Whitaker | first1=Jerry C. | date=3 October 2018 | publisher=CRC Press }}</ref>

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 apply write-enable signals, and special lock/unlock command codes. Modern NAND Flash can be used to achieve the highest write speeds of any rewritable ROM technology, with speeds as high as 10 [[Gigabyte|GB]]/[[second|s]] in an SSD. This has been enabled by 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 by increasing parallelism both in controller design and of storage, the use of large DRAM read/write caches and the implementation of memory cells which can store more than one bit (DLC, TLC and MLC). The latter approach is more failure prone but this has been largely mitigated by overprovisioning (the inclusion of spare capacity in a product which is visible only to the drive controller) and by increasingly sophisticated read/write algorithms in drive firmware.

! [[MOSFET]]

|µCOMμCOM-43 (PMOS)

|<ref name="ucom-43">{{cite book |title=µCOMμCOM-43 SINGLE CHIP MICROCOMPUTER: USERS' MANUAL |date=January 1978 |publisher=[[NEC|NEC Microcomputers]] |url=https://en.wikichip.org/w/images/9/9c/%C2%B5COM-43_SINGLE_CHIP_MICROCOMPUTER_USERS_MANUAL.pdf |access-date=27 June 2019}}</ref>

|µCOMμCOM-43 (CMOS)

* [[Random -access memory]]