Read-only memory
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).
Programmable read-only memory (PROM), invented by Wen Tsing Chow in 1956,[5][6] 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.
[citation needed] In 1967, Dawon Kahng and Simon Sze of Bell Labs proposed that the floating gate of a MOS semiconductor device could be used for the cell of a reprogrammable ROM, which led to Dov Frohman of Intel inventing erasable programmable read-only memory (EPROM) in 1971.
Electrically erasable programmable read-only memory (EEPROM), developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at the Electrotechnical Laboratory in 1972,[10] 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 by Fujio Masuoka at Toshiba in the early 1980s and commercialized in the late 1980s, is a form of EEPROM that makes very efficient use of chip area and can be erased and reprogrammed thousands of times without damage.
Its designers explicitly broke from past practice, stating plainly that "the aim of NAND flash is to replace hard disks,"[13] rather than the traditional use of ROM as a form of non-volatile primary storage.
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.
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).
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).
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.
Since ROM (at least in hard-wired mask form) cannot be modified, it is only suitable for storing data which is not expected to need modification for the life of the device.
[16] It is common practice to use rewritable non-volatile memory – such as UV-EPROM or EEPROM – for the development phase of a project, and to switch to mask ROM when the code has been finalized.
Since the cost of an integrated circuit strongly depends on its size, mask ROM is significantly cheaper than any other kind of semiconductor memory.
[17] As of 2003[update], four companies produce most such mask ROM chips: Samsung Electronics, NEC Corporation, Oki Electric Industry, and Macronix.
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.
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 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 data retention of EPROM, EAROM, EEPROM, and Flash may be time-limited by charge leaking from the floating gates of the memory cell transistors.
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.
Another example is making backups of firmware/OS ROMs from older computers or other devices - for archival purposes, as in many cases, the original chips are PROMs and thus at risk of exceeding their usable data lifetime.
