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Read-only memory

From Academic Kids

Read-only memory (ROM) is used as a storage medium in computers. Because it cannot (easily) be written to, its main uses lie in the distribution of firmware (software that is very closely related to hardware, and not likely to need frequent upgrading).

Modern semiconductor ROMs typically take the shape of IC packages, i.e. "computer chips", not immediately distinguishable from other chips like RAMs but for the text printed on the chips. Template:Spoken Wikipedia

Contents

Applications

Many microcontrollers include the program ROM, a CPU core, some on-board peripherals, and RAM, all on one chip. Practically all microprogrammed CPUs also include a kind of ROM on the same chip to hold the control store.

Home computers of the early 1980s came with their complete operating system in ROM, often including a BASIC programming language interpreter. There was no reasonable alternative because floppy disk drives were generally optional. Upgrading to a newer version meant replacing the old ROM chip with a new one -- using either a soldering iron or a set of DIP sockets.

Most video game consoles from the 1970s to the mid-1990s used ROMs as their primary software distribution medium. Such ROMs are sealed into plastic cases suitable for handling and repeated insertion, known as cartridges or "carts" (called "Game Paks" by Nintendo). Some home computers also used ROM cartridges for distributing games and other types of software. The Nintendo 64 was the last major console to use ROM cartridges for software distribution. As of the 2000s, only handheld consoles like the Game Boy Advance and Nintendo DS continue using ROM cartridges.

There is a trend to put less and less software into static ROMs, and more on disk storage. This makes changes easier. By the 2000s, operating systems for desktop computers are not generally on ROM anymore. Computers may still rely on some software in ROMs, like their BIOS, but even that is more likely to reside on a Flash-ROM (see below). Mobile phones and personal digital assistants are likely to have software in ROM (or at least flash memory).

One reason why some data still sits in ROMs is speed – disks are an order of magnitude slower. Even more important, though, is that you cannot read software that is needed to drive a disk from the disk itself – see bootstrap. Hence the BIOS or a bootloader for a computer is often stored in ROM. Also, a networking or graphics card may implement some basic functionality through software contained on a ROM chip. Another application for ROMs is in storing software for embedded systems operating in physically demanding environments (exposed to, say, vibration, or high acceleration), where disks are too fragile.

Types of ROMs

Classic mask-programmed ROM chips are written to during production and cannot change content afterwards. But there are other types of non-volatile solid-state memory:

  • PROMs (Programmable Read-Only Memory) can be written to (programmed) via a special device, a PROM programmer. The writing often takes the form of destroying internal links with the result that a PROM can only be programmed once.
  • EPROMs (Erasable Programmable Read-Only Memory) can be erased by exposure to ultraviolet light then rewritten via an EPROM programmer. Repeated exposure to ultraviolet light will eventually destroy the EPROM but it generally takes many exposures before the EPROM becomes unusable.
  • EAROMs (Electrically Alterable Read-Only Memory) can be modified a bit at a time, but writing is intended to be an infrequent operation; most of the time the memory is used as a ROM. EAROM may be used to store critical system setup information in a non-volatile way. For many applications, EAROM has been supplanted by CMOS RAM backed-up by a lithium battery.
  • EEPROM such as Flash memory (Electrically Erasable Read-Only Memory) allow the entire ROM (or selected banks of the ROM) to be electrically erased (flashed back to zero) then written to without taking them out of the computer (camera, MP3 player, etc.). Flashing is much slower than writing to RAM (Random Access Memory) (or reading from any ROM).
  • By applying write protection, read/write memory may be turned (temporarily) into read-only memory.
  • A CD-ROM isn't a form of electrical ROM in the traditional sense, but rather a kind of compact disk. A CD-ROM is Read Only, hence the name, while CD-R is Write-Once-Read-Many (analogous to a PROM), and CD-RW is Read/Write (analogous to an EEPROM).
  • Diode matrix ROM, which was used in small amounts in many computers in the 1960s as well as keyboard 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.
  • core rope was a form of pre-semiconductor ROM technology. This was used in the Apollo spacecraft computers, Digital Equipment Corporation's PDP-8 computers, and other places. This ROM was programmed by hand by weaving word line wires inside or outside of ferrite transformer cores.

Speed of ROMs

Reading speed

Although this has varied over time, today, large RAMs can be read faster than most large ROMs. Therefore ROM content that is used often is sometimes copied to RAM and subsequently read from there (shadowed).

Writing speed

For those types of ROMs that can be written to or electrically altered, the writing speed is always much slower than the reading speed and it may require unusual voltages or the movement of jumper plugs to apply write-enable signals to the EAROM/Flash ROM.

EEPROM/EAROM lifetime

Because they are written by forcing electrons through an layer of electrical insulation onto a floating transistor gate, re-writeable ROMs will only withstand a limited number of write cycles before the insulation is permanently damaged. In very old EAROMs, this damage could occur in as few as 1,000 write cycles. In modern Flash EEPROMs, the life may be in excess of 10,000 or even 100,000 cycles, but it is by no means infinite. It is partially for this reason (as well as their limited, more-expensive capacity) that Flash ROMs are unlikely to completely supplant magnetic disk drives in the near future.

The reading life of a ROM is not limited by the number of cycles performed. The reading life of EAROMs and EEPROMs may be limited by charge leakage from the floating gates of the storage transistors, especially at high temperatures or in high radiation environments. Masked ROMs or fused EPROMs do not suffer from this effect; once a 1 or 0, always a 1 or 0 (although fuse re-growth was once a problem in early fused PROMs).

ROM images

Main article: ROM image

The contents of video game console ROM cartridges can be extracted with special software, creating files known as "ROM images" for use in emulators. Illegal distribution and trade of these "ROM images" is hugely popular in software piracy circles, where the files are commonly referred to as "roms" (or even "romz", which has a connotation with "warez") even though the term is technically incorrect. The term is so widespread that it has even started to refer to computer games that do not come on ROM cartridges, but instead on floppy disks or CD-ROMs. Usage of such files usually is a violation of copyright or sui generis mask work rights unless your jurisdiction has a fair use protection. Many people believe that games that are no longer sold by official companies should be allowed for download, as the company would not suffer profit loss.


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