Read-only memory (usually known by its acronym, ROM) is
a class of storage media used in computers and other electronic devices. Because
data stored in ROM cannot be modified (at least not very quickly or easily), it
is mainly used to distribute firmware (software that is very closely tied to
specific hardware, and unlikely to require frequent updates).
In its strictest sense, ROM refers only to mask ROM (the oldest type of solid state ROM), which
is fabricated with the desired data permanently stored in it, and thus can never
be modified. However, more modern types such as EPROM and flash EEPROM can be
erased and re-programmed multiple times; they are still described as "read-only
memory"(ROM) because the reprogramming process is generally infrequent,
comparatively slow, and often does not permit random access writes to individual
memory locations. Despite the simplicity of mask ROM, economies of scale and
field-programmability often make reprogrammable technologies more flexible and
inexpensive, so mask ROM is rarely used in new products as of 2007.
History
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 circuit came 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 predictable propagation 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:
It is only economical to buy mask ROM in large quantities, since users
must contract with a foundry to produce a custom design.
The turnaround time between completing the design for a mask ROM and
receiving the finished product is long, for the same reason.
Mask ROM is impractical for R&D work since designers frequently need to
modify the contents of memory as they refine a design.
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.
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 (e.g. 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.) Furthermore,
despite the fact that newer technologies were increasingly less "read-only,"
most were envisioned only as replacements for the traditional use of mask ROM.
The most recent development is NAND flash, also invented by Toshiba. Its
designers explicitly broke from past practice, stating plainly that "the aim of
NAND Flash is to replace hard disks,"[1] rather than the traditional use of ROM
as a form of non-volatile primary storage. As of 2007[update], 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 of ROM for program storage
Every stored-program computer requires some form of non-volatile, or
erasable, storage 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 requires 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 required 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 requires 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 a single transistor connecting a bit line to a word
line. Consequently, ROM could be implemented at a lower cost-per-bit than RAM
for many years.
Mosthome 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 expensive. For example, the Commodore 64 included 64 KiB of RAM
and 20 KiB of ROM contained a BASIC interpreter and the "KERNAL" (sic) 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 ICs 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. This governs 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 nonetheless based on general-purpose microprocessors
in most cases. With software usually tightly coupled to hardware, program
changes are rarely needed in such devices (which typically lack devices such as
hard disks for reasons of cost, size, and/or power consumption). As of 2008,
most products use Flash rather than mask ROM, and many provide some means for
connection to a PC for firmware updates; a digital audio player's might be
updated to support a new file format for instance. 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.
ROM for data storage
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.
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