Computer Memory
What is Computer Memory?
Computer memory, device that is used to store data or programs (sequences
of instructions) on a temporary or permanent basis for use in an
electronic digital computer. Computers represent
information in binary code, written as sequences of 0s and 1s.
Each binary digit (or “bit”) may be stored by any physical system that
can be in either of two stable states, to represent 0 and 1. Such a system is
called bistable. This could be an on-off switch, an electrical capacitor that
can store or lose a charge, a magnet with its polarity up or down, or a surface
that can have a pit or not. Today capacitors and transistors,
functioning as tiny electrical switches, are used for temporary storage, and
either disks or tape with a magnetic coating, or plastic discs with patterns of
pits are used for long-term storage.
Computer
memory is divided into main (or primary) memory and auxiliary (or secondary) memory. Main
memory holds instructions and data when a program is executing, while auxiliary
memory holds data and programs not currently in use and provides long-term
storage.
Main Memory
The
earliest memory devices were electro-mechanical switches, or relays (seecomputers: The first computer), and electron tubes (see computers: The first stored-program machines).
In the late 1940s the first stored-program computers used ultrasonic waves in
tubes of mercury or charges in special electron
tubes as main memory. The latter were the first random-access memory (RAM). RAM contains
storage cells that can be accessed directly for read and write operations, as
opposed to serial access memory, such as magnetic tape, in which each cell in
sequence must be accessed till the required cell is located.
Magnetic drums, which had fixed
read/write heads for each of many tracks on the outside surface of a rotating
cylinder coated with a ferromagnetic material, were used for both main and
auxiliary memory in the 1950s, although their data access was serial.
About
1952 the first relatively cheap RAM was developed: magnetic core memory, an
arrangement of tiny ferrite cores on a wire grid through which current could be
directed to change individual core alignments. Because of the inherentadvantage of RAM, core memory was the
principal form of main memory until superseded by semiconductor memory
in the late 1960s.
There
are two basic kinds of semiconductor memory. Static RAM (SRAM) consists
of flip-flops, a bistable circuit composed of four to six transistors.
Once a flip-flop stores a bit, it keeps that value until the opposite value is
stored in it. SRAM gives fast access to data, but it is physically relatively
large. It is used primarily for small amounts of memory called registers in a
computer’s central processing unit(CPU) and for fast
“cache” memory. Dynamic RAM (DRAM) stores each bit in an
electrical capacitor rather than in a flip-flop, using a transistor as
a switch to charge or discharge the capacitor. Because it has fewer electrical
components, a DRAM storage cell is smaller than SRAM. However, access to its
value is slower and, because capacitors gradually leak charges, stored values
must be recharged approximately 50 times per second. Nonetheless, DRAM is
generally used for main memory because the same size chip can hold several times as much DRAM
as SRAM.
Storage cells in RAM have addresses. It is common to
organize RAM into “words” of 8 to 64 bits, or 1 to 8 bytes (8 bits = 1 byte).
The size of a word is generally the number of bits that can be transferred at a
time between main memory and the CPU. Every word, and usually every byte, has
an address. A memory chip must have additional decoding circuits that select
the set of storage cells that are at a particular address and either store a
value at that address or fetch what is stored there. The main memory of a
modern computer consists of a number of memory chips, each of which might hold
many megabytes (millions of bytes), and still further addressing circuitry
selects the appropriate chip for each address. In addition, DRAM requires
circuits to detect its stored values and refresh them periodically.
Main
memories take longer to access data than CPUs take to operate on them. For
instance, DRAM memory access typically takes 20 to 80 nanoseconds (billionths
of a second), but CPU arithmetic operations may take only a nanosecond or less.
There are several ways in which this disparity is handled. CPUs have a small number
of registers, very fast SRAM that hold current instructions and the data on
which they operate. Cache memory
is a larger amount (up to several megabytes) of fast SRAM on the CPU chip. Data
and instructions from main memory are transferred to the cache, and since
programs frequently exhibit “locality of reference”—that is, they execute the
same instruction sequence for a while in a repetitive loop and operate on sets
of related data—memory references can be made to the fast cache once values are
copied into it from main memory.
Much of the DRAM access time goes into decoding the address
to select the appropriate storage cells. The locality of reference property
means that a sequence of memory addresses will frequently be used, and fast
DRAM is designed to speed access to subsequent addresses after the first one.
Synchronous DRAM (SDRAM) and EDO (extended data output) are two such types of
fast memory.
Nonvolatile
semiconductor memories, unlike SRAM and DRAM, do not lose their contents when
power is turned off. Some nonvolatile memories, such as read-only memory
(ROM), are not rewritable once manufactured or written. Each memory cell of a
ROM chip has either a transistor for a 1 bit or none for a 0 bit. ROMs are used
for programs that are essential parts of a computer’s operation, such as the
bootstrap program that starts a computer and loads its operating system or the BIOS (basic
input/output system) that addresses external devices in a personal computer (PC).
EPROM (erasable
programmable ROM), EAROM (electrically alterable ROM), and flash memory are types of nonvolatile
memories that are rewritable, though the rewriting is far more time-consuming
than reading. They are thus used as special-purpose memories where writing is
seldom necessary—if used for the BIOS, for example, they may be changed to correct
errors or update features.
Auxiliary memory
Auxiliary
memory units are among computer peripheral equipment. They trade
slower access rates for greater storage capacity and data stability. Auxiliary
memory holds programs and data for future use, and, because it is nonvolatile
(like ROM), it is used to store inactive programs and to archive data. Early
forms of auxiliary storage included punched paper tape, punched cards, and
magnetic drums. Since the 1980s, the most common forms of auxiliary storage
have been magnetic disks, magnetic tapes, and optical discs.
Magnetic disk drives
Magnetic
disks are coated with a magnetic material such as iron oxide. There are two
types: hard disks made of rigid aluminum or glass, and removable diskettes made
of flexible plastic. In 1956 the first magnetic hard drive (HD)
was invented at IBM; consisting of 50 21-inch (53-cm) disks, it
had a storage capacity of 5 megabytes. By the 1990s the standard HD diameter
for PCs had shrunk to 3.5 inches (about 8.9 cm), with storage capacities in
excess of 100 gigabytes (billions of bytes); the standard size HD for portable
PCs (“laptops”) was 2.5 inches (about 6.4 cm). Since the invention of the floppy disk drive (FDD) at IBM by Alan
Shugart in 1967, diskettes have shrunk from 8 inches (about 20 cm) to the
current standard of 3.5 inches (about 8.9 cm). FDDs have low capacity—generally
less than two megabytes—and have become obsolete since the introduction of
optical disc drives in the 1990s.
Hard
drives generally have several disks, or platters, with an electromagnetic read/write head for each
surface; the entire assembly is called a comb. A microprocessor in the drive controls the
motion of the heads and also contains RAM to store data for transfer to and
from the disks. The heads move across the disk surface as it spins up to 15,000
revolutions per minute; the drives are hermetically sealed, permitting the
heads to float on a thin film of air very close to the disk’s surface. A small
current is applied to the head to magnetize tiny spots on the disk surface for
storage; similarly, magnetized spots on the disk generate currents in the head
as it moves by, enabling data to be read. FDDs function similarly, but the removable
diskettes spin at only a few hundred revolutions per minute.
Data
are stored in close concentric tracks that require very precise control of the
read/write heads. Refinements in controlling the heads have enabled smaller and
closer packing of tracks—up to 20,000 tracks per inch (8,000 tracks per cm) by
the start of the 21st century—which has resulted in the storage capacity of
these devices growing nearly 30 percent per year since the 1980s. RAID
(redundant array of inexpensive disks) combines multiple disk drives to store
data redundantly for greater reliability and faster access. They are used in
high-performance computer network servers.
Magnetic tape, similar to the tape
used in tape recorders, has also been used for auxiliary storage, primarily for
archiving data. Tape is cheap, but access time is far slower than that of a
magnetic disk because it is sequential-access memory—i.e., data must be
sequentially read and written as a tape is unwound, rather than retrieved
directly from the desired point on the tape. Servers may also use large
collections of tapes or optical discs, with robotic devices to select and load
them, rather like old-fashioned jukeboxes.
Another
form of largely read-only memory is the optical compact disc, developed from videodisc technology during
the early 1980s. Data are recorded as tiny pits in a single spiral track on
plastic discs that range from 3 to 12 inches (7.6 to 30 cm) in diameter, though
a diameter of 4.8 inches (12 cm) is most common. The pits are produced by a
laser or by a stamping machine and
are read by a low-power laser and a photocell that generates an electrical
signal from the varying light reflected from the pattern of pits. Optical discs
are removable and have a far greater memory capacity than diskettes; the
largest ones can store many gigabytes of information.
A
common optical disc is the CD-ROM (compact
disc read-only memory). It holds about 700 megabytes of data, recorded with an
error-correcting code that can correct bursts of errors caused by dust or
imperfections. CD-ROMs are used to distribute software,
encyclopaedias, and multimedia text with audio and images. CD-R
(CD-recordable), or WORM (write-once read-many), is a
variation of CD-ROM on which a user may record information but not subsequently
change it. CD-RW (CD-rewritable) disks can be re-recorded. DVDs (digital video, or versatile, discs),
developed for recording movies, store data more densely than does CD-ROM, with
more powerful error correction. Though the same size as CDs, DVDs typically
hold 5 to 17 gigabytes—several hours of video or several million text pages.
Magneto-optical discs
Magneto-optical
discs are a hybrid storage medium. In reading, spots with different directions
of magnetization give different polarization in the reflected light of a
low-power laser beam. In writing, every spot on the disk is first heated by a
strong laser beam and then cooled under a magnetic field, magnetizing every spot in one
direction, to store all 0s. The writing process then reverses the direction of
the magnetic field to store 1s where desired.
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