Memory
Main
memory- The
main memory of the computer is also known as RAM, standing for Random Access
Memory. It is constructed from integrated circuits and needs to have electrical
power in order to maintain its information. When power is lost, the information
is lost too! It can be directly accessed by the CPU. The access time to read or
write any particular byte are independent of whereabouts in the memory that
byte is, and currently is approximately 50 nanoseconds (a thousand millionth of
a second). This is broadly comparable with the speed at which the CPU will need
to access data. Main memory is expensive compared to external memory so it has
limited capacity. The capacity available for a given price is increasing all
the time. For example many home Personal Computers now have a capacity of 16
megabytes (million bytes), while 64 megabytes is commonplace on commercial
workstations. The CPU will normally transfer data to and from the main memory
in groups of two, four or eight bytes, even if the operation it is undertaking
only requires a single byte.
RAM- Random-access memory (RAM)
is
a form of computer data
storage. A random-access device allows stored data to be accessed quickly in any random
order. In contrast, other data storage media such as hard disks, CDs, DVDs and magnetic tape, as well as
early primary memory types such as drum memory, read and
write data only in a predetermined order, consecutively, because of mechanical
design limitations. Therefore the time to access a given data location varies
significantly depending on its physical location.
Today, random-access memory takes the form of integrated circuits.
Strictly speaking, modern types of DRAM
are not random access, as data is read in bursts, although the name DRAM / RAM has stuck. However, many types
of SRAM, ROM, OTP,
and NOR flash are still random access even in a
strict sense. RAM is often associated with volatile types of memory
(such as DRAM memory modules), where its stored information
is lost if the power is removed. Many other types of non-volatile memory are
RAM as well, including most types of ROM and a type of flash memory called NOR-Flash. The first RAM
modules to come into the market were created in 1951 and were sold until the
late 1960s and early 1970s.
static
and dynamic- Readers who are familiar
with the concepts of dynamic memory and pointers may wish to skip to the next
section of this chapter. In this one, the general concepts of static and
dynamic memory is outlined. How these issues are specifically handled in
Modula-2 is not taken up again until after the rest of the preliminary
discussions are complete.
The distinction made in this section is based on
the timing and manner for the setting aside of memory for the use of a program.
Some memory use is predetermined by the compiler and will always be set aside
for the program in exactly the same manner at the beginning of every run of a
given program. Other memory is obtained by the program at various points during
the time it is running. This memory may only be used by the program temporarily
and then released for other uses.
ROM-
Read-only memory (ROM) is
a class of storage
medium used in computers
and other electronic devices. Data stored in ROM cannot be modified, or can be
modified only slowly or with difficulty, so it is mainly used to distribute firmware (software that is very
closely tied to specific hardware, and unlikely to
need 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.
Despite the simplicity, speed and economies of scale of mask
ROM, field-programmability
often make reprogrammable memories more flexible and inexpensive. As of 2007,
actual ROM circuitry is therefore mainly used for applications such as microcode, and similar
structures, on various kinds of processors.
Other types of non-volatile memory
such as erasable programmable read only
memory (EPROM) and electrically erasable programmable
read-only memory (EEPROM or Flash ROM) are sometimes referred to, in
an abbreviated way, as "read-only memory" (ROM); although these types
of memory can be erased and re-programmed multiple times, writing to this
memory takes longer and may require different procedures than reading the
memory.[1]
When used in this less precise way, "ROM" indicates a non-volatile
memory which serves functions typically provided by mask ROM, such as storage
of program code and nonvolatile data.
EPROM-
An
EPROM. The small quartz window admits UV light for erasure.
An EPROM (rarely EROM), or erasable programmable read only memory, is a
type of memory chip
that retains its data when its power supply is switched off. In other words, it
is non-volatile. It is an
array of floating-gate
transistors individually programmed by an electronic device that
supplies higher voltages than those normally used in digital circuits. Once
programmed, an EPROM can be erased by exposing it to strong ultraviolet light source
(such as from a mercury-vapor light). EPROMs are easily recognizable by the
transparent fused quartz
window in the top of the package, through which the silicon chip is visible,
and which permits exposure to UV light
during erasing.
EEPROM- EEPROM (also
written E2PROM and pronounced "e-e-prom," "double-e prom,"
"e-squared," or simply "e-prom") stands for Electrically Erasable
Programmable Read-Only Memory and is a type of non-volatile memory
used in computers and other electronic devices to store small amounts of data
that must be saved when power is removed, e.g., calibration tables or device
configuration.
When larger amounts of static data are to be stored
(such as in USB flash drives)
a specific type of EEPROM such as flash memory is more
economical than traditional EEPROM devices. EEPROMs are realized as arrays of floating-gate
transistors.
EEPROM is user-modifiable read-only memory (ROM)
that can be erased and reprogrammed (written to) repeatedly through the
application of higher than normal electrical voltage generated externally or
internally in the case of modern EEPROMs. EPROM usually must be removed from the
device for erasing and programming, whereas EEPROMs can be programmed and
erased in-circuit. Originally, EEPROMs were limited to single byte operations
which made them slower, but modern EEPROMs allow multi-byte page operations. It
also has a limited life - that is, the number of times it could be reprogrammed
was limited to tens or hundreds of thousands of times. That limitation has been
extended to a million write operations in modern EEPROMs. In an EEPROM that is
frequently reprogrammed while the computer is in use, the life of the EEPROM
can be an important design consideration. It is for this reason that EEPROMs
were used for configuration information, rather than random access memory.
EAROM-(Electrically Alterable
ROM) See EEPROM.
Computer Desktop Encyclopedia copyright ©1981-2013
by The Computer Language Company Inc.
All Right reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other
reproduction is strictly prohibited without permission from the publisher.
Cache memory-
·
it
is a part of the cpu i.e, it is in built inside the processor.
·
It
is pieced between the main memory and the processor.
·
It
has an access time of 100 neno second which is compatible to the processor
wheres the access time of main memory is 700 neno second.
·
Its
storage capacity Is only 512 KB. whereas the storage capecity of main memory is
128 MB or more.
·
It
is very expansive and hence limited in capacity.
·
Earlier
cache memories were available separately but the latest microprocessor contain
the cache memory on the chip(processor) itself.
·
The
need for cache memory is due to the speed mismatch between the CPU and the main
memory.
The CPU clock is very
fast whereas the main memory access time is combatively slower. Hence no matter
how fast the processor is the processing speed depands more on the speed of the
main memory. It is because of this
reason the cache memory is introduced which memory Is having access time closer
to the processor. The cache memory stores the program that are currently
begging executed or which are required to be executed writhen a very short
period of the time. It also stores the temporary data. That the CPU may require
frequently for manipulation, it works according to various algorithm, which
decide what information it has to store.
Virtual memory- this technique is useful when
application or program which are having a capacity very much more then the
capacity of main memory.
The application that is to
be executed resides on secondary storage
and when it is to be storage and when it is to be executed, since the main
memory cannot accomplish the whole capecity of the application, this
application is divided into segment/pages/forms and each segment in sequence is
loaded into the main memory and is executed via the CPU . once the segment has
been executed it is transferred back to the secondary storage via the main
memory. This process i.e. loaded of segment form secondary storage via main
memory and then executed. This technique gives the users an illusion that the
capacity remains the same. Also in this technique a lot of CPU’s ideal time is
wasted i.e. full utilization of CPU doesn’t take place.