A memory is just like a human brain. It is used to store data
and instruction. Computer memory is the storage space in computer where data is
to be processed and instructions required for processing are stored.
The memory is divided into large number of small parts. Each
part is called a cell. Each location or cell has a unique address which varies
from zero to memory size minus one.
For example if computer has 64k words, then this memory unit
has 64 * 1024 = 65536 memory location. The address of these locations varies
from 0 to 65535.
Memory is primarily of two types
·
Internal Memory − cache memory and
primary/main memory
·
External Memory − magnetic disk / optical
disk etc.
Internal memory
That temporarily memorises data while programs are running.
Internal memory uses microconductors, i.e. fast specialized electronic
circuits. Internal memory corresponds to what we call random access memory
(RAM).
External/Physical/Auxiliary memory
That stores information over the long term, including after
the computer is turned off. Auxiliary memory corresponds to magnetic storage
devices such as the hard drive, optical storage devices such as CD-ROMs and DVD-ROMs,
as well as read-only memories.
RAM
A RAM constitutes the internal memory of the CPU for storing
data, program and program result. It is read/write memory. It is called random
access memory (RAM).
Since access time in RAM is independent of the address to
the word that is, each storage location inside the memory is as easy to reach
as other location & takes the same amount of time. We can reach into the
memory at random & extremely fast but can also be quite expensive.
RAM is volatile, i.e. data stored in it is lost when we
switch off the computer or if there is a power failure. Hence, a backup
uninterruptible power system (UPS) is often used with computers. RAM is small,
both in terms of its physical size and in the amount of data it can hold.
RAM is of two types
- Static RAM (SRAM)
- Dynamic RAM (DRAM)
Static RAM (SRAM)
The word static indicates that the memory retains its
contents as long as power remains applied. However, data is lost when the power
gets down due to volatile nature. SRAM chips use a matrix of 6-transistors and
no capacitors. Transistors do not require power to prevent leakage, so SRAM
need not have to be refreshed on a regular basis.
Because of the extra space in the matrix, SRAM uses more
chips than DRAM for the same amount of storage space, thus making the
manufacturing costs higher.
Static RAM is used as cache memory needs to be very fast and
small.
Dynamic RAM (DRAM)
DRAM, unlike SRAM, must be continually refreshed in order
for it to maintain the data. This is done by placing the memory on a refresh
circuit that rewrites the data several hundred times per second. DRAM is used
for most system memory because it is cheap and small. All DRAMs are made up of
memory cells. These cells are composed of one capacitor and one transistor.
ROM
ROM stands for Read Only Memory. The memory from which we
can only read but cannot write on it. This type of memory is non-volatile. The
information is stored permanently in such memories during manufacture.
A ROM, stores such instruction as are required to start
computer when electricity is first turned on, this operation is referred to as
bootstrap. ROM chip are not only used in the computer but also in other
electronic items like washing machine and microwave oven.
Following are the various types of ROM −
MROM (Masked ROM)
The very first ROMs were hard-wired devices that contained a
pre-programmed set of data or instructions. These kind of ROMs are known as
masked ROMs. It is inexpensive ROM.
PROM (Programmable Read Only Memory)
PROM is read-only memory that can be modified only once by a
user. The user buys a blank PROM and enters the desired contents using a PROM
programmer. Inside the PROM chip there are small fuses which are burnt open
during programming. It can be programmed only once and is not erasable.
EPROM (Erasable and Programmable Read Only Memory)
The EPROM can be erased by exposing it to ultra-violet light
for a duration of upto 40 minutes. Usually, an EPROM eraser achieves this
function. During programming an electrical charge is trapped in an insulated
gate region. The charge is retained for more than ten years because the charge
has no leakage path. For erasing this charge, ultra-violet light is passed
through a quartz crystal window (lid). This exposure to ultra-violet light
dissipates the charge. During normal use the quartz lid is sealed with a
sticker.
EEPROM (Electrically Erasable and Programmable Read Only
Memory)
The EEPROM is programmed and erased electrically. It can be
erased and reprogrammed about ten thousand times. Both erasing and programming
take about 4 to 10 ms (millisecond). In EEPROM, any location can be selectively
erased and programmed. EEPROMs can be erased one byte at a time, rather than
erasing the entire chip. Hence, the process of re-programming is flexible but
slow.
Serial Access Memory
Sequential access means the system must search the storage
device from the beginning of the memory address until it finds the required
piece of data. Memory device which supports such access is called a Sequential
Access Memory or Serial Access Memory. Magnetic tape is an example of serial
access memory.
Direct Access Memory
Direct access memory or Random Access Memory, refers to
conditions in which a system can go directly to the information that the user
wants. Memory device which supports such access is called a Direct Access
Memory. Magnetic disks, optical disks are examples of direct access memory.
Cache Memory
Cache memory is a very high speed semiconductor memory which
can speed up CPU. It acts as a buffer between the CPU and main memory. It is
used to hold those parts of data and program which are most frequently used by
CPU. The parts of data and programs, are transferred from disk to cache memory
by operating system, from where CPU can access them.
Advantages
- Cache memory is faster than main memory.
- It consumes less access time as compared to main memory.
- It stores the program that can be executed within a short period of time.
- It stores data for temporary use.
Disadvantages
- Cache memory has limited capacity.
- It is very expensive.
Virtual memory is a technique that allows the execution of processes which
are not completely available in memory. The main visible advantage of this
scheme is that programs can be larger than physical memory. Virtual memory is
the separation of user logical memory from physical memory.
This separation allows an extremely large virtual memory to
be provided for programmers when only a smaller physical memory is available.
Following are the situations, when entire program is not required to be loaded
fully in main memory.
- User written error handling routines are used only when an error occurred in the data or computation.
- Certain options and features of a program may be used rarely.
- Many tables are assigned a fixed amount of address space even though only a small amount of the table is actually used.
- The ability to execute a program that is only partially in memory would counter many benefits.
- Less number of I/O would be needed to load or swap each user program into memory.
- A program would no longer be constrained by the amount of physical memory that is available.
- Each user program could take less physical memory, more programs could be run the same time, with a corresponding increase in CPU utilization and throughput.
Auxiliary Memory
Auxiliary memory is much larger in size than main memory but
is slower. It normally stores system programs, instruction and data files. It
is also known as secondary memory. It can also be used as an overflow/virtual
memory in case the main memory capacity has been exceeded. Secondary memories
cannot be accessed directly by a processor. First the data/information of
auxiliary memory is transferred to the main memory and then that information
can be accessed by the CPU. Characteristics of Auxiliary Memory are following –
- Non-volatile memory − Data is not lost when power is cut off.
- Reusable − The data stays in the secondary storage on permanent basis until it is not overwritten or deleted by the user.
- Reliable − Data in secondary storage is safe because of high physical stability of secondary storage device.
- Convenience − With the help of a computer software, authorised people can locate and access the data quickly.
- Capacity − Secondary storage can store large volumes of data in sets of multiple disks.
- Cost − It is much lesser expensive to store data on a tape or disk than primary memory.
Data storage device
A data
storage device is a device for recording (storing) information (data).
Recording can be done using virtually any form of energy, spanning from manual
muscle power in handwriting, to acoustic vibrations in phonographic recording,
to electromagnetic energy modulating magnetic tape and optical discs.
A storage device may hold information, process information, or
both. A device that only holds information is a recording medium. Devices that
process information (data storage equipment) may either access a separate
portable (removable) recording medium or a permanent component to store and
retrieve data.
Portable Device
A portable device is any device that can easily be carried. It
is a small form factor of a computing device that is designed to be held and
used in the hands. Portable devices are becoming an increasingly important part
of personal computing as the capabilities of devices like laptops, tablets and
smartphones continue to improve.
A portable device may also be called a handheld device or
mobile device.
Hard disk drive
A hard disk drive (HDD),
hard disk, hard drive or fixed disk is a data storage
device used for storing and retrieving digital information using one or more
rigid ("hard") rapidly rotating disks (platters) coated with magnetic
material. The platters are paired with magnetic heads arranged on a moving actuator
arm, which read and write data to the platter surfaces. Data is accessed in a random-access
manner, meaning that individual blocks of data can be stored or retrieved in
any order rather than sequentially. HDDs retain stored data even when powered
off.
