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CONTENT

CHAPTER 1

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MEMORY CELL

1.1  SEMICONDUCTOR MEMORY

            A memory cell is used to store the
data in the semiconductor-based integrated circuits. It is a storage device
that stores data electronically either temporarily or permanently. These things
are implemented differently by using various VLSI techniques. At present memory
cell is an essential requirement of electronics. Based upon the technology
memory is used in any equipment which has a processor which converts it to one
form to any other form. The fundamental building block of any system is Memory
cell. It sets to logic 1 and resets itself to logic 0.

           Most of the semiconductor memory has
the property of Random Access, which indicates that the memory is being
accessed at the same amount of time; hence the data could be efficiently
accessed. Examples of this include data that is being stored in hard disks and
CDs, which read and write the data simultaneously. Semiconductor memory has the
highest speed than another type of storage elements but it is slow to access
the program storage and data storage.

            The term “memory” means that
“primary storage” or “main memory” is always addressed to the semiconductor
memory.  There are generally two kinds of
memory. They are:

1.
Volatile memory

2.
Nonvolatile memory

            Volatile
Memory is a computer memory that is capable of storing the information until
the power is on. When the power is interrupted then the data that is stored
gets lost immediately. It is mainly used in main memory, also it is faster
hence used in storing data in hard disks. This protects the sensitive
information. It is also termed as temporary memory. The operating system loads
the memory from RAM when once there is a sudden fluctuation in power or when
power is off everything is lost from the RAM that is why we need to restart our
system and wait for loading of OS to work further on the system. It is the
primary source of memory in the system and is widely used. Transfer of data is
not that easy in this kind of memory.

            Nonvolatile Memory is a type of
memory that is capable of storing the information even though power is lost. This
memory is used in ROM, flash memory and in many other memories. It has
continuous flow of power and doesn’t require any kind of refreshing. So it can
be termed as permanent memory, as the system can retrieve the data even the
power is off. This kind of storage is used to store data persistently for long
term. The memory cost is more and it gives less performance as it takes time to
access the memory of the system.

 

 

 

 

           

 

1.2 
READ ONLY MEMORY

          ROM is a non-volatile memory which is widely used in
all electronic equipment’s like computer and home appliances to store the
program. The data that is stored in ROM can be modified but it is difficult, so
it is used to store application software. ROM refers to the memory that is hard
wired (which cannot be altered after manufacture).  Nowadays ROM is manufactured in such a way
that they can be reprogrammable and is used to store the data as well in a
specific way.

            EPROM (Erasable programmable
read-only memory) and EEPROM (Electrically erasable programmable read-only
memory) could be erased and reprogramed but requires a special circuit and it
is a lag process. The limitation is that it can be reprogrammed for certain
number of times only. This model of ROM is as old as the semiconductor memory.
Mask ROM contains a grid like structure which has word lines and bit lines
making a transistor switch which represents a look up tables and could predict
the delay that is being propagated.

            PROM was invented in 1956 and it
allows its contents to be manipulated only once with the impulses of high
voltage. As the problem persists a company could dump their desired code and
order a large amount of fresh PROM cells. Due to the invention of EPROM in 1971
the problem mentioned can be solved as it can repeat reset the values under the
effect of ultrasonic rays. And again due to the invention of EEPROM in 1983
which again solved a problem occurring in PROM and EPROM as it is able to
reprogram the device in the condition after receiving the signal from an
external source. Toshiba invented flash memory which provided a much better
efficiency and good usage of area, in addition to this without leading to
damage the devices can be reprogrammed thousands number of times.

            All the technologies that were
mentioned above provided a flexible ROM but with different cost, since many
years masked PROM remained available to everyone economically. Other
technologies made new improvements as the replacement of masked PROM.

            Toshiba recently developed NAND
Flash memory so that hard disks that are using the ROM cells can be replaced. SD
cards and USB flash drives can run with a low power consumption using this
technology.  

            Other technologies related to ROM
are not based upon solid state IC technology, where CD-ROM is a read only. CD-R
means write once and read as many times as possible whereas CD-RW is used to read
and write data to it.

 

 

 

 

 

 

 

 

 

 

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