Introduction, Memory Hierarchy, Random Access Memory (RAM), Types of RAM, Read Only Memory (ROM), Types of ROM. Introduction, Classification of Secondary Storage Devices, Magnetic Tape, Magnetic Disk, Optical Disk, Magneto Optical disk.

1. Introduction to Computer System
Primary Memory & Secondary Memory
Prepared By
S.K. Barnwal
Assistant Professor
Arka Jain University
Research Scholar (NIT, Jamshedpur)

2. Memory
Computer memory is just like a human brain used to store data and
instructions. It is the storage space in the computer, where data is to
be processed and instructions required for processing are stored. The
memory is divided into large number of small parts called cells. Each
location or cell has a unique address, which varies from zero to
memory size minus one. For example, if the computer has 64k words,
then this memory unit has 64 * 1024 = 65536 memory locations. The
address of these locations varies from 0 to 65535.
In actual memory is the most essential element of a computing system
because without it computer can’t perform simple tasks. Computer
memory is of two basic type – Primary memory(RAM and ROM) and
Secondary memory(hard drive, CD, etc.). Random Access Memory
(RAM) is primary-volatile memory and Read Only Memory (ROM)
is primary-non-volatile memory.

3. Memory
Memory unit is the amount of data that can be stored in the storage
unit. This storage capacity is expressed in terms of Bytes.
• Bit (Binary Digit): A binary digit is logical 0 and 1 representing a passive or an
active state of a component in an electric circuit.
• Nibble: A group of 4 bits is called nibble.
• Byte: A group of 8 bits is called byte. A byte is the smallest unit, which
can represent a data item or a character.
• Word: A computer word, like a byte, is a group of fixed number of bits
processed as a unit, which varies from computer to computer but is fixed
for each computer.
• 1 KB = 1024 Bytes
• 1 MB = 1024 KB
• 1 GB = 1024 MB
• 1 TB = 1024 GB
Memory is primarily of three types −
• Cache Memory
• Primary Memory/Main Memory
• Secondary Memory

4. Memory
Cache Memory: Cache memory is a very high speed semiconductor
memory which can speed up the CPU. It acts as a buffer between the
CPU and the main memory. It is used to hold those parts of data and
program which are most frequently used by the CPU. The parts of
data and programs are transferred from the disk to cache memory by
the operating system, from where the CPU can access them.
Advantages
• The advantages of cache memory are as follows −
• 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
• The disadvantages of cache memory are as follows −
• Cache memory has limited capacity.
• It is very expensive.

5. Memory
Primary Memory (Main Memory): Primary memory holds only
those data and instructions on which the computer is currently
working. It has a limited capacity and data is lost when power is
switched off. It is generally made up of semiconductor device. These
memories are not as fast as registers. The data and instruction required
to be processed resides in the main memory. It is divided into two
subcategories RAM and ROM.
Characteristics of Main Memory
• These are semiconductor memories.
• It is known as the main memory.
• Usually volatile memory.
• Data is lost in case power is switched off.
• It is the working memory of the computer.
• Faster than secondary memories.
• A computer cannot run without the primary memory.

6. Memory
Secondary Memory: This type of memory is also known as external
memory or non-volatile. It is slower than the main memory. These are
used for storing data/information permanently. CPU directly does not
access these memories, instead they are accessed via input-output
routines. The contents of secondary memories are first transferred to
the main memory, and then the CPU can access it. For example, disk,
CD-ROM, DVD, etc.
Characteristics of Secondary Memory
• These are magnetic and optical memories.
• It is known as the backup memory.
• It is a non-volatile memory.
• Data is permanently stored even if power is switched off.
• It is used for storage of data in a computer.
• Computer may run without the secondary memory.
• Slower than primary memories.

7. Memory Hierarchy
Speed Increases
Memory Space Decreases Speed Decreases
Memory Space Increases

8. RAM
RAM (Random Access Memory): It is the internal memory of the
CPU for storing data, program, and program result. It is a read/write
memory which stores data until the machine is working. As soon as
the machine is switched off, data is erased.
Access time in RAM is independent of the address, that is, each
storage location inside the memory is as easy to reach as other
locations and takes the same amount of time. Data in the RAM can be
accessed randomly but it is very expensive. It is volatile, data stored
in it is lost when switch off the computer or if there is a power failure.
Hence, a backup UPS (Uninterruptible Power System) 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)

9. RAM
Static RAM (SRAM): The word static indicates that the memory
retains its contents as long as power is being supplied. 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 be refreshed on
a regular basis.
Characteristic of Static RAM
• Long life
• No need to refresh
• Faster
• Used as cache memory
• Large size
• Expensive
• High power consumption

10. RAM
Dynamic RAM (DRAM): It is unlike SRAM, must be
continually refreshed in order 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 as
it is cheap and small.
Characteristics of Dynamic RAM
• Short data lifetime
• Needs to be refreshed continuously
• Slower as compared to SRAM
• Used as RAM
• Smaller in size
• Less expensive
• Less power consumption

11. ROM
Read Only Memory (ROM): The memory from which data can only
read but cannot write on it. This type of memory is non-volatile in
nature. The information is stored permanently in such memories
during manufacture. A ROM stores such instructions that are required
to start a computer. This operation is referred to as bootstrap. ROM
chips are not only used in the computer but also in other electronic
items like washing machine and microwave oven.
Types of Read Only Memory (ROM) –
• PROM (Programmable read-only memory) – It can be programmed by
user. Once programmed, the data and instructions in it cannot be changed.
• EPROM (Erasable Programmable read only memory) – It can be
reprogrammed. To erase data from it, expose it to ultra violet light. To
reprogram it, erase all the previous data.
• EEPROM (Electrically erasable programmable read only memory) –
The data can be erased by applying electric field, no need of ultra violet
light. We can erase only portions of the chip.

12. Classification of Secondary Storage Devices
As primary memory, is expensive as well as limited. The faster
primary memory is also volatile. If we need to store large amount of
data or programs permanently, we need a cheaper and permanent
memory. Such memory is called secondary memory that provides
permanent storage of data and in bulk quantity.
The Secondary storage media can be fixed or removable. Fixed
Storage media is an internal storage medium like hard disk that is
fixed inside the computer. Storage medium that are portable and can
be taken outside the computer are termed as removable storage media.
Applications:
• Permanent Storage: Primary Memory (RAM) is volatile, i.e. it
loses all information when the electricity is turned off, so in order to
secure the data permanently in the device, Secondary storage
devices are needed.
• Portability: Storage medium, like the CDs, flash drives can be used
to transfer the data from one device to another.

13. Secondary Storage Devices
Magnetic Tape, Magnetic Disk, Optical Disk, Magneto Optical
disk:
These are the storage devices provides a way to store data for a long
duration. These disks differ in many characteristics; firstly magnetic
disk works by using magnetic material over the disk whereas in
optical disk polycarbonate plastic is used in its construction and laser
is used to store and retrieve the data. The magnetic and optical disk
comes under the category of secondary storage devices. The need to
devise these devices has emerged because the previous semiconductor
storage devices have very limited capabilities, for example, the cost of
storing the information in such devices is very high.

14. Secondary Storage Devices
Magnetic Tape, Magnetic Disk, Optical Disk, Magneto Optical Disk

15. Secondary Storage Devices
Magnetic Tape memory: In magnetic tape only one side of the
ribbon is used for storing data. It is sequential memory which contains
thin plastic ribbon to store data and coated by magnetic oxide. Data
read/write speed is slower because of sequential access. It is highly
reliable which requires magnetic tape drive writing and reading data.
The width of the ribbon varies from 4mm to 1 Inch and it has storage
capacity 100 MB to 200 GB.
Advantages :
• These are inexpensive, i.e., low cost memories.
• It provides backup or archival storage.
• It can be used for large files.
• It can be used for copying from disk files.
• It is a reusable memory.
• It is compact and easy to store on racks.
Disadvantages :
• Sequential access not allowed,
• It requires caring to store, i.e., vulnerable humidity, dust free, and suitable
environment.
• stored data cannot be easily updated or modified

16. Secondary Storage Devices
Magnetic Disk: It is a type of secondary memory which is a flat disc
covered with magnetic coating to hold information. It is used to store
various programs and files. The polarized information in one direction
is represented by 1, and vice versa. The direction is indicated by 0.
It can store large amounts of data, but data access rate is slower than
main memory because of secondary memory. Data can be modified or
can be deleted easily in the magnetic disk memory. It also allows
random access of data.
• Advantages :-
These are economical memory
The easy and direct access of data possible.
It can store large amounts of data.
It has better data transfer rate than magnetic tapes.
It has less prone to corruption of data as compared to tapes.
• Disadvantages :-
These are less expensive than RAM but more expensive than magnetic tape.
It need clean and dust free environment to store.
These are not suitable for sequential access.

17. Secondary Storage Devices
Optical Disc: It is an electronic data storage medium that can be
written to and read from using a low-powered laser beam. Originally
developed in the late 1960s, the first optical disc, created by James T.
Russell, stored data as micron-wide dots of light and dark. In this a
laser read the dots and the data was converted to an electrical signal.
Then the data was converted to audio or visual output could hold
much more data than similarly sized magnetic storage media.
Magneto-optical Disc: It is 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.