Definition of Memory
i. Memory unit is that part of the computer that holds data and instructions for processing.
ii. It may also be defined as the storage space in computer where data to be processed and instructions
required for processing are stored.
Although closely associated with the central processing unit, memory is separated from it.
Types of Memory
Memory is of two types:
i.Primary memory/main memory/internal memory
ii.Secondary memory/auxiliary memory/external memory
Primary Memory: Primary memory is the memory that can be directly accessed by the CPU, the CPU
constantly interact with it, reads instructions stored there and executes them as required.
Types of Primary memory
There are basically two types of primary memory:
i. RAM
ii. ROM
RAM (Radom Access Memory):
Random access memory, also called the Read/Write memory, is the temporary memory of a computer. It
is said to be ‘volatile’ since its contents are accessible only as long as the computer is on. The contents of
RAM are cleared once the computer is turned off or if there is a power cut.
Types of RAM
1. Dynamic RAM: Dynamic RAM (DRAM) is a type of physical memory used in most personal
computers. The term dynamic indicates that the memory must be constantly refreshed (reenergized) or it
will lose its contents.
2. Static RAM: Static RAM (SRAM) is a type of RAM that holds its data without external refresh, for as
long as power is supplied to the circuit. This is contrasted to dynamic RAM (DRAM), which must be
refreshed many times per second in order to hold its data contents
ROM (Read Only Memory)
Read Only Memory is a special type of memory which can only be read and contents of which are not
lost even when the computer is switched off or if there is a power cut. It typically contains manufacturer’s
instructions.
Types of ROM
Programmable Read-Only Memory (PROM): This type of ROM can be re-programmed by using a
special device called a PROM programmer. Generally, a PROM can only be changed/updated once.
Erasable Programmable Read-Only Memory (EPROM): This type of ROM can have its contents erased
by ultraviolet light and then reprogrammed by an RPROM programmer. This procedure can be carried
out many times; however, the constant erasing and rewriting will eventually render the chip useless.
Electrically Erasable Programmable Read-Only Memory (EEPROM): These are also erasable like
EPROM, but the same work of erasing is performed with electric current. Thus, it provides the ease of
erasing it even if the memory is positioned in the computer. It stores computer system’s BIOS. Unlike
EPROM, the entire chip does not have to be erased for changing some portion of it. Thus, it even gets rid
of some biggest challenges faced by using EPROMs.
Differences between RAM and ROM
RAM ROM
It is volatile (It loses data when power is turned It is non-volatile (retains data even when power is
off) off)
Data in RAM can be changed or deleted ROM is fixed or data cannot be modified.
RAM chip are bigger in size ROM chip are smaller in size
RAM chips are relatively expensive ROM chips are relatively less expensive
RAM stores all the applications and data when the ROM usually stores instructions that are required to
computer is running for booting the computer
Secondary Memory
This type of memory is a non-volatile memory. It is slower than 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. Contents of secondary memories are first transferred to main memory, and then
CPU can access it.
Types of Secondary Memory
Secondary memory can be of following types:
Magnetic: Data and information are stored and retrieved using magnetism
Examples
Magnetic disk
Magnetic tape
Hard disk
floppy disks
Optical: Optical storage devices employ light (laser beams) to store and retrieve data and information
Examples
CD
VCD
DVD
Electronic: This holds data and information in the form of electric voltage
Example
Flash Memory
Others include Punch card
Secondary Storage Devices
1. Floppy Diskette
There are two standard sizes used these days that are 5 □((1 )/4) inches and 3 □(1/2) inches, in size,
commonly referred to as the mini-floppy and microfloppy. The capacity of 5 □((1 )/4) inches floppy is 1.2
MB and that of 3 □(1/2) inches is 1.44 MB.
2. CD-ROM drives
They are of three types
a. CD-ROM
CD-ROM stands for (Compact Disc Read Only Memory), and it is mainly used to mass produce audio
CD's and computer games. Computer users can only read data and music from the discs, but they cannot
burn their own information onto the discs, from their personal computers.
b. CD-R
CD-R (Compact Disc Recordable) also known as WORM (Write Once Read Many) is a blank disc that
users can put into a CD-ROM drive to burn or make a copy of their personal data, music, videos and
information. CD-Recordable discs are designed for one-time recording only.
3. CD-RW
Unlike a CD-R, the CD-RW (Compact Disc Rewritable) can be erased and returned to its original blank
state. New files can then be copied onto the rewritable disk. CD-RW never became as popular as the CD-
R's because they are not compatible with most disc players to listen to music. They are primarily used to
move data from one computer to another, or to copy files that are only needed a few times.
3. DVD (Digital Versatile Disk)
DVD is very similar to a CD but it has a much larger capacity. A standard DVD can hold 4.7 GB of data.
DVD’s are of the same types as CD’s along with different formats as explained about CD’s.
4. Combo Drives
These drives combine the function of a DVD drive and a CD-RW drive.
5. Hard disk: Largest in capacity
Differences between Primary and Secondary Memory
Primary Memory Secondary Memory
These devices are temporary (Volatile) These devices are permanent (Non-volatile)
These devices are expensive These devices are cheaper
They have less storage capacity They have storage capacities
They are usually faster They are slower
Directly accessed by the CPU Not directly accessed by the CPU
Internal memory External Memory
Units of Storage in Computer
Bits: The Smallest Unit of data on a binary computer. A single bit consist of 0 (zero) or 1 (one).
Nibbles: A nibble is a collection of four bits.
Bytes: A byte consists of eight bits. It is the smallest item that can be individually accessed by a program.
It is the most important data structure used by the microcomputer.
Word: A word is a group of 16 bits
Others include
Name Symbol Value (bytes) Base 10 Base 16
Kilobyte KB 2 10 >10 3 =16 2.5
Megabyte MB 2 20 >10 6 =16 5
Gigabyte GB 2 30 >10 9 =16 7.5
Terabyte TB 2 40 >10 12 =16 10
Petabyte PB 2 50 >10 15 =16 12.5
Exabyte EB 2 60 >10 1 =16 15
Zettabyte ZB 2 70 >10 15 =16 17.5
Yottabyte PB 2 80 >10 24 =16 20
Conversion From One Unit To Another
The conversion process from one unit to another can be done using the following relationships below.
1 bit = 0 or 1
1 nibble = 4 bits
1 bytes = 8 bits
1 word = 16 bits
1 kilobyte = 1024 bytes
1 MB = 1024 KB
1 GB = 1024 MB
1 TB = 1024 GB
1 PB = 1024 TB
1 EB = 1024 PB
1ZB = 1024 EB
1 YB = 1024 ZB
Example 1
Convert the following:
12 bits to byte
50 bits to word
1420 Bytes to Kilobytes
2 Gigabytes to Megabyte
1.44 MB to byte
