Memory
chips store data in the form of bytes. Bytes represent information such as
letters, numbers, and symbols. A byte is a grouping of digital information in
computing. A byte is most commonly a block of eight bits. Each bit is stored as
either 0 or 1 in the memory chip.
The
computer memory units start from bit. A bit is the smallest memory unit to
measure data stored in main memory and storage devices. A bit can have only one
binary value out of 0 and 1.
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.
Characteristics
of Memory Hierarchy are following when we go from top to bottom.
1.
Capacity
in terms of storage increases.
2.
Cost
per bit of storage decreases.
3.
Frequency
of access of the memory by the CPU decreases.
4.
Access
time by the CPU increases.
Primary
Memory:
Primary
memory is computer memory that is accessed directly by the CPU.
Primary
memory is the main memory of the computer system. Accessing data from primary
memory is faster because it is an internal memory of the computer.
Secondary
Memory:
All
secondary storage devices which are capable of storing high volume data is
referred to secondary memory. It's slower than primary memory. However, it can
save a substantial amount of data, in the range of gigabytes to terabytes. This
memory is also called backup storage or mass storage media.
The
secondary memory is accessed indirectly via input/output operations. It is
non-volatile, so permanently stores the data even when the computer is turned
off or until this data is overwritten or deleted. The CPU can't directly access
the secondary memory. First, the secondary memory data is transferred to
primary memory then the CPU can access it.
Primary
Memory Vs Secondary Memory:
|
Parameter |
Primary
memory |
Secondary
memory |
|
Nature |
The
primary memory is categorized as volatile & nonvolatile memories. |
The
secondary memory is always a non-volatile memory. |
|
Alias |
These
memories are also called internal memory. |
Secondary
memory is known as a Backup memory or Additional memory or Auxiliary memory. |
|
Access |
Data
is directly accessed by the processing unit. |
Data
cannot be accessed directly by the processor. It is first copied from
secondary memory to primary memory. Only then CPU can access it. |
|
Formation |
It's
a volatile memory meaning data cannot be retained in case of power failure. |
It's
a non-volatile memory so that that data can be retained even after power
failure. |
|
Storage |
It
holds data or information that is currently being used by the processing
unit. Capacity is usually in 16 to 32 GB |
It
stores a substantial amount of data and information. Capacity is generally
from 200GB to terabytes. |
|
Accesses |
Primary
memory can be accessed by the data bus. |
Secondary
memory is accessed by I/O channels. |
|
Expense |
Primary
memory is costlier than secondary memory. |
Secondary
memory is cheaper than primary memory. |
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