Memory Hierarchy
Design and its Characteristics
In the Computer
System Design, Memory Hierarchy is an enhancement to organize the memory such
that it can minimize the access time. The Memory Hierarchy was developed based
on a program behaviour known as locality of references. The figure below
clearly demonstrates the different levels of memory hierarchy:
This Memory
Hierarchy Design is divided into 2 main types:
External Memory or
Secondary Memory –
Comprising of Magnetic Disk, Optical Disk, Magnetic Tape i.e. peripheral
storage devices which are accessible by the processor via I/O Module.
Internal Memory or
Primary Memory –
Comprising of Main Memory, Cache Memory & CPU registers. This is directly
accessible by the processor.
We can infer the
following characteristics of Memory Hierarchy Design from above figure:
Capacity:
It is the global volume of information the memory can store. As we move from
top to bottom in the Hierarchy, the capacity increases.
Access Time:
It is the time interval between the read/write request and the availability of
the data. As we move from top to bottom in the Hierarchy, the access time increases.
Performance:
Earlier when the computer system was designed without Memory Hierarchy design,
the speed gap increases between the CPU registers and Main Memory due to large
difference in access time. This results in lower performance of the system and
thus, enhancement was required. This enhancement was made in the form of Memory
Hierarchy Design because of which the performance of the system increases. One
of the most significant ways to increase system performance is minimizing how
far down the memory hierarchy one has to go to manipulate data.
Cost per bit:
As we move from bottom to top in the Hierarchy, the cost per bit increases i.e.
Internal Memory is costlier than External Memory.