primary and secondary memory

1. COMPUTER - MEMORY
A memory is just like a humanbrain. It is used to store data and instruction. Computer memory is the storage
space incomputer where data is to be processed and instructions required for processing are stored.
The memory is divided into large number of smallparts. Eachpart is called cell. Eachlocationor cellhas a unique
address whichvaries fromzero to memory size minus one.
For example if computer has 64k words, thenthis memory unit has 64 * 1024=65536 memory location. The
address of these locations varies from0 to 65535.
Memory is primarily of three types
Cache Memory
Primary Memory/MainMemory
Secondary Memory
Cache Memory
Cache memory is a very highspeed semiconductor memory whichcanspeed up CPU. It acts as a buffer
betweenthe CPU and mainmemory.
It is used to hold those parts of data and programwhichare most frequently used by CPU. The parts of data and
programs are transferred fromdisk to cache memory by operating system, fromwhere CPU canaccess them.
Advantage
Cache memory is faster thanmainmemory.
It consumes less access time as compared to mainmemory.
It stores the programthat canbe executed withina short period of time.
It stores data for temporary use.
Disadvantage:
Cache memory has limited capacity.
It is very expensive.

2. Primary Memory (Main Memory)
Primary memory holds only those data and instructions onwhichcomputer is currently working. It has limited
capacity and data get lost whenpower is switched off.
It is generally made up of semiconductor device. These memories are not as fast as registers. The data and
instructionrequired to be processed earlier reside inmainmemory. It is divided into two subcategories RAM
and ROM.
Characteristic of MainMemory
These are semiconductor memories.
It knownas mainmemory.
Usually volatile memory.
Data is lost incase power is switchoff.
It is working memory of the computer.
Faster thansecondary memories.
A computer cannot runwithout primary memory.

3. Secondary Memory
This type of memory is also knownas externalmemory or non-volatile. It is slower thanmainmemory. These are
used for storing Data/Informationpermanently.
CPU directly does not access these memories instead they are accessed via input-output routines. Contents of
secondary memories are first transferred to mainmemory, and thenCPU canaccess it.For example: disk, CD-
ROM,DVD etc.
Characteristic of Secondary Memory
These are magnetic and opticalmemories.
It is knownas backup memory.
It is non-volatile memory.
Data is permanently stored evenif power is switched off.
It is used for storage of the data inthe computer.
Computer may runwithout secondary memory.
Slower thanprimary memories.