RAM ( RANDOM ACCESS MEMORY )

What is a RAM?

The memory system in the computer that is easily read from

and written to by the processor in the RAM. In the RAM,

any address may be accessed at any time, i.e, any memory

location can be accessed in a random manner without going

through any other memory location. The access search time

for all the memory locations in the same. RAM is the main

memory of computer system. The RAM is a very fast memory,

both for reading and writing information. The information

 written in it is retained as long as the power supply is on.

All information stored in the RAM is lost when the power

supply is switched off.

The two main classes of RAM are Static ( SRAM ) and

Dynamice RAM ( SRAM )

Static RAM ( SRAM )

A static RAM is made from an array of flip-flops, where each

flip-flop maintains a single bit of data within a single memory

address or location.

A static RAM retains its data without external refresh as long

as electricity is available.The other features of SRAM are as

follows:-

• It is a type of semiconductor memory.
• It does not require any external refresh to keep its data intact.
• It is used for high-speed registers,caches and small memory  

banks such as router buffers.

• It has access times in the range of 10 to 30 nanoseconds and,

hence, allows for very fast access.

• It is very expensive.

Dynamic RAM ( DRAM )

A dynamic RAM keeps its data only if is accessed by special logic 

called refresh circuit - on a continuous basic . This circuitry reads

the data of each memory cell very fast, irrespective of weather 

the memory cell is being is being used at that time by the computer

or not. The memory cell are constructed in such a way that the 

reading action itself refreshes the contents of the memory. If not 

done on a regular basic , the DRAM will lose its contents of the 

memory . If it has uninterrupted power supply . Because of this

refreshing action , the memory is called dynamic. The other features

of DRAM are as follows:-

• It is the most general type of memory . It is used in all PCs 

for their main memory system.

• It has a much higher capacity.
• It is cheaper than SRAM.
• Due to the refresh circuitry, it is slower than SRAM.

Read Only Memory ( ROM )

The Read Only Memory , which is a secure memory , is not affected

by any interruption in the power supply . It is a non-volatile memory, 

i.e, information stored in it is not lost even if the power supply goes 

off. It is used for permanent storage of information and possesses 

random access properties.

The essential purpose of the ROM is to store the Basic Input/Output

system ( BIOS ) of the computer. The BIOS commands the processors

to access its resources on receiving power supply to the system.

The different types of ROMs are as follows:-

Programmable Read Only Memory ( PRAM )

Data is written into a ROM at the time of manufacture. However, a user
can program the contents with a special PROM programmer. PROM
provides flexible and economical storage for fixed programs and data.

Erasable Programmable Read Only Memory ( EPROM )

This lets the programmer erase the contents of the ROM and reprogram it.
The contents of EPROM cells can be erased with ultraviolet light using an
EPROM  programmer. This type of ROM provides more flexibility than a
ROM during the development of digital systems. Since these are able to
retain the stored information for a longer duration duration, any change can
be easily made.

Electrically Erasable Programable Read Only Memory ( EEPROM )

In this type of ROM , the contents of the cell can be erased electrically by
applying a high voltage. The EEPROM need not removed physically for
 reprogramming.