Computer Memories

 Computer Memories

Memory

The memory of computer is where the program and data are stored before the calculations begin. The memory is therefore one of the most active parts of the computer. The memory is equivalent to thousands of registers, each storing a binary word.

Memory Cell

A device or electrical circuit a single bits is called a memory cell.

For Example : Flip flop,a charged and a single spot on a magnetic tape or disk.

Memory Word

A group of bits in a memory that represent instrucion or data of same type is called a memory word.

Example : If  Register has 8 Flip flop, It stores 8-bit word.
In the computer word size range is 4 to 64 bit.

Byte

A group of 8-bits is termed as byte.

Memory capacity or Density

By memory capacity, we mean that how can we specify the number of bits that can be stored in th memory.

Example : Memory stores 512 word in the 8-bit,memory capacity will  be  512 X 8. 512 is the no of words and 8 is word-size. The memory of big-size is represent in the "Kilo" or "Mega". 1k = 2 = 1024 and 1M = 2=1,048,576.
If the size of memory is 2048 X 8. It writes 2k X 8.

Address

Address is a number that identifies the location of word in the memory. Each word stored in the memory has a unique binary address.
 To store 8 word store, each location address will be 3 Bit.
                                                                                

                                                 Address                            Words
                                                
                                                  0 0 0                                Word 0 

                                                  0 0 1                                Word 1

                                                  0 1 0                                Word 2

                                                  0 1 1                               Word 3               

                                                  1 0 0                               Word 4

                                                  1 0 1                               Word 5

                                                  1 1 0                               Word 6

                                                  1 1 1                               Word 7     

        

 Read Operation

Read opration is the by which the binary word stored in a specific memory location is sensed and then transfered to another device.The read operation says fetch operation.

Write Operation

The operation where a new word is placed or stored into a particular memory location is called a write operation.

Access time

It is a measure of operating speed of the memory. It is amount of time required to perform a read operation. It is the time between memory receiving a new address input and data becoming available at the memory output.

Volatile Memories

Volatile memories can store information with electrical power.

Non-Volatile Memories

Non volatile memories can store information without electrical power. All magnetic memories are non-volatile.

Random Access and Sequential Memory(RAM and SAM)

Ram is a type of memory in which the access time is same for any address in the memory. SAM is a type of memory in which the access time is not constant but varies depending on address location.

Read/Write Memory and Read only Memory (RWM and ROM)

Any memory that can be read from or written into with equal ease is called a Read/Write Memory. In a ROM, the data is written into only once, and this operation is performed at the factory (or by programmer). Thereafter, the information can only be read from the memory.

Static memory 

Semiconductor memory devices in which the stored data will remain permanently stored as long as the power is applied, wihout the need for periodically rewritting the data into memory, are called static memory.

Dynamic memory
Semiconductor memory devices in which the stored data will not remain permantently stored, even with power applied, unless the data are periodically rewritten(refreshed) into the memory are called dynamic memory.

Internal memory

Internal memory stores instruction and data CPU is currently working on.

Auxiliary memory or External memory

Auxiliary memory stores massive amount of information eternal to internal memory.

Semi conductor and Non-semi conductor memory

Semi conductor memory is made of semiconductor materials while non-semiconductor memory is made of magnetic materials. The example of magnetic memory are Floppy disc, Cassette, Hard disc, Magnetic tapes, Magnetic core and Magnetic drums.

Destructive memory

During read operation, if data is lost from the memory than the read out is called a destructive read out. Memories with destructive read out are called destructive memories.

Non-destructive memory

Memories having a non destructive read out i.e, data remains intact during a readout are termed non destructive memories.