CENTRAL PROCESSING UNIT II - MEMORY UNIT (SS2 FIRSTTERM)

 Memory Unit

The memory unit is the part of the computer that holds data and instructions for processing. Although it is closely associated with the CPU. In actual fact, it is separate from it. Memory unit associated with CPU is also called Primary Storage

and main storage, internal storage and main memory. When we load software from a floppy disk, hard disk or CD-ROM, it is stored in the main memory.

There are two types of primary memory (RAM) and (ROM).

RAM – stands for Random Access Memory. This is the main storage and is the place where the programs and software we load gets stored.

• This is really the main store and is the place where the programs and software we load gets stored. When the Central Processing Unit runs a program, it fetches the program instructions from the RAM and carries them out.
• If the Central Processing Unit needs to store the results of calculations it can store them in RAM.
• Random Access Memory can have instructions READ from it by the CPU and also it can have numbers or other computer data WRITTEN to it by the CPU.
• The more RAM in your computer, the larger the programs you can run.
• When we switch a computer off, whatever is stored in the RAM gets erased.

ROM – stands for Read Only Memory. The CPU can read instructions from read only memory. ROM comes with instructions permanently stored inside and these instructions cannot be over written by the computer CPU. ROM is used for storing special sets of instruction which the computer needs when starting up. When we switch the computer off, the content of the ROM does not become erased but remain stored permanently. Therefore, it can’t be deleted.

The CPU can only fetch or read instructions from Read Only Memory (or ROM). ROM comes with instructions permanently stored inside it. Therefore it is non-volatile it.

Types of memory

• Primary memory (main memory)
• Secondary memory (auxiliary memory)

PRIMARY MEMORY

It's also known as the main memory is the storage in the computer in which data is stored for quick access by the CPU and are connected via a memory bus.

SECONDARY MEMORY

This is the permanent, non-volatile memory that is not directly accessed by the computer/processor. The secondary storage is the slowest and cheapest form of memory. Examples of secondary storage include Hard Disk (Local Disk), Optical disk (CD, DVD), Floppy Disk, USB flash drive, memory card etc.

Differences between primary and secondary memory

Primary	Secondary
It is the internal memory	It is the external memory
It is under the direct control of the CPU	It is not directly under the control of the CPU
It cannot be used for massive data storage	It can be used for massive data storage
It is faster than the secondary storage	It is slower than the primary storage
It does not supplement the secondary storage	It supplements the primary storage

 

Exercises

1. Write the difference(s) between CD-R and CD-RW, DVD-R and DVD-RW

2.  2. Write the size, and technology of the following storage devices:

1.  ·    Hard disk

2.  ·    Floppy disk

3.  ·    USB flash drive

4.  ·    Magnetic tape

     5.       Compact Disc