WHAT IS CPU ? Full Form of CPU in English

 What is CPU ?

A computer needs some sort of ‘brain‘ or ‘calculator‘. At the core of every computer, there is a device roughly the size of a large postage stamp. This device is known as the Central Processing Unit (CPU). This is the ‘brain’ of the computer; it reads and executes program instructions, performs calculations and makes decisions. The CPU is reponsible for storing and retrieving information on disks and other media. It also handles information from one part of the computer to another like a central switching station that directs the flow of traffic throughout the computer system.

What is Full form of CPU ?



The CPU carries out instructions and tells the rest of the computer system ‘what to do’. This is done by the Control Unit of the CPU which sends command signals to the other components of the system.

cpu diagram

  • It also performs arithmetic calculations and data manipulation, eg. comparisons, sorting, combining, etc. This is performed by a part of the CPU known as the Arithmetic Logic Unit.
  • It holds data and instructions which are in current use. These are kept in the Main Store or Memory.

Control Unit :-

The control unit directs the entire computer system to carry out stored program instructions. The control unit
must communicate with both the arithmetic logic unit and main memory. The control unit uses the instruction contained in the Instruction Register to decide which circuits need to be activated.

The control unit co-ordinates the activities of the other two units as well as all peripheral and auxiliary storage devices linked to the computer. The control unit instructs the arithmetic logic unit which arithmetic operations or
logical operation is to be performed.

Specialised electronic circuitry in the control unit is designed to decode program instructions held in the main memory. Each instruction is read from the memory into the instruction register. The process of reading an instruction, is often referred to as the fetch-execute process.

Arithmetic Logic Unit :-

The arithmetic logic unit executes arithmetic and logical operations. Arithmetic operations include addition, subtration, multiplication and division. Logical operations compare numbers, letters and special characters.

Comparison operations test for three conditions:

  • equal-to condition in which two values are the same,
  • less-than condition in which one value is smaller than the other, and
  • greater-than condition in which one value is larger than the other.

These operations (=, <, >) are used to describe the comparison operations used by the arithmetic logic unit.

On the other hand, ALU also performs logic functions such as AND or and NOT. the accumulator is used to accumulate results. It is the place where the answers from many operations are stored temporarily before being put out to the computer’s memory. The other general-purpose registers hold data on which operations are to be performed by the arithmetic logic unit.

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, but in actual fact, it is seperate . Memory associated with the CPU is also called primary storage, primary memory, 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. It’s amazing ‘how many different types of electronic memory encounter in daily life.’ Many of them have become an integral part of the vocabulary: RAM, ROM, Cache, Dynamic RAM, Static RAM, Flash memory, Memory sticks, Volatile memory, Virtual memory, Video memory, BIOS.

We already know that computer has memory. What we may not know, is that most of the electronic items, we use every day, have some form of memory also. Here are just a few examples of the many items that use memory : Computers, Cell phones, Personal Digital Assistants (PDAs), Game consoles, Car radios, VCRs, TVs.

Each of these devices uses different types of memory in different ways.

There are two basic types of computer memory inside the computer, RAM and ROM.

(a) Random Access Memory (RAM): 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.RAM क्या है


RAM is the best known form of computer memory. RAM is considered ‘random access’ because anyone can access any memory cell directly if we know the row and column that intersect at that cell. The opposite of RAM is Serial Access Memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the needed data is found. SAM works very well for memory buffers, where the data is normally stored in the order in which it will be used (a good example is the texture buffer memory on a video card). RAM data, on the other hand, can be accessed in any order.

(b) Read Only Memory (ROM): The CPU can only fetch or read instructions from Read Only Memory (or ROM). ROM comes with instructions permanently stored inside and these instructions cannot be over-written by the computer’s CPU. ROM is used for storing special sets of instructions which the computer needs when it starts up.

ROM क्या है

The CPU can only fetch or read instructions from Read Only Memory (or ROM). ROM comes with instructions permanently stored inside and these instructions cannot be over-written by the computer’s CPU. ROM is used for storing special sets of instructions which the computer needs when it starts up.

the relationship between the Central Processing Unit and the Main Memory (RAM and ROM).

(c) Cache Memory: Caching is a technology based on the memory subsystem of the computer. The main purpose of a cache is to accelerate the computer while keeping the price of the computer low. Caching allows to do the computer tasks more rapidly.


Cache technology is the use of a faster but smaller memory type to accelerate a slower but larger memory type. When using a cache, we must check the cache to see if an item is in there. If it is there, it’s called a cache hit. If not, it is called a cache miss and the computer must wait for a round trip from the larger, slower memory area. A cache has some maximum size that is much smaller than the larger storage area. It is possible to have multiple layers of cache.

There are a lot of subsystems in a computer; one can put cache between many of them to improve performance. Here’s an example. We have the microprocessor (the fastest thing in the computer).
Then there’s the L1 cache that caches the L2 cache that caches the main memory which can be used (and is often used) as a cache for even slower peripherals like hard disks and CD-ROMs.
The hard disks are also used to cache an even slower medium — Internet connection. A list of different types of cache is given as follows:

  • L1 cache – Memory accesses at full microprocessor speed (10 nanoseconds, 4 kilobytes to 16 kilobytes in size)
  • L2 cache – Memory access of type SRAM (around 20 to 30 nanoseconds, 128 kilobytes to 512 kilobytes in size)
  • Main memory – Memory access of type RAM (around 60 nanoseconds, 32 megabytes to 128 megabytes in size)
  • Hard disk – Mechanical, slow (around 12 milliseconds, 1 gigabyte to 10 gigabytes in size)

(d) Flash Memory: Electronic memory comes in a variety of forms to serve a variety of purposes. Flash memory is used for easy and fast information storage in such devices as digital cameras and home video game consoles. It is used more as a hard drive than as RAM. In fact, Flash memory is considered a solid state storage device. Solid state means that there are no moving parts — everything is electronic instead of mechanical.

Here are a few examples of Flash memory:

  • Computer’s BIOS chip
  • CompactFlash (most often found in digital cameras)
  • Smart Media (most often found in digital cameras)
  • Memory Stick (most often found in digital cameras)
  • PCMCIA Type I and Type II memory cards (used as solid-state disks in laptops)
  • Memory cards for video game consoles

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