Introduction to Central Processing Unit

Central Processing Unit

Introduction to Central Processing Unit

Central Processing unit widely known as CPU, is the brain of a computer and it executes program codes instruction after instruction in the logical sequence it is written. It runs system software (Operating system) and application software. It takes inputs from users and other active programs, processes the data, stores intermediate results in memory and displays final output in the Computer screen or stores it in external memory. CPU is an electronic circuitry and it performs logical, basic arithmetic, input/output and controlling functions as per the program flow. CPU is also known as Processor mainly referring to processing and controlling unit excluding main memory and I/O units.

History of Central Processing Unit

Given below is the history of Central Processing Unit:

1. Custom Design CPU

In the initial days during 1940s, computers were wired to perform various tasks and there were no software codes. CPUs were custom designed and used as part of distinctive computers. These computers were known as fixed program computer or stored program computer. Vacuum tubes and relays were used in these room sized and water cooled computers.

2. Transistor CPU

During 1950s and 1960s, the unreliable heavy vacuum tubes and relays were replaced with small sized transistors. More complex CPUs were built onto printed circuit board along with other key components. IBM system 360 was the first mainframe computer to be introduced by IBM in the year 1964. IBM used micro programming concept to make computer compatible and IBMS 360 architecture was so popular and it ruled mainframe market for many years.

Transistor based CPU had improved performance level, high reliability, lower power consumption and achieved clock rate of 10 Mhz. IC CPUs paved way for new processors like SIMD vector processors and facilitated era of super computers from Cray and Fujitsu.

3. Small Scale Integrations

Multiple interconnected transistors were accommodated onto a compact semiconductor space called Chip. Initially non specialized digital circuits have appeared in these chips and such ICs are called as small scale integration devices (SSI). SSI ICs were initially used in Apollo computer and later in IBM S/370, PDP- 10. It may need hundreds of chips to build a CPU still it saves lot of power and space.

4. Large Scale Integrations

Metal oxide semiconductor field effect transistor (MOSFET) facilitated development of large scale integrations circuit. Very low power consumption, high scalability, ability to accommodate higher density of transistors are the unique features of LSI circuits. PDP-11 computers was the first LSI implementations. Popular Course in this categoryAll in One Software Development Bundle (600+ Courses, 50+ projects)600+ Online Courses | 3000+ Hours | Verifiable Certificates | Lifetime Access
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5. Microprocessors

In Microprocessors, all the CPU components are accommodated on a single small IC and due to this design clock rates of these CPU goes up to several GHz. MOS IC technology triggered Microprocessor invention in 1970s. Intel 4004 in 1971 and Intel 8080 in 1974 dominated all other CPU implementation. Its instruction set enabled backward compatibility with earlier hardware and software versions in main frame computers. Microprocessor became synonym for CPU.

Components of Central Processing Unit

Given below are the three major basic components in Central Processing Unit:

  • Arithmetic Logic Unit(ALU)
  • Control Unit
  • Registers

With few add on components like Address generation units, Memory management unit, Cache.

1. Arithmetic Logic Unit

ALU is responsible for arithmetical and logical operations.

Inputs to these operations are given below:

  • Data(Operands): Supplied by the registers which stores the result of previous operations or fresh from external memory or constant value managed internally
  • Code (Operations): Fed by the control unit as an instruction.

At the end of the operation the results are stored in internal register for next operation or in the external memory through control unit if steps are complete and the status registers are updated accordingly. The speed with which these operations are performed is called as clock rate and CPUs are categorized based on the clock rate.

2. Control Unit

Component are as follows:

  • Controls the operation of the CPU.
  • Issues signals to ALU, memory and I/O devices at the right time to all the units in CPU in executing instructions.
  • Moves the data from the external memory to ALU.
  • Stores the output of the processing in the external memory.

3. Registers

Registers provide storage space for holding a.intermediate result, b.transit data before it is processed by ALU, c.Output data before it is moved to external memory, d.Instructions, e.Memory address. There are some registers used for internal purposes which cannot be accessed from outside and there are some accessible to users.

Given below are the several registers:

  • Instruction register brings instructions from program counter for processor to execute.
  • Memory buffer register.
  • Memory data register.
  • Memory address register.

4. Address Generation Unit

This unit computes the memory address from where the data need to be fetched. This unit functions in parallel to the others and saves time for execution and enhance the performance.

5. Memory Management Unit

This unit handles conversion of logical address into physical address, mange memory paging and virtual memory.

6. Cache

Frequently accessed data are stored in cache memory located near to the processor as part of CPU to avoid unwanted I/O operations and reduce processing time.

Working of Central Processing Unit

Any CPU irrespective of its size, form and complexity executes program that contains a set of instructions in a logical sequence. CPU process the first instruction and look for next instruction in the program counter. If the next instruction is a jump statement the corresponding instruction address to where the control need to be transferred is stored in program counter and the program jumps to that location and execution continues from there. In some computers with multiple processors, parallel execution is allowed where in multiple instructions simultaneously.

Various steps in the execution are common to most of the computers and they are fetch, decode and execute.

1. Fetch

This step gets the instruction from memory where program is stored. Address of the instruction is stored in program counter and once the address is processed it gets indexed to the next instruction in the logical sequence. Fetching of instruction may take some time due to slow memory and it is resolved by storing them in cache.

2. Decode

The instruction is decoded and type of operation to be performed and the operand on which the operation to be performed are culled out.

3. Execute

Execute step is performed using operation code and operand and the results are stored in registers.

Advantages of Central Processing Unit

Given below are the advantages mentioned:

  • CPU is the heart and brain of the computer and it is the primary unit in the computer.
  • It is small in size and fits well into its slot in motherboard. It facilitates design of small computers such as mobile phone, Tab and embedded computers etc.
  • Power of arithmetic and logical operations can be enhanced by having multiple processors in a computer.
  • It crunches mathematical and business data faster.


Computers size is miniaturized continuously and they find place in watches, toys, machines and on everything on earth. This is made possible with matching improvisations in CPU design and functionalities. CPU is an essential and critical part of the computer.

Recommended Articles

This is a guide to Central Processing Unit. Here we discuss the introduction, history of CPU, components, working and advantages respectively. You may also have a look at the following articles to learn more –

  1. Logical Address
  2. What is Register?
  3. Types of Computer Architecture
  4. Types of Computer Hardware

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