Introduction to the Central Processing Unit

The CPU (central processing unit) is commonly known as the brain of a computer. It executes all instructions in the order they are written. It runs both system software (Operating System) and application software. It receives inputs from users as well as other active programs. Then it processes them, stores intermediate results in memory, and displays the final output on the Computer screen. The CPU is an electronic circuitry that performs basic, logical and control functions according to the program flow. The CPU, also called Processor, refers to the processing and controlling unit that is not included in main memory or I/O units. Buy second-hand server CPU online in India, Shop all kinds of Refurfbished IT spare parts online at

History of Central Processing Unit

Below is the history of the Central Processing Unit.

Computers were wired to perform different tasks in the 1940s. There were no software codes at that time. Computers with CPUs were made to order and included in unique computers. These computers were also known as fixed-program computers or stored-program computers. These computers were room-sized and water-cooled and used vacuum tubes and relays.

Transistor CPU

In the 1950s and 1960s, unreliable heavy vacuum tubes were replaced by relays and relays. Printed circuit boards were used to house more complex CPUs and other key components. IBM system 360 was introduced in 1964 as the first mainframe computer. IBM introduced micro programming to make computers compatible. IBMS 360 architecture was very popular and ruled the mainframe market for many decades.

A Transistor-based CPU offered higher performance, reliability, and lower power consumption. It also achieved a clock rate of 10 Mhz. IC CPUs opened the door to new processors such as SIMD vector processors, and facilitated the era of supercomputers from Fujitsu and Cray.

Integrations at a Small Scale

Chip is a small semiconductor space that houses multiple interconnected transistors. These chips have seen non-specialized digital circuits in the beginning. Small scale integration devices (SSI) are used for such ICs. SSI ICs first appeared in Apollo computer, then later in IBM S/370 and PDP-10. Although it may take hundreds of chips to make a CPU, it is very efficient and saves a lot of space and power.

Large-Scale Integrations

Large-scale integrations were possible thanks to the metal oxide semiconductor field effect transistor (MOSFET). LSI circuits have unique characteristics such as low power consumption, high scalability and the ability to accommodate higher transistor density. PDP-11 computers were the first LSI implementations.

Microprocessors have all of the CPU components on a single small IC. This design clock rate allows for several GHz. Microprocessor invention was triggered by MOS IC technology in the 1970s. All other CPU implementations were dominated by Intel 4004 and Intel 8080, both in 1971 and 1974. Its instruction set allowed backward compatibility to earlier hardware and software versions on mainframe computers. The term microprocessor was later shortened to CPU.

Central Processing Unit Components

Below are the three main components of Central Processing Unit.

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

A few components can be added such as Address generation units, Memory management units, and Cache.

Arithmetic Logic Unit

ALU is responsible to perform arithmetical as well as logical operations.

Below are the inputs for these operations:

Code (Operations),: The code is fed by the control unit to be used as an instruction.

The operation is completed when the results are saved in an internal register. If steps have been completed, they are also stored in an external memory. Clock rate is the speed at which these operations are performed. CPUs are classified based on this clock rate.

These components are:

This controls the operation of CPU.

Sends signals to ALU, memory, and I/O devices when the appropriate time is available to all units in CPU for execution of instructions.

Transfers data from external memory to ALU

The output of the processing is stored in the external memory.


Registers are used to store a.intermediate results, b.transit information before it’s processed by ALU, and c.Output Data before it’s moved to external memory. d.Instructions, and e.Memory addresses. Some registers are only accessible from within the organization, while others are not available to outsiders.

Below are the various registers:

Instruction register holds instructions for the processor to execute.

Register memory buffer.

Register memory data

Register memory addresses

This unit calculates the memory address where data must be fetched. This unit can be used in parallel with the others to save time and improve performance.

Memory Management Unit

This unit is responsible for converting logical addresses into physical addresses, managing memory paging, and virtual memory.

To avoid I/O errors and speed up processing, frequently accessed data is stored in cache memory near the processor.

Central Processing Unit

No matter how large or complex the CPU is, it can execute a program that contains a sequence of instructions. The CPU will process the first instruction and then look for the next instruction in program counter. If the next instruction is an instruction that is a jump statement, the instruction address to which the control needs to be transferred is stored in the program counter. The program then jumps to the location and continues execution. Parallel execution is possible on some computers that have multiple processors.

Most computers have the same steps for execution: fetch, decode, execute.

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