Central Processing Unit (CPU) :

Introduction & Features

    • The task of performing operations like arithmetic,logical and non-logical/arithmetic operations is called Processing.
    • The CPU is a complex IC chip having millions of transistors.
    • The CPU or central processing unit takes data and instructions from the storage/ memory unit and makes all sorts of calculations based on the instructions given and the type of data provided. It is then sent back to the storage unit to store it.
    • In order to carry out the operations, the computer allocates the task among its various operational units i.e. arithmetic logical unit, control unit, and register.
    • The term CPU relates to a specific chip or the processor.
    • Central Processing Unit (CPU) is considered as one of the most important component of a computer system.
    • The Central Processing Unit (CPU) is like the brain of the computer. This is because it takes all major decisions, makes all sorts of calculations and directs different parts of the computer functions by activating and controlling the operations.
    • The fundamental operation of most CPU is to execute a series of instructions called as a program.
    • The main function of a CPU is to execute a series of instructions called as program in a specific sequence. Normally there are four steps that all CPU use in order to perform their operation these are: fetch, decode, execute and output.
    • A dual-core processors has two cores, a quad-core processor contains four cores and a hexa-core processor contains six cores on a single chip. Multi-core processors implement multi-processing which improves performance and speed of executions.
    • The different chip manufacturers use different measuring standards to measure the processor‟s speed. It depends on the circuit board that the chip is housed in, or the motherboard. 
    • It is responsible for executing instructions.
    • It controls and coordinates the execution of instructions.
    • Execution of an instruction involves almost all parts (CU, ALU & Registers) of the CPU. Hence, CPU is known as the most vital component of a computer system. 
    • CPU mainly involves in processing of input data.Processing the data thus means manipulation of letters, numbers and symbols in a specific manner. The processing may include calculations, decision making, comparisons, classification, sorting, aligning & formatting etc. The processing of data results in some meaningful values/output, often termed as ‘information’.
    • As we know that, computer takes the raw data as input and performs several operations on these data in order to produce the desired output by the CPU so it acts as a processing unit for those data.
    • The CPU chip is connected with other components of the computer through a system bus (printed wires on the mother board) which has three sets of wires forming Control Bus, Data Bus and Address Bus.

Components of CPU

    • A CPU typically comprised of a (A)Control Unit (CU), (B)an Arithmetic & Logic Unit (ALU), (C) Set of Registers and (D)Memory Unit(MU) 

(A)Control Unit (CU) – 

    • The Control Unit of the processor is a unit which controls and coordinates the execution of instructions by the processor.
    • The CU controls the execution of instructions by decoding the instruction and generating micro-operations to be performed for executing that instruction. 
    • Control Unit (CU) is the unit which manages and coordinates the entire operation of a computer system. It controls the operation of the other components of a computer system.
    • It acts like the supervisor and monitors that the operations are done in proper systematic fashion or not.
    • The control unit is responsible for overall control and coordination of instruction execution i.e. It is responsible for defining and controlling the instruction cycle.
    • It issues control signals external to the processor to cause data exchange with memory and I/O modules.
    • It also issues control signals internal to the processor to move data between registers, to cause the ALU to perform a specified function, and to regulate other internal operations.
    • It generates timing signals and initiates the Fetch cycle of instruction execution. When the instruction is fetched, it generates the sequence of micro-operations which need to be executed in order to execute the instruction.
    • The control unit determines the sequence in which computer programs and instructions are executed. 
    • It also acts as a switch board operator when several users access the computer simultaneously. Thereby it coordinates the activities of computer‟s peripheral equipment as they perform the input and output. Therefore, it is the manager of all operations mentioned in the previous section.
    • Most of the modern CISC processors use microprogram control unit.

(B)Arithmetic & Logic Unit (ALU)

    • The ALU is responsible for performing arithmetic and logic operations on data.The CU, CPU registers and memory help in bringing the data into the ALU and then taking the results back.
    • Data are presented to ALU in registers and the results are also stored in registers. Accumulator is one such register which is very frequently used during the ALU operation.
    • ALU has many other registers such as flags and status register, which indicate information about the operation and its result.
    • ALU has logic implemented to perform operations like addition, multiplication, division, shifting, complement etc. The operations are performed on represented numbers, both integer and floating point numbers.
    • Modern processors work through two ways which improve ALU performance to a much higher level. These are use of on chip Cache memory and having more than one processor core on the same IC chip. Here, Cache memory is a fast semiconductor memory which can be used to temporarily store instructions and data that are frequently referred by the processor thus, the wait cycles introduced due to memory references are minimized and hence the processor performance improves a lot. Another modern technique of having use of more than one processor core on the same IC chip tries to perform the execution of instructions in parallel and hence the performance of the processor improves a lot.
    • The co-processor or the arithmetic-logic unit does arithmetic, non-arithmetic and logical operations.
    • The Arithmetic Logical Unit is an important component of the CPU, which carry the actual execution of the instructions.
    • The brief working mechanism of ALU is – After entering the data through the input device it is stored in the primary storage unit/RAM. Then processing of the data and instruction are performed by Arithmetic Logical Unit. After that output is sent to the monitor screen to display or printer to print on paper as hard copy or hard disk to store it permanently via main memory or RAM.
    • The major operations performed by the ALU during processing are addition, subtraction, multiplication, division, logic and comparison.
    • Data is transferred to ALU from storage unit when required. After processing, the output is returned to the storage unit for further processing or getting stored.

(C)Registers/CPU Registers/Processor Memory

    • CPU has a set of Registers which is used to store some data/address/instruction temporarily.
    • Register lies at the top of Cache and Main memory in memory hierarchy of the system.
    • The registers in CPU has two major categories : –
      (i) User-visible registers : – Used to store temporary data items and other user accessible information useful for machine or assembly language programmers.
      (ii) Control & Status Registers : – Used by control unit to control and coordinate the operation of the processor.
    • A processor memory is the fastest accessible memory present inside a cpu and is available to the processor during certain operations.
    • Registers are typically addressed by mechanisms other than main memory.
    • They store data temporarily i.e. volatile memory.
    • They are smallest in size.
    • They are normally at the top of the memory hierarchy, and provide the fastest way to access data.
    • They store small amount of data, normally 1 to 2 byte per register but in some cases it may store 4-8 bytes or even more.

(D)Memory Unit(MU)

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