CPU CPU ORGANISATIOORGANISATIO

NN

CPU (CPU (Central Processing UnitCentral Processing Unit))

Acts as the brain of computer.

Performs all the calculations & controls all the

components.

Carries out the instructions of a computer

program, performs the basic arithmetical,

logical, and input and output operations of the

system.

CPU Has Two Main CPU Has Two Main ComponentsComponents

Control Unit (CU)

Arithmetic & Logic Unit (ALU)

Control UnitControl Unit

Selects program instructions & coordinates their execution.

Regulates timing of processor.

Sends control signal to and receives control signal from peripheral devices.

Acts as central nervous system for all other components.

It obtains instruction from memory, interprets the instruction & issues signals to the required units to carry out the execution.

ALUALUActual execution of instruction takes place

in ALU.

It consists of a complicated set of logic circuit, registers and accumulator to carry out all operations.

Depends on the commands given by the Control Unit.

RegistersRegistersIt holds data temporarily during the execution

of instruction.They are a part of CPU.Length of register = No. of bits in can store.

Two types - Special Purpose Registers General Purpose Registers

List of Special Purpose RegistersList of Special Purpose Registers

Decoder

Program Control Register

Memory Address Register

Memory Buffer Register

Instruction Register

I/O Register

Accumulator

Decoder It interprets the instruction to be executed.

Program Control Register• It holds the address of next instruction to be executed.• In case of branch instruction, system passes the address part of branch instruction to PC.

Memory Address Register It holds the address of the active memory

location.

Memory Buffer Register

It holds the content of accessed memory word.

It holds the data word when it is transferred to and from the main memory.

Instruction Register It holds the instruction being executed. Instruction is divided into two parts :

Operation Part - Given to MAR

Address part - Given to Decoder

I/O Register

It is used for communication with the I/O devices. An input unit transfers all the contents to this register

& then system picks up the data & vice-versa.

Accumulator It holds the data under execution, intermediate results

& result of operations. Result Accumulator MBR Main Memory

Main MemoryMain MemoryRAM (Random Access Memory)Volatile MemoryEvery file or application opened is placed

in RAMUses Random Access Technique, i.e., the

data can be directly jumped to the middle if necessary without having to read the data sequentially.

ROM (Read Only Memory)ROM (Read Only Memory)Non-volat ile Memor yThe inst r uct ions ar e st or ed by

manuf act ur er sSpecial pr ogr ams ar e st or ed

called mi cro-programs which deal wit h low-level machine f unct ions.

Types of ROM --• PROM (Programmabl e ROM)• EPROM (Eras abl e Programmabl e ROM)

Cache MemoryCache MemoryExt r emely f ast & small memor yPlaced bet ween CPU & Main

memor yUsed f or t empor ar ily st or ing

act ive dat a & f or minimizing t he dat a f et ch t ime

CPU

Cache Memory Main

Memory

Steps of Execution Of InstructionSteps of Execution Of Instruction1 ) The CU ta ke s a ddre s s o f the ne x t ins truc tio n

fro m the PC Re g is te r & re a d s the ins truc tio n fro m c o rre s p o nd ing m e m o ry a ddre s s into the Ins truc tio n Re g is te r.

2 ) CU the n s e nd s the o p e ra tio n & a ddre s s p a rt to De c o d e r & MAR re s p e c tive ly .

3 ) De c o d e r inte rp re ts the ins truc tio n s e nd s c o m m a nd s ig na ls to the a p p ro p ria te units to c a rry o ut the ta s k s p e c ifie d .

4) As the s y s te m e x e c ute s e a ch ins truc tio n, it lo a d s the a ddre s s o f ne x t ins truc tio n in PC Re g is te r & re p e a ts s te p s 1 to 4.

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