The CPU is the central processing unit of a computer that controls and coordinates all operations. It consists of two main components - the control unit and the arithmetic logic unit. The control unit fetches and decodes instructions, and signals other components to perform tasks. The ALU performs arithmetic and logic operations on data. The document also discusses registers, processor speed, instruction sets, and types of processors like CISC and RISC.

1. Ashis Talukder, MIS, DU. 1
Lecture 8
The Processor and Its
Component

2. Ashis Talukder, MIS, DU. 2
The CPU
 The brain.
 Performs all major calculations.
 Controls and manages the operations of other
components of the computer.
 Determines much of the performance of the
computer.
 The two basic components of the CPU are:
 The Control Unit (CU).
 The Arithmetic Logic Unit (ALU).

3. Ashis Talukder, MIS, DU. 3
The CPU

4. Ashis Talukder, MIS, DU. 4
The CPU
 Fetches instruction from
main memory.
 Decodes instruction to
determine which action is
required to be done.
 Based on instruction,
fetches data from
memory or I/O.
 Execution of instruction.
 Stores Result
Start
Fetch Instruction
Decode Instruction
Fetch Operand
Execute Instruction
Store Result
More
instructio
n?
Y
N
End

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The CPU
Start
Fetch Instruction
Decode Instruction
Fetch Operand
Execute Instruction
Store Result
More
instructio
n?
Y
N
End

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The Control Unit
 It selects and interprets program instructions
and then sees that they are executed.
 Contains:
 Special purpose registers.
 instruction register (IR)
 program control register (PCR).
 A decoder to perform various activities.
 IR holds the current instruction to be executed.
 PCR holds the address of the next instruction
to be executed.

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The Control Unit
 The decoder has necessary circuitry to
interpret the meaning of every instruction
supported by the CPU.
 Each instruction is determined by microcode –
which tell the CPU how to execute the
instruction.
 CU does no processing by itself.
 It coordinates the entire computer system
including I/O units.

8. Ashis Talukder, MIS, DU. 8
The Control Unit
 Basic task – summary:
 Obtain instructions from program stored in
memory.
 Interpret/decode those instructions.
 Issue signals.
 The signals cause other units of the system to
do their tasks.

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The ALU
 Actual data processing takes place here.
 CU hands control over to the ALU when it
encounters instructions such as add, divide etc.
 Contains:
 Special purpose registers.
 Circuitry to perform arithmetic & logic
operations such as adder.
 Example: CU might load 2 numbers into the
ALU registers and then tell the ALU to add
them.

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Instruction Set
 A set of machine instructions supported by the
processor such as add, compare etc.
 Each different processor has different
instruction set. So machine language programs
for one processor will not execute in another
processor.
 When a manufacturer develops a CPU, they try
to maintain upward compatibility.

11. Ashis Talukder, MIS, DU. 11
Registers
 Temporary storage of data. Not a part of main
memory.
 CPU always puts data from RAM to internal
register before it processes that data
 Most CPU registers of today are of 32 or 64
bits. Known as word size.
 The bigger the word size the faster the CPU
can process data.

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Registers
 Number of registers vary from computer to
computer.
 Some are common to all computers:
 Memory buffer register (MBR)
 Temporary storage for memory contents.
 Instructions are transferred to IR.
 Data are accessible to the ALU registers and can also be
sent to I/O registers.
 Data needed to be written in memory is placed here
first.

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Registers
 Memory address register (MAR)
 Holds address of active memory location.
 Loaded from PCR.
 Accumulator register (A)
 Holds data and result after processing.
 Mostly used.
 Result that is to be written in memory is
transferred from here to MBR.

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Registers
 Program control register (PCR)
 Holds memory address of next instruction to be
executed.
 Instruction register (IR)
 Holds current instruction to be executed.
 The operation part is sent to CU.
 Address is part sent to MAR.
 I/O register (I/O)
 Used to talk to I/O devices.
 Data & instruction from/to I/O devices are first put
in this register.

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Processor Speed
 Operations of ALU and CU are synchronized by
an internal clock called system clock.
 Generates electrical pulses.
 Several clock cycles are needed to execute a
single instruction. (fetch, decode, execute).
 Measured in megahertz. MHz and now-a-days
GHz.

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Types of Processor
 CISC (complex instruction set computer)
 Supports a large number of instructions.
 Makes it easier to translate high level languages.
 Adds more circuitry and complexity to the overall
design of the CPU.
 Supports variable length instructions.
 Supports various addressing modes.
 Expensive.
 Makes the job of machine level programmer easier.
 Most processors of today are CISC. Intel’s Pentium.

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Types of Processor
 RISC (reduced instruction set computers)
 Low design complexity.
 Supports few number of instructions.
 Complex instructions are performed by the basic
instructions. E.g. multiplication is performed by
repeated addition.
 Fixed length instructions.
 Supports small number of addressing modes.
 Puts extra burden on machine language
programmers.
 Less expensive.
 Faster for most applications.
 Motorola’s processors.