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Lecture 2

CPU BASICS

Central Processor Unit

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FigureFigure 1-1 Block Diagram of a Digital Computer

Memory

Input Device Output DeviceInterface Port

Or IOP

CPU

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A typical CPU has three major components:

yregister set

y

arithmetic logic unit (AL

U)y control unit (CU)

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A CPU contains very fast memory called registers.The register can do one of two things: it can

parallel load the inputs, that is, it can read in allfour inputs and store the four bits into the array.Each element of the array can store a single bit.

Register

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The black box shown has four inputs or (8, 16, 32 inputs).

Data Inputs

b3 b2 b1 b0

1 0 1 0

z3 z2 z1 z0

Outputs

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Register Types

The register set differs from one computerarchitecture to another. It is usually a

combination of general-purpose andspecial purpose registers.

General-purpose registers are used for

any purpose, hence the name generalpurpose. For example (A, B, C, D, R1, R2, R3)

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Special-purpose registers have specificfunctions within the CPU.

For example of special-purpose registers:

The program counter (PC) is a special-purpose register that is used to hold theaddress of the instruction to be executed

next.The instruction register (IR) which isused to hold the instruction that is currentlyexecuted.

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Other processor registers used by a program instruction.

Instruction Decoder (ID)

MAR (Memory Address Register)

MDR (Memory Data Register) .

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The ALU provides the circuitry needed toperform the arithmetic, logical and shift

operations demanded of the instructionset.

ALU

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The control unit is the entity responsiblefor fetching the instruction to be

executed from the main memory anddecoding and then executing it.

Control Unit

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A typical and simple execution cycle can be summarized as follows:

y The next instruction to be executed, whoseaddress is obtained from the PC, is fetched fromthe memory and stored in the IR.

y The instruction is decoded.y Operands are fetched from the memory and

stored in CPU registers, if needed.

y The instruction is executed.

y Results are transferred from CPU registers to thememory, if needed.

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The CPU fetches instructions frommemory, reads and writes data from and

to memory, and transfers data from andto input/output devices.

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Question

-List the main components of CPU.

-Define the following:

y Register

y ALU

y Control Unit