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Principles of Computer Architecture

ALU & MICROARHITECTURE

DESIGN

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High Level View of Microarchitecture

The microarchitecture consists of the control unit andthe programmer-visible registers, functional units such as

the ALU, and any additional registers that may be required

by the control unit.

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Basics of the Microarchitecture

The Fetch-Execute Cycle

The steps that the control unit carries out in executing a

program are:

(1) Fetch the next instruction to be executed from memory.

(2) Decode the opcode.

(3) Read operand(s) from main memory, if any.

(4) Execute the instruction and store results, if any.

(5) Go to step 1.

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Designing A Microarchitecture

Hardware component:-

ALU

Register for Source Operand

Register for Destination Operand

Bus for Data Transfer Register to ALU(read)

Bus for Data Transfer ALU to Register(write)

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High Level View

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Basics of the Microarchitecture

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Designing A Microarchitecture Using

ALU 74181R0

R1

R2

R3

ENB

ENB

ENB

ENB

ENB

ENB

ENB

ENB

ABus BBus

ALU74181

S3S2S1S0

CBus

WriteEnable

C Decoder A Decoder BDecoder

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ALU 74181

Refer toALU74181.doc

Microword formatCmux M C S3S2S1S0 Abus Bbus Cbus

0 x x xxxx xx xx 00 Load R0

0 x x xxxx xx xx 01 Load R1

1 0 0 1001 0 1 10 R2=R0 plus R1

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Expanding the ALUALU

1BitY1A1

B1

ALU

1BitY2A2

B2

ALU

1BitY3A3

B3

ALU1Bit

Y4A4

B4

F1F0

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Creating the DatapathR0n

2

1. Input 2 to Cbus.

2. Select 0 to choose R0.

3. Press S1 to load to R0.

R1 n 3

1. Input 3 to Cbus.

2. Select 1 to choose R1.3. Press S1 to load to R1.

Abus Decodern R0

Select 0 to choose R0.

Bbus Decodern R1Select 1 to choose R1.

R0

R1

R2

R3

ENB

ENB

ENB

ENB

ENB

ENB

ENB

ENB

ABus BBus

ALU

74181

S3S2S1S0

CBus

Write

Enable

C Decoder A Decoder BDecoder

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Creating the control sequence

Cmux = to choose data from memory or ALU 0 input to Cbus

1 ALU output to Cbus

M, C & S3S2S1S0 = input for ALU 74181 Abus = register to Abus.

Bbus = register to Bbus.

Cbus = Cbus to Register.

Cmux M C S3S2S1S0 Abus Bbus Cbus

1bit 1bit 1bit 4bit 2bit 2bit 2bit

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Example 1 Determine the control sequence for

R2

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Example 2

Determine the control sequence forR3

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Example 3

Determine the control sequence for R2

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or

2R1R2R1plus0R2R

0R0R

21R

30R

n

n

n

n

n