Lecture 10ALU and Control Unit Design

Presented ByDr. Shazzad Hosain

Asst. Prof. EECS, NSU

ALU Design• Arithmetic Unit• Logic Unit

4-bit parallel adder

Cin

Cout

x3

x2

x1

x0

y3

y2

y1

y0

X

Y

Arithmetic Unit

Ff3

f2

f1

f0

A3

A2

A1

A0

B3

B2

B1

B0

Arithmetic UnitLets take the following two operations

F = X + Y F = X – Y

4-bit parallel adder

Cin

Cout

XF

Y

S0

01

= X + Y + 0= X + (-Y) = X + (Y + 1) = X + Y + 1

4-bit parallel adder

MUX0

1

MUX0

1

MUX0

1

MUX0

1

Cin

Cout

x3

x2

x1

x0

y3

y2

y1

y0

S0

Ff3

f2

f1

f0

Block Diagram of Arithmetic Unit

Arithmetic Unit

4

4

X

Y

4F

S0

Logic UnitS0 Operation

0 X AND Y

1 X XOR Y

Block Diagram of Logic Unit

LogicUnit

4

4

X

Y

4G

S0

Block Diagram of ALU

LogicUnit

4

4X

Y

4 G

S0

Arithmetic Unit

4

4

X

Y

4 F

S0

4

Mul

tiple

xer 4

Z

S1

Design of the Control Unit

• Two types of control unit design– Hardwired Control Design– Micro-programmed Control Unit Design

Basic Concepts Transfer to

Transfer 14th bit of to 1st bit of

Transfer to if E is enabled

Multiplication Using Addition

Microprocessor speed depends on bus architecture

Bus Architecture

• Three types of bus architectures– Single-bus architecture– Two-bus architecture– Three-bus architecture

Single Bus Architecture

Two-bus Architecture

• One of the main task of control unit is to sequence a set of operations properly

• Use a ring counter to implement this

Timing Signals

Hardwired Control Design• Eight Steps to follow

Step 1: Derive the Flow ChartStart

Stop

R 0M Multiplicand via inbus

Q Multiplier via inbus

R R + MQ Q – 1

Q=0

Outbus R

no

yes

Step 2: Obtain register transfer description

Step 3: Specify processing hardware along with various components

Step 4: Complete the design of the processing section

M

L

CD

C0: R ← 0C1: M ← inbusC2: Q ← inbusC3: F ← r + lC4: Q ← Q – 1 C5: outbus ← RC6: R ← F

R

CL

DQ

LD

C

4 4

C0

44

4-bitadder

F

C1 C2

C3

C4

C5

C6

Z

Step 5: Determine the block diagram of the controller

0

1

2

3

4

5

6

References

• Chapter 7, Fundamental of Digital Logic and Microcomputer Design – by M. Rafiquzzaman