CSEN402: Computer OrganizationLecture 1: Register Transfer Level I

Nada Sharaf

Spring Semester 2020

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General Information

Lecturer: Nada Sharaf, office: C7.211, nada.hamed@guc.edu.eg.

TAs:Omnia Ayman, Sama El Baroudy, Fatma El Azab and NadaNasser

Textbook: Computer System Architecture, Third Edition, M. Morris,Mano

Share your questions through piazza:https://piazza.com/#spring2020/csen402csis402

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Credits and a Huge Thank You to Dr. MohammedEl-Mahdy

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Tentative Grading

Assignments (project) (15%)

Quizzes (15%)

Midterm (25%)

Final (45%)

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What

You can write algorithms

You can write programs

How do they work

How is the machine code acutally used

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Why

Computing is everywhereI Embedded systemsI Internet of Things

Optimizations could b achievedusing:

I ArchitectureI Better programsI Enhanced hardware

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How

Study of the Basic Computer and how it works

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Let us Start :)

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Register Transfer Level

Digital systems are designed using a modular approach

Modules are connected with data and control paths

A module is best describes using:I The registers it containsI Operations done on data stored in them Microoperations

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Microoperations

Basic operation performed onthe data in one or more registers

Result could clear contents of aregister or transfer data toanother register

ExampleI ShiftI ClearI LoadI Count

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Microoperations

Done in one clock cycle

R ← f (R,R)

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Digita Computer Description

The internal hardware of a digital computer is specified by:

1 The registers and their functions2 Sequence of microoperations on registers

I What are the allowed and available operations

3 Control of microopertionsI Signals to control and initiate the operations

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Register Transfer Language

Instead of using words to describe what the operations do in terms ofthe sequence of transfers between regsiters

Special syntax and symbols

Register Transfer Language

Symbolically describes the microoperations sequence of betweenregisters

Any functionality of the system could be describes using thisdescription level

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Registers

A register is described by anupper case letter

The letter could be followed bya numeral e.g. R3, R5, ... etc

There are some special registernames that indicate thefunctionality

I MAR: Memory AddressRegister

I PC: Program CounterI IR: Instruction Register

To save the bits of a register,flip flops are used

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Register Representation in Block Diagram

Rectangular box with name inside it

Show the individual bits

Show the numbering of bitsIndividual bits could be distinguished

Show the partitionsPC(L) or PC(0-7) or PC(H) or PC(8-15)

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Register Transfer

R2← R1

Used to copy contents from one register to the other

In this example contents of R1 are copied to R2

Which bits? All bits are being transferred

What happens to R1? It is unchanged i.e. non-destructive operation

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Register Transfer: How does it Happen

R2← R1

Contents of R2 are replaced by the current content of R1

R1 is unchanged

Circuits are available from output of source to input of destination

Transfer could also be controlled

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Control Signals

Control structures are crucial in all types of programming

We usually want actions (register transfer in our case) to happenunder certain conditions

We introduce if-then-else statements

if (P = 1) then (R2← R1)I P is our control signalI In case the control signal P holds the value 1I then load the contents of R1 into R2

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Control Functions

A control function can be used

A control function is a Boolean variable

This variable is equal to 1 or 0.

terminated with a colon

P : R2← R1

Equivalent to if (P = 1) then (R2← R1)

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Block Diagram

P : R2← R1

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Timing Diagram

P : R2← R1

Control variable is synchronized using the same clock

P is activated by the rising edge of the clock pulse at time t.

On the next positive edge of the clock. the load input is active andthe data is loaded

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Simultaneous Operations

P : R2← R1,R1← R2

What does this do? Sli.do J762

It swaps the contents

Block Diagram:

Assume registers have edge triggered flip flops

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Other Examples: Draw Block Diagrams

A + B : R2← R1,R1← R2

AB : R2← R1,R1← R2

P : R2← R1 + R3

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Register Transfer Symbols

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Bus and Memory Transfers

A digital system has many registers that have to be connected

If separate lines are used to connected individual registers, A Lot ofwires will be used

A common bus system is usually used

Common lines

One for every every bit of a register

Binary information is transferred one at a time

How to choose which register is selected by the bus? Control Signals

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Bus System

Multiple sources and destinations

Bus ← R

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Multiplexer

f depends on the value of the selection lines

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Multiplexers and Bus System

Multiplexer selects the source register

Binary information is then placed on the bus

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Bus System with Multiplexers

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Bus System with Multiplexers

MUX0 has the 0-bits of all 4 registers

MUX1 has the 1-bits of all 4 registers

MUX2 has the 2-bits of all 4 registers

MUX3 has the 3-bits of all 4 registers

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Bus System with Multiplexers

Same selection lines are used

Output is formed from the four output bits of the four multiplexers

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Bus System with Multiplexers

S1 S0 Register0 0 A0 1 B1 0 C1 1 D

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Multiplexers Numbers

How many multiplexers are required for a bus system with eightregisters each having 16 bits

How many data input lines

How many selection lines: Sli.do J762

16, 8 and 3

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Bus System

At any time, one register can place its content

Bus could be mentioned or notI

R2← R1

I

Bus ← R1,R2← Bus

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LogiSim

You can download it throughhttps://sourceforge.net/projects/circuit/

Online with limited capablities throughhttps://www.rollapp.com/launch/logisim

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References

Chapter 4

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Thank you :)

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