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Pertemuan 20Microprogrammed Control

Matakuliah : H0344/Organisasi dan Arsitektur Komputer

Tahun : 2005

Versi : 1/1

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa

akan mampu :

• Menjelaskan konsep dasar microprogrammed control

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Outline Materi

• Basic Concepts

• Microinstruction Sequencing

• Microinstruction Execution

• Application of Microprogramming

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Basic conceptsMicroinstructions

Typical microinstruction formats

Microinstruction Address

Jump condition

- Unconditional

- Zero

- Overflow

- Indirect bit

System bus control signals

Internal CPU control signal

Microinstruction Address

Jump condition

Function codes

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Basic conceptsOrganization of

control memory

Jump to indirect or execute

Jump to execute

Jump to fetch

Jump to opcode routine

Jump to fetch or interrupt

Jump to fetch or interrupt

Jump to fetch or interrupt

Fetch cycleroutine

Indirectcycle routine

Interrputcycle routine

Execute cycle beginning

AND routine

ADD cycleroutine

IOF cycleroutine

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Basic conceptsControl unit microinstruction

Sequencinglogic

Control addressregister

Control unit

Control bufferregister

Read

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Basic conceptsFunctioning of

microprogrammed

control unit

Sequencinglogic

Control addressregister

Control unit

Control bufferregister

Read

Decoder

Instruction register

Decoder

Control signalswithin CPU

Control signalsto system bus

Control Unit

ALU Flag

Clock

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Basic conceptsWilkes’s microprogrammed control unit

Register II

Register I

Addressdecoder

Clock

Clock

From instructionregsiter

Control signals

Conditionalsignal

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Basic conceptsAdvantages and disadvantage

The principal advantage of the use of microprogramming to implement a control unit is that it simplifies the design of control unit. Thus it is both cheaper and less error-prone to implement. A hardwired control unit must contain complex logic for sequencing through the many micro-operations of instruction cycle. On the other hand, the decoder and sequencing logic unit of a microprogrammed control unit are very simple pieces of logic.

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Microinstruction sequencing

The two basic tasks performed by microprogrammed control unit as follows:

1. Microinstruction sequencing

2. Microinstruction execution

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Microinstruction sequencing

Two concerns are involved in he design of microinstruction sequencing technique: the size of microinstruction and address-generation time. The first concern is obvious: minimizing the size of the control memory reduces the cost of that component. The second concern is simply a desire to execute microinstruction as fast possible.

Design considerations

In executing, a microprogram, the address of the next microinstruction to be executed is in one of these categories:

1. Determined by instruction register

2. Next sequential address

3. Branch

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Microinstruction sequencingSequencing techniques

1. Two address fields

2. Single address field

3. Variable format

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CAR

Address decoder

Controlmemory

ControlAddress

1Address

2

Branchlogic

Multiplexer

IR

Flags

CBR

Addressselection

Microinstruction sequencingBranch control logic,

two address fields

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Microinstruction sequencingBranch control logic,

single address field

Address decoder

Controlmemory

Control Address 1

Branchlogic

Multiplexer

IR

Flags

CBR

Addressselection

CAR+1

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Microinstruction sequencingBranch control logic,

Variable format

Address decoder

Controlmemory

Branchlogic

Multiplexer

IR

Flags

CBR

Addressselection

CAR+1Gate andfunction

logicEnable

Branchcontrol

field

Entirefield

Addressfield

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Microinstruction sequencingAddress generation

Explicit Implicit

Two field Mapping

Unconditional branch Addition

Conditional branch Residual control

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Microinstruction executingControl unit organization

Sequencinglogic

Control addressregister

Control memory

Control bufferregister

Instruction register

Control logic

Internalcontrolsignals

Externalcontrolsignals

ALU Flag

Clock

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Microinstruction executingThe microinstruction spectrum

Characteristics

Unencoded Highly encoded

Many bits Few bits

Detailed view hardware Aggregated view of hardware

Difficult to program Easy to program

Concurrency fully exploited Concurrency not fully exploited

Little or no control logic Complex control logic

Fast execution Slow execution

Optimize performance Optimize programming

Terminology

Unpacked Packed

Horizontal Vertical

Hard Soft

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Field Field Field

Decodelogic

Decodelogic

Decodelogic

Control signals

Field Field Field

Decodelogic

Decodelogic

Decodelogic

Control signals

Decodelogic

Decodelogic

(a) Direct encoding (b) Indirect encoding

Microinstruction executingMicroinstruction encoding

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0 0 0 0 0 0

0 0 0 0 0 1

0 0 0 0 1 0

Register select

Simple register transfers

MDR Register

Register MDR

MAR Register

0 0 1 0 0 0

0 0 1 0 0 1

Memory operation

Read

Write

0 0

0 0

0 1 0 0 0 0

0 1 0 0 0 1

0 1 0 0 1 0

Special sequencing operations

CSAR Decoded MDR

CSAR Constant (in next byte)

Skip

0 0

0 0

0 0

0 1 1 0 0 0

0 1 1 0 0 1

0 1 1 0 1 0

ALU operations

ACC ACC + Register

ACC ACC - Register

ACC Register

0 1 1 0 1 1 Regsiter ACC

0 1 1 1 0 0 ACC Register + 1

Register select

Microinstruction executingVertical microinstruction repertoire

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Microinstruction executingHorizontal microinstruction repertoire

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1 2 3 4 5 6Field

Field definition1 - Register transfer2 - Memory operation3 - Sequencing operation4 - ALU operation5 - Register selection6 - Constant

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Applications of microprogramming

The set of current applications for microprogramming includes

• Realization of computers

• Emulation

• Operating system support

• Realization of special-purpose devices

• High level language support

• Micro diagnostics

• User tailoring