1. Control Unit 1 Lecture 20
CSE 211, Computer Organization and Architecture Harjeet Kaur, CSE/IT
Overview
 Control Memory
 Comparison of Implementations
Sequencing Microinstructions
Design of Control Unit
 Address Sequencer

2. Basic Computer Orgsnization and Design 2 Lecture 20
CSE 211, Computer Organization and Architecture Harjeet Kaur, CSE/IT
Control Unit
 Control unit (CU) of a processor translates from machine
instructions to the control signals for the microoperations
that implement them
 Control units are implemented in one of two ways
Hardwired Control
CU is made up of sequential and combinational circuits to generate
the control signals
Microprogrammed Control
A control memory on the processor contains microprograms that
activate the necessary control signals

3. Control Unit 3 Lecture 20
CSE 211, Computer Organization and Architecture Harjeet Kaur, CSE/IT
Control Unit Implementations
Combinational Logic Circuits (Hard-wired)
Microprogram
I R Status F/Fs
Control Data
Combinational
Logic Circuits
Control
Points
CPU
Memory
Timing State
Ins. Cycle State
Control Unit's State
Status F/Fs
Control Data
Next Address
Generation
Logic
C
S
A
R
Control
Storage
(-program
memory)
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I R
C
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CPUD
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4. Control Unit 4 Lecture 20
CSE 211, Computer Organization and Architecture Harjeet Kaur, CSE/IT
Terminology
Microprogram
- Program stored in memory that generates all the control signals required
to execute the instruction set correctly
- Consists of microinstructions
Microinstruction
- Contains a control word and a sequencing word
Control Word - All the control information required for one clock cycle
Sequencing Word - Information needed to decide
the next microinstruction address
- Vocabulary to write a microprogram
Control Memory(Control Storage: CS)
- Storage in the microprogrammed control unit to store the microprogram
Writeable Control Memory(Writeable Control Storage:WCS)
- CS whose contents can be modified
-> Allows the microprogram can be changed
-> Instruction set can be changed or modified
Dynamic Microprogramming
- Computer system whose control unit is implemented with
a microprogram in WCS
- Microprogram can be changed by a systems programmer or a user

5. Control Unit 5 Lecture 20
CSE 211, Computer Organization and Architecture Harjeet Kaur, CSE/IT
Terminology
Sequencer (Microprogram Sequencer)
A Microprogram Control Unit that determines the Microinstruction Address to
be executed in the next clock cycle
- In-line Sequencing
- Branch
- Conditional Branch
- Subroutine
- Loop
- Instruction OP-code mapping