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MANINDER KAUR
                              professormaninder@gmail.com



Maninder Kaur
                                            www.eazynotes.com   1
professormaninder@gmail.com
CONTROL UNIT
  CPU is partitioned into Arithmetic Logic Unit
     (ALU) and Control Unit (CU).

   The function of control unit is to generate
     relevant timing and control signals to all
     operations in the computer.

   It controls the flow of data between the processor
     and memory and peripherals
Maninder Kaur
                                             www.eazynotes.com   2
professormaninder@gmail.com
FUNCTIONS OF CONTROL UNIT
  The control unit directs the entire computer system to carry out
     stored program instructions.

  The control unit must communicate with both the arithmetic
     logic unit (ALU) and main memory.

  The control unit instructs the arithmetic logic unit that which
     logical or arithmetic operation is to be performed.

  The control unit co-ordinates the activities of the other two
     units as well as all peripherals and auxiliary storage devices
     linked to the computer.

Maninder Kaur
                                                         www.eazynotes.com   3
professormaninder@gmail.com
DESIGN OF CONTROL UNIT

     Control unit generates control signals using one of
     the two organizations:

  Hardwired Control Unit


  Micro-programmed Control Unit



Maninder Kaur
                                               www.eazynotes.com   4
professormaninder@gmail.com
HARDWIRED CONTROL UNIT
  It is implemented as logic circuits (gates, flip-flops,
     decoders etc.) in the hardware.

  This organization is very complicated if we have a large
     control unit.

  In this organization, if the design has to be modified or
     changed, requires changes in the wiring among the
     various components. Thus the modification of all the
     combinational circuits may be very difficult.

Maninder Kaur
                                                   www.eazynotes.com   5
professormaninder@gmail.com
HARDWIRED CONTROL UNIT

  ADVANTAGES

  Hardwired Control Unit is fast because control
     signals are generated by combinational circuits.

  The delay in generation of control signals depends
     upon the number of gates.


Maninder Kaur
                                               www.eazynotes.com   6
professormaninder@gmail.com
HARDWIRED CONTROL UNIT
  DISADVANTAGES

  More is the control signals required by CPU; more
     complex will be the design of control unit.

  Modifications in control signal are very difficult. That
     means it requires rearranging of wires in the hardware
     circuit.

  It is difficult to correct mistake in original design or
     adding new feature in existing design of control unit.

Maninder Kaur
                                                    www.eazynotes.com   7
professormaninder@gmail.com
ARCHITECTURE OF HARDWIRED CONTROL UNIT

            I           Opcode             Address




Maninder Kaur
                                                                 www.eazynotes.com   8
professormaninder@gmail.com
HARDWIRED CONTROL UNIT

  Control unit consist of a:

  Instruction Register

  Number of Control Logic Gates,

  Two Decoders

  4-bit Sequence Counter
Maninder Kaur
                                      www.eazynotes.com   9
professormaninder@gmail.com
HARDWIRED CONTROL UNIT
  An instruction read from memory is placed in the
     instruction register (IR).

  The instruction register is divided into three parts: the I
     bit, operation code, and address part.

  First 12-bits (0-11) to specify an address, next 3-bits specify
     the operation code (opcode) field of the instruction and
     last left most bit specify the addressing mode I.
           I = 0 for direct address
           I = 1 for indirect address

Maninder Kaur
                                                    www.eazynotes.com   10
professormaninder@gmail.com
HARDWIRED CONTROL UNIT
  First 12-bits (0-11) are applied to the control logic gates.

  The operation code bits (12 – 14) are decoded with a 3 x 8
     decoder.

  The eight outputs ( D0 through D7) from a decoder goes to
     the control logic gates to perform specific operation.

  Last bit 15 is transferred to a I flip-flop designated by
     symbol I.
Maninder Kaur
                                                    www.eazynotes.com   11
professormaninder@gmail.com
HARDWIRED CONTROL UNIT

  The 4-bit sequence counter SC can count in binary
     from 0 through 15.

  The counter output is decoded into 16 timing pulses
     T0 through T15.

  The sequence counter can be incremented by INR
     input or clear by CLR input synchronously.
Maninder Kaur
                                             www.eazynotes.com   12
professormaninder@gmail.com
HARDWIRED CONTROL UNIT
  For example:
    Consider the case where SC is incremented to
   provide timing signals T0, T1, T 2 , T3, and T4 in sequence. At
   time T4 , SC is cleared to 0 if decoder output D3 is
   active. This is expressed symbolically by the
   statement:
                         D3 T4 : SC ← 0

  The timing diagram shows the time relationship of
     the control signals.
Maninder Kaur
                                                    www.eazynotes.com   13
professormaninder@gmail.com
TIMING DIAGRAM




Maninder Kaur
                                          www.eazynotes.com   14
professormaninder@gmail.com
MICRO-PROGRAMMED CONTROL UNIT
  A micro-programmed control unit is implemented
     using programming approach. A sequence of micro-
     operations are carried out by executing a program
     consisting of micro-instructions.

  Micro-program, consisting of micro-instructions is
     stored in the control memory of the control unit.

  Execution of a micro-instruction is responsible for
     generation of a set of control signals.

Maninder Kaur
                                               www.eazynotes.com   15
professormaninder@gmail.com
MICRO-PROGRAMMED CONTROL UNIT

  A micro-instruction consists of:
        One or more micro-operations to be executed.
        Address of next microinstruction to be executed.


     Micro-Operations: The operations performed on the
     data stored inside the registers are called micro-
     operations.

  Micro-Programs: Microprogramming is the concept for
     generating control signals using programs. These
     programs are called micro-programs.
Maninder Kaur
                                                        www.eazynotes.com   16
professormaninder@gmail.com
MICRO-PROGRAMMED CONTROL UNIT
  Micro-Instructions: The instructions that make
     micro-program are called micro-instructions.

  Micro-Code: Micro-program is a group of micro-
     instructions. The micro-program can also be termed
     as micro-code.

  Control Memory: Micro-programs are stored in the
     read only memory (ROM). That memory is called
     control memory.

Maninder Kaur
                                             www.eazynotes.com   17
professormaninder@gmail.com
MICRO-PROGRAMMED CONTROL UNIT
     ADVANTAGES

   The design of micro-program control unit is less complex because
     micro-programs are implemented using software routines.

   The micro-programmed control unit is more flexible because design
     modifications, correction and enhancement is easily possible.

   The new or modified instruction set of CPU can be easily
     implemented by simply rewriting or modifying the contents of control
     memory.

   The fault can be easily diagnosed in the micro-program control unit
     using diagnostics tools by maintaining the contents of flags, registers
     and counters.
Maninder Kaur
                                                             www.eazynotes.com   18
professormaninder@gmail.com
MICRO-PROGRAMMED CONTROL UNIT
     DISADVANTAGES

   The micro-program control unit is slower than hardwired control
     unit. That means to execute an instruction in micro-program control
     unit requires more time.

   The micro-program control unit is expensive than hardwired control
     unit in case of limited hardware resources.

   The design duration of micro-program control unit is more than
     hardwired control unit for smaller CPU.


Maninder Kaur
                                                         www.eazynotes.com   19
professormaninder@gmail.com
ARCHITECTURE OF MICRO-PROGRAMMED
             CONTROL UNIT




Maninder Kaur
                              www.eazynotes.com   20
professormaninder@gmail.com
ARCHITECTURE OF MICRO-PROGRAMMED
             CONTROL UNIT
   The address of micro-instruction that is to be executed is stored in the
     control address register (CAR).

   Micro-instruction corresponding to the address stored in CAR is
     fetched from control memory and is stored in the control data register
     (CDR).

   This micro-instruction contains control word to execute one or more
     micro-operations.

   After the execution of all micro-operations of micro-instruction, the
     address of next micro-instruction is located.

Maninder Kaur
                                                           www.eazynotes.com   21
professormaninder@gmail.com
COMPARISON BETWEEN HARDWIRED AND
  MICRO-PROGRAMMED CONTROL UNIT
        Attributes                      Hardwired Control               Micro-programmed Control

 Speed                        Fast                                     Slow
 Cost of
                              More                                     Cheaper
 Implementation
                              Not flexible, difficult to modify for new Flexible, new instructions can
 Flexibility
                              instruction                               easily be added
 Ability to Handle
                      Difficult                                        Easier
 Complex Instructions
 Decoding                     Complex                                  Easy

 Applications                 RISC Microprocessor                      CISC Microprocessor

 Instruction Set Size         Small                                    Large

 Control Memory               Absent                                   Present

 Chip Area Required           Less                                     More


Maninder Kaur
                                                                                 www.eazynotes.com   22
professormaninder@gmail.com

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Timing and-control-unit

  • 1. MANINDER KAUR professormaninder@gmail.com Maninder Kaur www.eazynotes.com 1 professormaninder@gmail.com
  • 2. CONTROL UNIT CPU is partitioned into Arithmetic Logic Unit (ALU) and Control Unit (CU).  The function of control unit is to generate relevant timing and control signals to all operations in the computer.  It controls the flow of data between the processor and memory and peripherals Maninder Kaur www.eazynotes.com 2 professormaninder@gmail.com
  • 3. FUNCTIONS OF CONTROL UNIT The control unit directs the entire computer system to carry out stored program instructions. The control unit must communicate with both the arithmetic logic unit (ALU) and main memory. The control unit instructs the arithmetic logic unit that which logical or arithmetic operation is to be performed. The control unit co-ordinates the activities of the other two units as well as all peripherals and auxiliary storage devices linked to the computer. Maninder Kaur www.eazynotes.com 3 professormaninder@gmail.com
  • 4. DESIGN OF CONTROL UNIT Control unit generates control signals using one of the two organizations: Hardwired Control Unit Micro-programmed Control Unit Maninder Kaur www.eazynotes.com 4 professormaninder@gmail.com
  • 5. HARDWIRED CONTROL UNIT It is implemented as logic circuits (gates, flip-flops, decoders etc.) in the hardware. This organization is very complicated if we have a large control unit. In this organization, if the design has to be modified or changed, requires changes in the wiring among the various components. Thus the modification of all the combinational circuits may be very difficult. Maninder Kaur www.eazynotes.com 5 professormaninder@gmail.com
  • 6. HARDWIRED CONTROL UNIT ADVANTAGES Hardwired Control Unit is fast because control signals are generated by combinational circuits. The delay in generation of control signals depends upon the number of gates. Maninder Kaur www.eazynotes.com 6 professormaninder@gmail.com
  • 7. HARDWIRED CONTROL UNIT DISADVANTAGES More is the control signals required by CPU; more complex will be the design of control unit. Modifications in control signal are very difficult. That means it requires rearranging of wires in the hardware circuit. It is difficult to correct mistake in original design or adding new feature in existing design of control unit. Maninder Kaur www.eazynotes.com 7 professormaninder@gmail.com
  • 8. ARCHITECTURE OF HARDWIRED CONTROL UNIT I Opcode Address Maninder Kaur www.eazynotes.com 8 professormaninder@gmail.com
  • 9. HARDWIRED CONTROL UNIT Control unit consist of a: Instruction Register Number of Control Logic Gates, Two Decoders 4-bit Sequence Counter Maninder Kaur www.eazynotes.com 9 professormaninder@gmail.com
  • 10. HARDWIRED CONTROL UNIT An instruction read from memory is placed in the instruction register (IR). The instruction register is divided into three parts: the I bit, operation code, and address part. First 12-bits (0-11) to specify an address, next 3-bits specify the operation code (opcode) field of the instruction and last left most bit specify the addressing mode I. I = 0 for direct address I = 1 for indirect address Maninder Kaur www.eazynotes.com 10 professormaninder@gmail.com
  • 11. HARDWIRED CONTROL UNIT First 12-bits (0-11) are applied to the control logic gates. The operation code bits (12 – 14) are decoded with a 3 x 8 decoder. The eight outputs ( D0 through D7) from a decoder goes to the control logic gates to perform specific operation. Last bit 15 is transferred to a I flip-flop designated by symbol I. Maninder Kaur www.eazynotes.com 11 professormaninder@gmail.com
  • 12. HARDWIRED CONTROL UNIT The 4-bit sequence counter SC can count in binary from 0 through 15. The counter output is decoded into 16 timing pulses T0 through T15. The sequence counter can be incremented by INR input or clear by CLR input synchronously. Maninder Kaur www.eazynotes.com 12 professormaninder@gmail.com
  • 13. HARDWIRED CONTROL UNIT For example: Consider the case where SC is incremented to provide timing signals T0, T1, T 2 , T3, and T4 in sequence. At time T4 , SC is cleared to 0 if decoder output D3 is active. This is expressed symbolically by the statement: D3 T4 : SC ← 0 The timing diagram shows the time relationship of the control signals. Maninder Kaur www.eazynotes.com 13 professormaninder@gmail.com
  • 14. TIMING DIAGRAM Maninder Kaur www.eazynotes.com 14 professormaninder@gmail.com
  • 15. MICRO-PROGRAMMED CONTROL UNIT A micro-programmed control unit is implemented using programming approach. A sequence of micro- operations are carried out by executing a program consisting of micro-instructions. Micro-program, consisting of micro-instructions is stored in the control memory of the control unit. Execution of a micro-instruction is responsible for generation of a set of control signals. Maninder Kaur www.eazynotes.com 15 professormaninder@gmail.com
  • 16. MICRO-PROGRAMMED CONTROL UNIT A micro-instruction consists of:  One or more micro-operations to be executed.  Address of next microinstruction to be executed. Micro-Operations: The operations performed on the data stored inside the registers are called micro- operations. Micro-Programs: Microprogramming is the concept for generating control signals using programs. These programs are called micro-programs. Maninder Kaur www.eazynotes.com 16 professormaninder@gmail.com
  • 17. MICRO-PROGRAMMED CONTROL UNIT Micro-Instructions: The instructions that make micro-program are called micro-instructions. Micro-Code: Micro-program is a group of micro- instructions. The micro-program can also be termed as micro-code. Control Memory: Micro-programs are stored in the read only memory (ROM). That memory is called control memory. Maninder Kaur www.eazynotes.com 17 professormaninder@gmail.com
  • 18. MICRO-PROGRAMMED CONTROL UNIT ADVANTAGES  The design of micro-program control unit is less complex because micro-programs are implemented using software routines.  The micro-programmed control unit is more flexible because design modifications, correction and enhancement is easily possible.  The new or modified instruction set of CPU can be easily implemented by simply rewriting or modifying the contents of control memory.  The fault can be easily diagnosed in the micro-program control unit using diagnostics tools by maintaining the contents of flags, registers and counters. Maninder Kaur www.eazynotes.com 18 professormaninder@gmail.com
  • 19. MICRO-PROGRAMMED CONTROL UNIT DISADVANTAGES  The micro-program control unit is slower than hardwired control unit. That means to execute an instruction in micro-program control unit requires more time.  The micro-program control unit is expensive than hardwired control unit in case of limited hardware resources.  The design duration of micro-program control unit is more than hardwired control unit for smaller CPU. Maninder Kaur www.eazynotes.com 19 professormaninder@gmail.com
  • 20. ARCHITECTURE OF MICRO-PROGRAMMED CONTROL UNIT Maninder Kaur www.eazynotes.com 20 professormaninder@gmail.com
  • 21. ARCHITECTURE OF MICRO-PROGRAMMED CONTROL UNIT  The address of micro-instruction that is to be executed is stored in the control address register (CAR).  Micro-instruction corresponding to the address stored in CAR is fetched from control memory and is stored in the control data register (CDR).  This micro-instruction contains control word to execute one or more micro-operations.  After the execution of all micro-operations of micro-instruction, the address of next micro-instruction is located. Maninder Kaur www.eazynotes.com 21 professormaninder@gmail.com
  • 22. COMPARISON BETWEEN HARDWIRED AND MICRO-PROGRAMMED CONTROL UNIT Attributes Hardwired Control Micro-programmed Control Speed Fast Slow Cost of More Cheaper Implementation Not flexible, difficult to modify for new Flexible, new instructions can Flexibility instruction easily be added Ability to Handle Difficult Easier Complex Instructions Decoding Complex Easy Applications RISC Microprocessor CISC Microprocessor Instruction Set Size Small Large Control Memory Absent Present Chip Area Required Less More Maninder Kaur www.eazynotes.com 22 professormaninder@gmail.com