Short Notes: Control Unit | Short Notes for Computer Science Engineering - Computer Science Engineering (CSE) PDF Download

 Page 1


The control unit is responsible for decoding the opcode and operand bits in the 
instruction register, and then generating the control signals necessary to drive all 
other hardware in the CPU to perform the sequence of micro-operations that 
comprise the instruction.
Design of Control Unit
The function of the control unit in a digital computer, is to initiate sequences of 
micro operations (the operations executed on data stored in registers are called 
micro operations).
• The number of different types of micro operations that are available in a given 
system is finite.
• When the control signals are generated by hardware using conventional logic 
design techniques, the control unit is said to be hardwired.
• Micro-programming is a second alternative for designing the control unit of a 
digital computer.
Block diagram of a control unit
• The control unit initiates a series of sequential steps of micro operations. 
During any given time certain micro operations are to be initiated, while others 
remain idle.
• The control variables at any given time can be represented by a string of 1 s 
and 0 s called a control word:
Approaches for Designing Control Unit
Page 2


The control unit is responsible for decoding the opcode and operand bits in the 
instruction register, and then generating the control signals necessary to drive all 
other hardware in the CPU to perform the sequence of micro-operations that 
comprise the instruction.
Design of Control Unit
The function of the control unit in a digital computer, is to initiate sequences of 
micro operations (the operations executed on data stored in registers are called 
micro operations).
• The number of different types of micro operations that are available in a given 
system is finite.
• When the control signals are generated by hardware using conventional logic 
design techniques, the control unit is said to be hardwired.
• Micro-programming is a second alternative for designing the control unit of a 
digital computer.
Block diagram of a control unit
• The control unit initiates a series of sequential steps of micro operations. 
During any given time certain micro operations are to be initiated, while others 
remain idle.
• The control variables at any given time can be represented by a string of 1 s 
and 0 s called a control word:
Approaches for Designing Control Unit
Hardwired Control
• Fixed logic circuits that correspond directly to the Boolean expressions are 
used to generate the control signals.
• Hardwired control is faster than microprogrammed control.
• A controller that uses this approach can operate at high speed.
• The control hardwire can be viewed as a state machine that changes from one 
state to another in every clock cycle, depending on the contents of the 
instruction register, the condition codes, and the external inputs. The outputs 
of the state machine are the control signals. The sequence of operations 
carried out by this machine is determined by the wiring of the logic elements, 
hence the name "hardwired".
Instruction register
Micro-programmed Control
• The control signals associated with operations are stored in special memory 
units inaccessible by the programmer as control words.
• Control signals are generated by a program similar to machine language 
programs.
• It leads to a slower operating speed because of the time it takes to fetch 
microinstructions from the control store.
• Control word: A control word is a word whose individual bits represent the 
various control signals.
• Micro-routine: A sequence of control words corresponding to the control 
sequence of a machine instruction constitutes the microroutine for that 
instruction,
• Micro-instruction: Individual control words in this microroutine are referred to 
as microinstructions.
• Micro-program: A sequence of microinstructions is called a microprogram, 
which is stored in a ROM or RAM called a control memory CM.
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The control unit is responsible for decoding the opcode and operand bits in the 
instruction register, and then generating the control signals necessary to drive all 
other hardware in the CPU to perform the sequence of micro-operations that 
comprise the instruction.
Design of Control Unit
The function of the control unit in a digital computer, is to initiate sequences of 
micro operations (the operations executed on data stored in registers are called 
micro operations).
• The number of different types of micro operations that are available in a given 
system is finite.
• When the control signals are generated by hardware using conventional logic 
design techniques, the control unit is said to be hardwired.
• Micro-programming is a second alternative for designing the control unit of a 
digital computer.
Block diagram of a control unit
• The control unit initiates a series of sequential steps of micro operations. 
During any given time certain micro operations are to be initiated, while others 
remain idle.
• The control variables at any given time can be represented by a string of 1 s 
and 0 s called a control word:
Approaches for Designing Control Unit
Hardwired Control
• Fixed logic circuits that correspond directly to the Boolean expressions are 
used to generate the control signals.
• Hardwired control is faster than microprogrammed control.
• A controller that uses this approach can operate at high speed.
• The control hardwire can be viewed as a state machine that changes from one 
state to another in every clock cycle, depending on the contents of the 
instruction register, the condition codes, and the external inputs. The outputs 
of the state machine are the control signals. The sequence of operations 
carried out by this machine is determined by the wiring of the logic elements, 
hence the name "hardwired".
Instruction register
Micro-programmed Control
• The control signals associated with operations are stored in special memory 
units inaccessible by the programmer as control words.
• Control signals are generated by a program similar to machine language 
programs.
• It leads to a slower operating speed because of the time it takes to fetch 
microinstructions from the control store.
• Control word: A control word is a word whose individual bits represent the 
various control signals.
• Micro-routine: A sequence of control words corresponding to the control 
sequence of a machine instruction constitutes the microroutine for that 
instruction,
• Micro-instruction: Individual control words in this microroutine are referred to 
as microinstructions.
• Micro-program: A sequence of microinstructions is called a microprogram, 
which is stored in a ROM or RAM called a control memory CM.
Microinstruction From main memory
Control signals
• Control Store: The micro-routines for all instructions in the instruction set of a 
computer are stored in a special memory called the control store.
• The format for control word is as follows:
Branch Condition Flag Control Field
Control Memory 
Address
Micro-instructions are commonly divided into two categories: horizontal and 
vertical.
Horizontal Micro-Programmimg Control: Each micro-instruction is a series of bits 
each of which represents a single control line.
• In this design, Control signal is expressed with decoded binary format per 
Control Signal.
• There is no need of external decoder to generate the control signal.
• It is bit flexible as compared to hardwired control unit since it allows 
modification easily as compared to hardwired control unit.
• This concept is not in practice since managing the signals in the decoded 
format is not easy as it leads to longer instructions.
Horizontal micro-instructions are characterized by:
• Long formats;
• Ability to express a high degree of parallelism;
• Minimally encoded scheme
• Many resources can be controlled
• Operating speed is low
Vertical Micro-Programming Control: Groups of bits in the micro-instruction 
represent commands. This requires decoding or de-multiplexing before the control 
commands can be issued. •
• In this Control Unit, Control signal are expressed in encode binary format.
• it requires external decoder to decode the signal and process them.
• It provides the ease for implementation of new instructions because of 
encoded binary format
Vertical micro-instructions are characterized by:
Short formats;
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The control unit is responsible for decoding the opcode and operand bits in the 
instruction register, and then generating the control signals necessary to drive all 
other hardware in the CPU to perform the sequence of micro-operations that 
comprise the instruction.
Design of Control Unit
The function of the control unit in a digital computer, is to initiate sequences of 
micro operations (the operations executed on data stored in registers are called 
micro operations).
• The number of different types of micro operations that are available in a given 
system is finite.
• When the control signals are generated by hardware using conventional logic 
design techniques, the control unit is said to be hardwired.
• Micro-programming is a second alternative for designing the control unit of a 
digital computer.
Block diagram of a control unit
• The control unit initiates a series of sequential steps of micro operations. 
During any given time certain micro operations are to be initiated, while others 
remain idle.
• The control variables at any given time can be represented by a string of 1 s 
and 0 s called a control word:
Approaches for Designing Control Unit
Hardwired Control
• Fixed logic circuits that correspond directly to the Boolean expressions are 
used to generate the control signals.
• Hardwired control is faster than microprogrammed control.
• A controller that uses this approach can operate at high speed.
• The control hardwire can be viewed as a state machine that changes from one 
state to another in every clock cycle, depending on the contents of the 
instruction register, the condition codes, and the external inputs. The outputs 
of the state machine are the control signals. The sequence of operations 
carried out by this machine is determined by the wiring of the logic elements, 
hence the name "hardwired".
Instruction register
Micro-programmed Control
• The control signals associated with operations are stored in special memory 
units inaccessible by the programmer as control words.
• Control signals are generated by a program similar to machine language 
programs.
• It leads to a slower operating speed because of the time it takes to fetch 
microinstructions from the control store.
• Control word: A control word is a word whose individual bits represent the 
various control signals.
• Micro-routine: A sequence of control words corresponding to the control 
sequence of a machine instruction constitutes the microroutine for that 
instruction,
• Micro-instruction: Individual control words in this microroutine are referred to 
as microinstructions.
• Micro-program: A sequence of microinstructions is called a microprogram, 
which is stored in a ROM or RAM called a control memory CM.
Microinstruction From main memory
Control signals
• Control Store: The micro-routines for all instructions in the instruction set of a 
computer are stored in a special memory called the control store.
• The format for control word is as follows:
Branch Condition Flag Control Field
Control Memory 
Address
Micro-instructions are commonly divided into two categories: horizontal and 
vertical.
Horizontal Micro-Programmimg Control: Each micro-instruction is a series of bits 
each of which represents a single control line.
• In this design, Control signal is expressed with decoded binary format per 
Control Signal.
• There is no need of external decoder to generate the control signal.
• It is bit flexible as compared to hardwired control unit since it allows 
modification easily as compared to hardwired control unit.
• This concept is not in practice since managing the signals in the decoded 
format is not easy as it leads to longer instructions.
Horizontal micro-instructions are characterized by:
• Long formats;
• Ability to express a high degree of parallelism;
• Minimally encoded scheme
• Many resources can be controlled
• Operating speed is low
Vertical Micro-Programming Control: Groups of bits in the micro-instruction 
represent commands. This requires decoding or de-multiplexing before the control 
commands can be issued. •
• In this Control Unit, Control signal are expressed in encode binary format.
• it requires external decoder to decode the signal and process them.
• It provides the ease for implementation of new instructions because of 
encoded binary format
Vertical micro-instructions are characterized by:
Short formats;
• Limited ability to express microoperation parallelism;
• Highly encoded schemes, hence considerable lightly encoded.
• Operating speed is high
Comparison between Horizontal and Vertical Micro-programming
• Horizontal microinstruction control is faster but requires greater length 
microinstructions.
• Horizontal programming allows more than one signal to be activated at a 
same time without any hardware modifications but vertical microproramming 
require modification to the instruction format.
Operations
Instruction Fetch Step: Read into Instruction Register from Memory at address 
indicated by Program Counter register. Then increment program counter to point to 
the next instruction.
IR <= lnstrCache[PC];
PC <= PC + 4;
Instruction Decode and Register Read Step:
RS <= RegFile[IR[25:21]];
RT <= RegFile[IR[20:16]];
Immediate <= sign-extend(IR[15:0]); 
pcALUOut <= PC + (Immediate « 2);
ALU Operation: This is instruction-specific, since control knows the instruction 
type.
R-Type Instruction: ALUOut <= RS op RT 
Memory-Reference Instr: ALUOut <= RS + Immediate 
Branch Instruction: If (RS/x compare RT/y) PC <= ALUOut 
Jump Instruction: PC <= (PC(31:28), (IR[25:0],2*b00))
Data Cache Access
R-Type Instruction: RegFile[IR[15:11]] <= ALUOut 
Memory-Reference Instr:
Load Instr: DataCacheOut <= DataCache[ALUOut]
Store Instr: DataCache[ALUOut] <=RT/Y 
Register Write-Back
Load Instr: RegFile[IR[20:16]] <= DataCacheOut 
Fetch Instruction: The following sequence of events in fetching an instruction:
1. The contents of the PC are loaded into the MAR.
2. The value in the PC is incremented. (This operation can be done in parallel 
with a memory access.)
3. As a result of a memory read operation, the instruction is loaded into the 
MDR.
4. The contents of the MDR are loaded into the IR.
Using single bus data path:
Page 5


The control unit is responsible for decoding the opcode and operand bits in the 
instruction register, and then generating the control signals necessary to drive all 
other hardware in the CPU to perform the sequence of micro-operations that 
comprise the instruction.
Design of Control Unit
The function of the control unit in a digital computer, is to initiate sequences of 
micro operations (the operations executed on data stored in registers are called 
micro operations).
• The number of different types of micro operations that are available in a given 
system is finite.
• When the control signals are generated by hardware using conventional logic 
design techniques, the control unit is said to be hardwired.
• Micro-programming is a second alternative for designing the control unit of a 
digital computer.
Block diagram of a control unit
• The control unit initiates a series of sequential steps of micro operations. 
During any given time certain micro operations are to be initiated, while others 
remain idle.
• The control variables at any given time can be represented by a string of 1 s 
and 0 s called a control word:
Approaches for Designing Control Unit
Hardwired Control
• Fixed logic circuits that correspond directly to the Boolean expressions are 
used to generate the control signals.
• Hardwired control is faster than microprogrammed control.
• A controller that uses this approach can operate at high speed.
• The control hardwire can be viewed as a state machine that changes from one 
state to another in every clock cycle, depending on the contents of the 
instruction register, the condition codes, and the external inputs. The outputs 
of the state machine are the control signals. The sequence of operations 
carried out by this machine is determined by the wiring of the logic elements, 
hence the name "hardwired".
Instruction register
Micro-programmed Control
• The control signals associated with operations are stored in special memory 
units inaccessible by the programmer as control words.
• Control signals are generated by a program similar to machine language 
programs.
• It leads to a slower operating speed because of the time it takes to fetch 
microinstructions from the control store.
• Control word: A control word is a word whose individual bits represent the 
various control signals.
• Micro-routine: A sequence of control words corresponding to the control 
sequence of a machine instruction constitutes the microroutine for that 
instruction,
• Micro-instruction: Individual control words in this microroutine are referred to 
as microinstructions.
• Micro-program: A sequence of microinstructions is called a microprogram, 
which is stored in a ROM or RAM called a control memory CM.
Microinstruction From main memory
Control signals
• Control Store: The micro-routines for all instructions in the instruction set of a 
computer are stored in a special memory called the control store.
• The format for control word is as follows:
Branch Condition Flag Control Field
Control Memory 
Address
Micro-instructions are commonly divided into two categories: horizontal and 
vertical.
Horizontal Micro-Programmimg Control: Each micro-instruction is a series of bits 
each of which represents a single control line.
• In this design, Control signal is expressed with decoded binary format per 
Control Signal.
• There is no need of external decoder to generate the control signal.
• It is bit flexible as compared to hardwired control unit since it allows 
modification easily as compared to hardwired control unit.
• This concept is not in practice since managing the signals in the decoded 
format is not easy as it leads to longer instructions.
Horizontal micro-instructions are characterized by:
• Long formats;
• Ability to express a high degree of parallelism;
• Minimally encoded scheme
• Many resources can be controlled
• Operating speed is low
Vertical Micro-Programming Control: Groups of bits in the micro-instruction 
represent commands. This requires decoding or de-multiplexing before the control 
commands can be issued. •
• In this Control Unit, Control signal are expressed in encode binary format.
• it requires external decoder to decode the signal and process them.
• It provides the ease for implementation of new instructions because of 
encoded binary format
Vertical micro-instructions are characterized by:
Short formats;
• Limited ability to express microoperation parallelism;
• Highly encoded schemes, hence considerable lightly encoded.
• Operating speed is high
Comparison between Horizontal and Vertical Micro-programming
• Horizontal microinstruction control is faster but requires greater length 
microinstructions.
• Horizontal programming allows more than one signal to be activated at a 
same time without any hardware modifications but vertical microproramming 
require modification to the instruction format.
Operations
Instruction Fetch Step: Read into Instruction Register from Memory at address 
indicated by Program Counter register. Then increment program counter to point to 
the next instruction.
IR <= lnstrCache[PC];
PC <= PC + 4;
Instruction Decode and Register Read Step:
RS <= RegFile[IR[25:21]];
RT <= RegFile[IR[20:16]];
Immediate <= sign-extend(IR[15:0]); 
pcALUOut <= PC + (Immediate « 2);
ALU Operation: This is instruction-specific, since control knows the instruction 
type.
R-Type Instruction: ALUOut <= RS op RT 
Memory-Reference Instr: ALUOut <= RS + Immediate 
Branch Instruction: If (RS/x compare RT/y) PC <= ALUOut 
Jump Instruction: PC <= (PC(31:28), (IR[25:0],2*b00))
Data Cache Access
R-Type Instruction: RegFile[IR[15:11]] <= ALUOut 
Memory-Reference Instr:
Load Instr: DataCacheOut <= DataCache[ALUOut]
Store Instr: DataCache[ALUOut] <=RT/Y 
Register Write-Back
Load Instr: RegFile[IR[20:16]] <= DataCacheOut 
Fetch Instruction: The following sequence of events in fetching an instruction:
1. The contents of the PC are loaded into the MAR.
2. The value in the PC is incremented. (This operation can be done in parallel 
with a memory access.)
3. As a result of a memory read operation, the instruction is loaded into the 
MDR.
4. The contents of the MDR are loaded into the IR.
Using single bus data path:
Step Micro-operation
to
M AR «— (PC); A < - (PC)
h
M DR McmfMARl; PC (A)+ 4
h IR (MDR)
Using Three bus data path:
Step M icro-operation
to
M AR < — (PC); PC (PC)+ 4
M DR Mem[M AR]
t2 IR + — (M DR)
Execution of an Instruction:
Simple Arithmetic Operation: Add R1, R2, RO
This instruction adds the contents of source registers R1 and R2, and stores the 
results in destination register RO.
This addition can be executed as in the following sequence:
1. The registers RO, R1, R2 , are extracted from the IR.
2. The contents of R1 and R2 are passed to the ALU for addition.
3. The output of the ALU is transferred to RO .
Using single bus datapath:
Step M icro-operation
to
A
(/f,)
B (R2)
t2
Ro
(A) + (£f)
Using two bus datapath:
Step Micro-operation
to A (/?, ) + (/?*)
tx R o < - (A)
Using three bus datapath:
Step Micro-operation
to Ro <tf,) + ( * 2)
Fetching a Word from memory