The document describes the phases of the instruction cycle in a CPU. It discusses the following phases: 1) Fetch - the next instruction is fetched from memory and stored in the instruction register using the program counter; 2) Decode - the instruction inside the register is decoded; 3) Execute - the control unit passes signals to function units like the ALU to perform the required actions like arithmetic or logic operations. It also describes common circuits used in a CPU like the program counter, memory address register, and instruction register.

1. Instruction Cycle
Phases of Instruction Cycle
Unit 3 - CONTROL UNIT DESIGN
Abhineet Anand
Computer Science and Engg. Department
University of Petroleum and Energy Studies, Dehradun
November 26, 2012
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

2. Instruction Cycle
Phases of Instruction Cycle
Introduction
Diagram
Circuit Used
Instruction Cycle
A program residing in the memory unit of the computer
consist of a sequence of Instructions.
The program is executed in the computer by going through
a cycle for each instruction.
Each instruction cycle in turn is subdivided into a sequence
of sub cycle or phase.
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

3. Instruction Cycle
Phases of Instruction Cycle
Introduction
Diagram
Circuit Used
Instruction Cycle
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

4. Instruction Cycle
Phases of Instruction Cycle
Introduction
Diagram
Circuit Used
Circuits Used
The circuits used in the CPU during the cycle are:
Program counter (PC) - an incrementing counter that
keeps track of the memory address of the instruction that is
to be executed next.
Memory address register (MAR) - holds the address of a
memory block to be read from or written to.
Memory data register (MDR) - a two-way register that
holds data fetched from memory (and ready for the CPU to
process) or data waiting to be stored in memory.
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

5. Instruction Cycle
Phases of Instruction Cycle
Introduction
Diagram
Circuit Used
Circuits Used Contd..
Instruction register (IR) - a temporary holding ground for
the instruction that has just been fetched from memory
Control unit (CU) - decodes the program instruction in the
IR, selecting machine resources such as a data source
register and a particular arithmetic operation, and
coordinates activation of those resources
Arithmetic logic unit (ALU) - performs mathematical and
logical operations
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

6. Instruction Cycle
Phases of Instruction Cycle
Fetching the instruction
Decode the instruction
In case of a memory instruction
Execute the instruction
Phases of Instruction Cycle
In basic computer, each instruction cycle consists of the
following phases:
Fetch an instruction from Memory.
Decode the instruction.
Read the effective address from memory if the instruction
has an indirect address.
Execute the instruction.
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

7. Instruction Cycle
Phases of Instruction Cycle
Fetching the instruction
Decode the instruction
In case of a memory instruction
Execute the instruction
Fetching the instruction
The next instruction is fetched from the memory address
that is currently stored in the program counter (PC), and
stored in the instruction register (IR).
Now, the PC points to the next instruction that will be read
at the next cycle.
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

8. Instruction Cycle
Phases of Instruction Cycle
Fetching the instruction
Decode the instruction
In case of a memory instruction
Execute the instruction
Decode the instruction
The decoder interprets the instruction.
During this phases the instruction inside the IR (instruction
register) gets decoded.
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

9. Instruction Cycle
Phases of Instruction Cycle
Fetching the instruction
Decode the instruction
In case of a memory instruction
Execute the instruction
In case of a memory instruction
In case of a memory instruction (direct or indirect) the
execution phase will be in the next clock pulse.
Required data is fetched from main memory to be
processed and then placed into data registers.
During this phases the instruction inside the IR (instruction
register) gets decoded.
If the instruction is direct, nothing is done at this clock
pulse.
If this is an I/O instruction or a Register instruction, the
operation is performed (executed) at clock Pulse.
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

10. Instruction Cycle
Phases of Instruction Cycle
Fetching the instruction
Decode the instruction
In case of a memory instruction
Execute the instruction
Execute the instruction
The control unit of CPU passes the decoded information
as a sequence of control signals.
Signals are passed to the relevant function units of the
CPU to perform the actions required.
These action may be :
Reading values from registers,
Passing them to the ALU to perform mathematical or logic
functions on them, or
Writing the result back to a register
The result generated by the operation is stored in the main
memory, or sent to an output device.
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

11. Instruction Cycle
Phases of Instruction Cycle
Fetching the instruction
Decode the instruction
In case of a memory instruction
Execute the instruction
The Fetch Execute cycle in Transfer Notation
Expressed in register transfer notation:
MAR <- PC
MDR<- Memory {MAR address}; PC <- [PC]+1 (Increment
the PC for next cycle at the same time)
IR<- MDR
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN

12. Instruction Cycle
Phases of Instruction Cycle
Fetching the instruction
Decode the instruction
In case of a memory instruction
Execute the instruction
THANK YOU
Abhineet Anand
Unit 3 - CONTROL UNIT DESIGN