The document describes the basic instruction cycle in a computer. It consists of 4 main phases: 1) Fetch - the next instruction is fetched from memory and stored in the instruction register. 2) Decode - the instruction is decoded to determine the operation. 3) Execute - if it is a memory instruction, the effective address is read from memory before execution. Otherwise, the instruction is directly executed. 4) The result is stored in memory or output to a device. The key components involved are the program counter, instruction register, memory, and control unit which coordinates the cycle.

2. COMPUTER ORGANIZATION//
INSTRUCTION CYCLE//
Made by:-
Panchal Parth R.

3. INSTRUCTION CYCLE
• A program residing in the memory unit of the computer consists 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-cycles or phases.

4. INSTRUCTION CYCLE
Fetch
Indirect
Execute
Interrupt

5. 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
• 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.

6. FLOW CHART OF INSTRUCTION CYCLE

7. TRANSFER NOTATION
• Step 1: Start SC <- 0
• Step 2: T0 :- AR <- PC
• Step 3: T1 :- IR <- M[AR], PC <- PC + 1
• Step 4: T2 :- Decode Opcode IR(12-14)
AR <- IR(0-4) , I <- IR(15)
• Step 5: T3 :- Decision , If I = 0 => Direct;
If I = 1 => Indirect => Memory Reference/ Register Reference Instruction/
IO Reference Instruction.
• Step 6: T4 :- Execute.

8. STEPS 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.

9. 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.

10. DECODE THE INSTRUCTION
• The decoder interprets the instruction.
• During this phases the instruction inside the IR (instruction register) gets
decoded.

11. 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.

12. 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
• aThe result generated by the operation is stored in the main memory, or sent to an output
device.

13. THANK YOU