An instruction tells the CPU to perform an operation by storing bits of code in memory that are read into the instruction register. Instructions are executed in three phases: fetching the instruction from memory, decoding what operation it specifies, and then executing it, which may involve reading an effective address or performing an input/output operation. Memory reference, register reference, and input/output instructions allow the CPU to access different parts of the computer's architecture during the instruction cycle.

1. -SHUBHAM JAIN
MCA( SEC 1)

2. WHAT IS INSTRUCTION ?
• A group of bits that tell the computer to perform a specific
operation.
• The instructions of a program are stored in memory.
• The CPU reads the next instruction from memory.
• It is placed in an Instruction Register (IR).

3. INSTRUCTION FORMAT

4. BASIC COMPUTER INSTRUCTIONS

5. CONTROL UNIT OF BASIC COMPUTER

6. INSTRUCTION CYCLE
A instruction is executed in three phases :
– Fetch an instruction from memory
– Decode the instruction.
– Read the effective address from memory if the
instruction has an indirect address. (OPTIONAL)
– Execute the instruction.

7. D'7IT3: AR M[AR]
D'7I'T3: Nothing
D7I'T3: Execute a register-reference instr.
D7IT3: Execute an input-output instr

8. MEMORY REFERENCE INSTRUCTIONS

9. REGISTER REFERENCE INSTRUCTIONS

10. INPUT – OUTPUT INTERRUPT

11. INPUT – OUTPUT INSTRUCTIONS

12. INTERRUPT CYCLE