Instruction Execution Cycle

Step by Step Instruction Execution

1. Instruction Execution 15MCA043

2. Instruction Execution Instruction is command which is given by the user to computer. Execution is the process by which a computer performs instruction. Instruction Execution means a program to be executed by a processor consists of a set of instructions stored in memory.

3. Terminologies Program Counter is a register in a computer processor that contains the address of the next instruction which will be executed. Memory Address Register (MAR) holds the Memory Location of data that needs to be accessed.

4. Instruction Register (IR) is a part of CPU control unit that stores the instruction currently being executed or decoded. Memory Buffer Register (MBR) stores the data being transferred to and from immediate access store also known as Memory Data Register (MDR) .

5. Control Unit (CU) decodes the program instruction in the IR, selecting machine resources such as a data source register and a particular arithmetic operation. Arithmetic Logic Unit (ALU) performs mathematical and logical operations. Accumulator (AC) means the processor contains the single data register.

6. Instruction Register Fetched instruction is placed in the instruction register Categories Processor-memory Transfer data between processor and memory Processor-I/O Data transferred to or from a peripheral device Data processing Arithmetic or logic operation on data Control Alter sequence of execution

7. Instruction Execution Cycle The time period during which one instruction is fetched from memory and execute when computer given an instruction in machine language. Each instruction is further divided into sequence of phases. After the execution of program counter is incremented to point to the next instruction.

8. Process Processor reads instruction from memory time . Decodes the instruction. Execute the instruction.

9. Fetch Cycle The Program Counter (PC) contains the address of the next instruction to be fetched. The address contained in the PC is copied to the Memory Address Register (MAR).

10. The instruction is copied from the memory location contained in the MAR and placed in the Memory Buffer Register (MBR) . The entire instruction is copied from the MBR and placed in the Current Instruction Register (CIR)

11. Execute Cycle The address part of the instruction is placed in the MAR The instruction is decoded and executed. The processor checks for interrupts and either branches to the relevant interrupt service routine or starts the cycle again.

12. Start Fetch Instruction Decode Instruction Execute Instruction Halt

13. Example