Instruction Execution

UNIT: IV

Chapter 7: Processor

Syllabus

Instruction Execution - Building a Data Path - Designing a Control Unit - Hardwired Control, Microprogrammed Control - Pipelining - Data Hazard - Control Hazards.

Instruction Execution

• Let us see how instruction is executed. The complete instruction cycle involves three operations: Instruction fetching, opcode decoding and instruction execution. 

• Fig. 7.1.1 shows the basic instruction cycle. After each instruction cycle, central processing unit checks for any valid interrupt request. If so, central processing unit fetches the instructions from the interrupt service routine and after completion of interrupt service routine, central processing unit starts the new instruction cycle from where it has been interrupted.

• Fig. 7.1.2 shows instruction cycle with interrupt cycle.

Instruction fetch cycle: In this cycle, the instruction is fetched from the memory location whose address is in the PC. This instruction is placed in the Instruction Register (IR) in the processor.

Instruction decode cycle: In this cycle, the opcode of the instruction stored in the instruction register is decoded/examined to determine which operation is to be performed.

Instruction execution cycle :In this cycle, the specified operation is performed by the processor. This often involves fetching operands from the memory or from processor registers, performing an arithmetic or logical operation and storing the result in the destination location. During the instruction execution, PC contents are incremented to point to the next instruction. After completion of execution of the current instruction, the PC contains the address of the next instruction and a new instruction fetch cycle can begin.

Review Question

1. Write a short note on instruction execution.