Computer Components:

Top Level View

Instruction Cycle :

The IAS computer operated by a repetitively performing an instruction cycle .Each instruction cycle consists of two sub cycles.

1. Fetch Cycle


2. Execute Cycle

Basic Instruction cycle:

Fetch Cycle :

During the Fetch cycle, the opcode of the next instruction is loaded into the IR and the address portion is loaded into the MAR

• Program Counter (PC) holds address of next instruction to fetch


• Processor fetches instruction from memory location pointed to by PC


• Increment PC

---Unless told otherwise

• Instruction loaded into Instruction Register (IR)


• Processor interprets instruction and performs required actions

Execute Cycle :

Once the opcode is in IR the Execute cycle is performed. control circuitry interpretes the opcode and executes the instruction by sending out the appropriate control signals to cause data to be moved or an operation to be performed by the ALU.

• Processor-memory

—   data transfer between CPU and main memory

• Processor I/O

—   Data transfer between CPU and I/O module

• Data processing

—   Some arithmetic or logical operation on data

  Example of Program Execution



Instruction Cycle - State Diagram