Fetch Decode Execute Cycle

Fetch Decode Execute Cycle

What is fetch decode execute cycle?

An instruction cycle (sometimes called fetch-decode-execute cycle) is the basic operation cycle of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions.

Steps

The steps in the processing cycle are:

[1] Fetch the next instruction. 

[1] Decode the instruction.
             [1] Get data if needed. 

[1] Execute the instruction.

Fetch

Fetch the Next Instruction The program counter (PC) contains the address of the next instruction to be executed, so the control unit goes to the address in memory specified in the PC, makes a copy of the contents, and places the copy in the instruction register. At this point the instruction register contains the instruction to be executed. Before going on to the next step in the cycle, the program counter must be updated to hold the address of the next instruction to be executed when the current instruction has been completed. Because the instructions are stored contiguously in memory, adding 1 to the program counter should put the address of the next instruction into the PC. So the control unit increments the program counter. It is possible that the PC may be changed later by the instruction being executed

Decode

Decode the Instruction In order to execute the instruction in the instruction register, the control unit has to determine what instruction it is. It might be an instruction to access data from an input device, to send data to an output device, or to perform some operation on a data value. At this phase, the instruction is decoded into control signals. That is, the logic of the circuitry in the CPU determines which operation is to be executed. This step shows why a computer can only execute instructions that are expressed in its own machine language. The instructions themselves are literally built into the circuits.

Get Data If Needed It may be that the instruction to be executed requires additional memory accesses in order to complete its task. For example, if the instruction says to add the contents of a memory location to a register, the control unit must get the contents of the memory location.

Execute

Execute the Instruction Once an instruction has been decoded and any operands (data) fetched, the control unit is ready to execute the instruction. Execution involves sending signals to the arithmetic/logic unit to carry out the processing. In the case of adding a number to a register, the operand is sent to the ALU and added to the contents of the register. When the execution is complete, the cycle begins again. If the last instruction was to add a value to the contents of a register, the next instruction probably says to store the results into a place in memory. However, the next instruction might be a control instruction: an instruction that asks a question about the result of the last instruction and perhaps changes the contents of the program counter.