The instruction cycle (also known as the fetch–decode–execute cycle, or simply the fetch-execute cycle) is the cycle that the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instructions. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage.
Role of components
The program counter (PC) is a special register that holds the memory address of the next instruction to be executed. During the fetch stage, the address stored in the PC is copied into the memory address register (MAR) and then the PC is incremented in order to "point" to the memory address of the next instruction to be executed. The CPU then takes the instruction at the memory address described by the MAR and copies it into the memory data register (MDR). The MDR also acts as a two-way register that holds data fetched from memory or data waiting to be stored in memory (it is also known as the memory buffer register (MBR) because of this). Eventually, the instruction in the MDR is copied into the current instruction register (CIR) which acts as a temporary holding ground for the instruction that has just been fetched from memory.
During the decode stage, the control unit (CU) will decode the instruction in the CIR. The CU then sends signals to other components within the CPU, such as the arithmetic logic unit (ALU) and the floating point unit (FPU). The ALU performs arithmetic operations such as addition and subtraction and also multiplication via repeated addition and division via repeated subtraction. It also performs logic operations such as AND, OR, NOT, and binary shifts as well. The FPU is reserved for performing floating-point operations.
Summary of stages
Each computer's CPU can have different cycles based on different instruction sets, but will be similar to the following cycle:
- Fetch Stage: The next instruction is fetched from the memory address that is currently stored in the program counter and stored into the instruction register. At the end of the fetch operation, the PC points to the next instruction that will be read at the next cycle.
- Decode Stage: During this stage, the encoded instruction presented in the instruction register is interpreted by the decoder.
- Read the effective address: In the case of a memory instruction (direct or indirect), the execution phase will be during the next clock pulse. If the instruction has an indirect address, the effective address is read from main memory, and any required data is fetched from main memory to be processed and then placed into data registers (clock pulse: T3). If the instruction is direct, nothing is done during this clock pulse. If this is an I/O instruction or a register instruction, the operation is performed during the clock pulse.
- Execute Stage: The control unit of the CPU passes the decoded information as a sequence of control signals to the relevant functional units of the CPU to perform the actions required by the instruction, such as reading values from registers, passing them to the ALU to perform mathematical or logic functions on them, and writing the result back to a register. If the ALU is involved, it sends a condition signal back to the CU. The result generated by the operation is stored in the main memory or sent to an output device. Based on the feedback from the ALU, the PC may be updated to a different address from which the next instruction will be fetched.
- Repeat Cycle
Registers Involved In Each Instruction Cycle:
- Memory address registers(MAR) : It is connected to the address lines of the system bus. It specifies the address in memory for a read or write operation.
- Memory Buffer Register(MBR) : It is connected to the data lines of the system bus. It contains the value to be stored in memory or the last value read from the memory.
- Program Counter(PC) : Holds the address of the next instruction to be fetched.
- Instruction Register(IR) : Holds the last instruction fetched.