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CPU Instruction Cycle

From when the computer is boot-up until shutdown, it follows the cycle to process instructions.

Components​

The program counter (PC) is the register that holds the memory address of the next instruction to be executed.

The memory address register (MAR) holds the address of the instruction to be executed.

The memory data register (MDR) acts as a two-way register that holds data fetched from memory or data waiting to be stored in memory (can also be known as MBR).

The current instruction register (CIR) acts as a temporary storage for the instruction fetched from memory.

The control unit (CU) decodes the instruction in the CIR and sends signals to the arithmetic logic unit (ALU) and the floating point unit (FPU)

Fetch​

  • The address of the PC is copied into the MAR.
  • The PC is incremented to point to the next instruction.
  • The instruction in address at MAR is copied to the MDR.
  • The instruction in MDR is copied to the CIR.

Decode​

  • The encoded instruction held in the CIR is decoded.

Execute​

  • The CU of the CPU passes the decoded information as signals to the CPU (FPU or ALU) to perform the instructions and storing the result back into memory, register or an output device.
  • Example operations: Add, Subtract, AND, OR, Branches of execution, XOR.

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