Ch04. Operation Principle of the CPU

04-1. ALU and Control Unit

ALU

• The ALU (Arithmetic Logic Unit) performs various operations, such as arithmetic and logical operations, using operands and control signals received from the registers and the CU(Control Unit) within the CPU.

  • The speed at which the CPU accesses memory is much slower than the speed at which it accesses registers.

    → The result of the ALU is first stored in a register rather than in memory.

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  • The ALU outputs flags along with the result of an operation

    Flags are used by the CPU to determine conditions such as overflow, zero result, or carry, which affect the flow of control during program execution.

    • These flags are stored in a flag register

CU(Control Unit)

• The CU(Control Unit) receives some information such as the clock signal, instructions, flags, and control signals; it generates control signals and decodes instructions.
  1. Receives the clock signal.
  2. Receives the instruction that it is responsible for decoding.
  3. Receives the flag values in the flag register.
  4. Receives control signals transmitted via the system bus, specifically through the control bus.

• The CU(Control Unit) generates control signal to both the internal and external components of the CPU via the control bus.
  • To external components of the CPU:
    • Control signals are sent to memory when the CU initiates read or write operations.
    • Control signals are sent to I/O device when the CU performs read or write operations with them.
  • To internal components of the CPU:
    • Control signals are sent to the ALU to specify the operation to be performed.
    • Control signal are sent to the registers to transfer data between them or to decode instructions stored in the registers.

04-2. Registers

Register is a small, temporary storage unit inside the CPU that holds instructions and data from a program before and after execution.

• PC; Program Counter (IP; Instruction Pointer):
  • Stores an address of the instruction to be fetched from memory.
• IR; Instruction Register
  • Stores the instruction just fetched from memory to be decoded.
  • The CU receives the instruction from the IR, decodes it, and then generates the appropriate control signals.
• MAR; Memory Address Register
  • Stores a memory address.
  • When the CPU sends an address to the address bus for fetching, it passes through the MAR.
• MBR; Memory Buffer Register (MDR; Memory Data Register)
  • Stores the value (data or instruction) to be transferred to or from memory.
  • The value to be written to memory or read from memory passes through the MBR.
• General Purpose Register
  • Can store both data and memory addresses.
  • As the name suggests, it can be used flexibly in various and general-purpose situations.
• Flag Register
  • Stores additional information about the result of an operation or the status of the CPU.

The Procedure of Executing Program in Memory