OCR A'Level Fetch decode execute cycle

Main Ideas and Concepts:

• Fetch-Decode-Execute Cycle:
  • The cycle consists of three main stages: fetching the instruction, decoding it, and executing it.
  • The Program Counter (PC) holds the address of the next instruction to be fetched.
• Registers Involved:
  • Program Counter (PC): Contains the address of the next instruction.
  • Memory Address Register (MAR): Holds the address of the instruction/data to be fetched from memory.
  • Current Instruction Register (CIR): Stores the fetched instruction.
  • Memory Data Register (MDR): Holds the data retrieved from memory.
  • Accumulator (ACC): Used to store intermediate results during execution.
• Instruction Fetching Process:
  • The address from the PC is copied to the MAR.
  • The address is sent via the address bus, and a control signal is sent to read the memory.
  • The fetched instruction is then transferred to the CIR, and the PC is incremented for the next instruction.
• Instruction Decoding:
  • The instruction in the CIR is decoded to identify the opcode (operation code) and the operand (address or data).
  • The system identifies what operation to perform and the data required.
• Data Fetching for Execution:
  • If the instruction requires additional data (e.g., a load operation), the address of that data is sent to the MAR.
  • A control signal is sent to read the data from memory, which is then loaded into the MDR and subsequently into the ACC.
• Branching in Programs:
  • The video explains how branching can alter the flow of execution, such as in loops or conditional statements.
  • If a branch instruction is encountered, the PC is updated to the address specified in the operand instead of incrementing.
• Use of Stack for Function Calls:
  • The video hints at the necessity of a Stack data structure to manage return addresses during function calls, which will be discussed in future content.

Methodology/Instructions:

• Cycle Steps:
  • Fetch:
    • Copy the address from the PC to the MAR.
    • Send the address to memory and request a read operation.
    • Transfer the fetched instruction to the CIR and increment the PC.
  • Decode:
    • Analyze the instruction in the CIR to identify the opcode and operand.
  • Execute:
    • If needed, fetch additional data using the operand address, update the MAR, and read from memory into the MDR, then transfer to the ACC.

Speakers/Sources Featured:

• The video appears to be presented by a single speaker, though their name is not mentioned in the provided subtitles.