Summary of "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.