Lesson 3

• What is the fetch-execute cycle in a traditional Von-Neumann computer?

  It is the process of fetching a program instruction from memory, decoding it, and executing it.
• What are the main components used in the fetch-execute cycle?

  • Program Counter (PC)
  • Memory Address Register (MAR)
  • Memory Data Register (MDR)
  • Current Instruction Register (CIR)
  • Control Unit (CU)
  • Arithmetic Logic Unit (ALU)
• What is the function of the Program Counter (PC) in the fetch-execute cycle?

  The PC keeps track of the memory address of the next instruction to be executed.
• What does the Memory Address Register (MAR) do?

  The MAR holds the address in main memory that is currently being read or written.
• What is the role of the Memory Data Register (MDR)?


  The MDR holds data fetched from memory or data waiting to be stored in memory.
• What does the Current Instruction Register (CIR) do?

  The CIR is a temporary holding ground for the instruction that has just been fetched from memory.
• What is the function of the Control Unit (CU)?

  The CU decodes the program instruction in the CIR and coordinates the activation of machine resources.
• What does the Arithmetic Logic Unit (ALU) perform?

  The ALU performs mathematical and logical operations.
• What are the basic instructions in the fetch-execute cycle?

  • LOAD xxx: Load the accumulator with the contents of the memory address given.
  • ADD xxx: Add the contents of the memory address to the accumulator.
  • STORE xxx: Store the contents of the accumulator at the given memory address.
  • HALT: Stop the code.
  • SUB xxx: Subtract the contents of the memory address from the accumulator.
  • CLR: Clear the contents of the accumulator.
• What does the binary instruction '0001000000001100' represent?

  It represents the instruction LOAD 12.
• How is assembly language related to binary instructions?

  Assembly language uses operator command words (mnemonics) and op-codes in decimal to represent binary instructions.
• What does the instruction 'LOAD 300' mean?

  It means to load into the accumulator the number in memory address 300.
• What does the instruction 'LOAD #300' signify?

  It means to load the number 300 directly into the accumulator.
• What happens during the fetch step of the fetch-execute cycle?

  The control unit moves the address from the PC into the MAR.
• What occurs when the address is sent from the MAR to the main memory?

  The instruction at that address is found and returned to the MDR.
• What does the control unit do when the MDR has a program instruction in it?

  The control unit loads the CIR from the MDR with the instruction to be executed.
• What is the purpose of decoding the instruction?

  The purpose is for the control unit to understand which instruction needs to be executed.
• What happens during the execution of an instruction?

  The instruction is carried out, such as loading data into the accumulator.
• What does the fetch-execute cycle do after executing an instruction?

  It goes back to the fetch step to repeat the process for the next instruction.
• What are the steps in the fetch-execute cycle?

  1. Fetch: Copy value in PC into MAR, set control bus to read, fetch data into MDR, increment PC.
  2. Decode: Control unit decodes the instruction in the CIR.
  3. Execute: Execute the instruction/command.
  4. Repeat from step 1.
• What does the control bus do during the fetch step?

  The control bus is set to read to fetch the instruction from memory.
• What happens to the PC after fetching an instruction?

  The contents of the PC are incremented by 1 to reference the next instruction.
• What is the significance of the address bus in the fetch-execute cycle?

  The address bus carries the address from the MAR to the main memory.
• What does the data bus do during the fetch-execute cycle?

  The data bus transfers both data and instructions from memory.
• What is the role of the control bus in the fetch-execute cycle?

  The control bus manages the operations of the CPU and memory during the fetch-execute cycle.
• What does the instruction 'ADD #205' mean?

  It means to add the number 205 to the accumulator.
• What does the instruction 'SUB #20' signify?

  It means to subtract the number 20 from the accumulator.
• What does the instruction 'CLR' do?

  It clears the contents of the accumulator.
• What does the instruction 'STORE 507' do?

  It stores the contents of the accumulator at memory address 507.
• What is the purpose of the HALT instruction?

  The HALT instruction stops the execution of the program.
• What is the process of loading data from memory into the accumulator?

  1. Set the MAR to the memory address where the data is stored.
  2. Send the address from the MAR along the address bus to memory with the control bus set to 'read'.
  3. Transfer the contents of the address into the MDR.
  4. Load the value from the MDR into the accumulator.
• What happens during the fetch step when the next address in the PC is moved into the MAR?

  The instruction at that address is fetched from memory and returned to the MDR.
• What does the instruction 'ADD #205' do to the accumulator?

  It adds the number 205 to the current value in the accumulator.
• What is the final step of the fetch-execute cycle?

  The cycle repeats by fetching the next instruction from memory.
• What are the key components of the fetch-execute cycle summary?

  1. PC holds the address of the first program instruction.
  2. Fetch: Copy value in PC into MAR, set control bus to read, fetch data into MDR, increment PC.
  3. Decode: Control unit decodes the instruction in the CIR.
  4. Execute: The instruction is executed.
  5. Repeat from step 2.
• What does the instruction 'STORE 205' do?

  It stores the contents of the accumulator at memory address 205.
• What is the significance of the instruction 'HALT' in a program?

  The 'HALT' instruction indicates the end of program execution.
• What does the instruction 'CLR' do to the accumulator?

  It clears the contents of the accumulator.
• What is the purpose of the instruction 'ADD #50'?

  It adds the number 50 to the current value in the accumulator.
• What does the instruction 'SUB #20' do?

  It subtracts the number 20 from the current value in the accumulator.