Special Purpose Registers (PC, ACC, MAR and MDR)

• What is the purpose of the CPU's register unit?

  It serves as very small, quick stores of data.
• What are the two categories of registers in a CPU?

  Special purpose registers and general purpose registers.
• Why are general purpose registers not the focus of this discussion?

  They are used for temporary data holding without specific reasons.
• What is the first special purpose register mentioned?

  The program counter (PC).
• What does the program counter (PC) hold?

  It holds the address of the next instruction to be executed.
• How does the program counter (PC) update after fetching an instruction?

  It usually increments by one to point to the next instruction.
• What is the accumulator abbreviated to?

  ACC.
• What happens if there is a branching instruction?

  The program counter (PC) changes to a completely different address.
• What does the accumulator (ACC) hold?

  It holds the result of a calculation performed by the ALU.
• Why is it faster to store results in the accumulator (ACC) than in main memory?

  Because accessing the accumulator is much quicker than accessing main memory.
• What is the function of the memory address register (MAR)?

  It holds the memory address to be accessed by the CPU.
• What does the memory data register (MDR) hold?

  It holds the actual data being transferred to or from the memory location.
• How are the MAR and MDR linked during memory operations?

  The MAR holds the address, while the MDR holds the data corresponding to that address.
• Why do the MAR and MDR exist in the CPU?

  They act as buffers to compensate for the speed difference between the CPU and main memory.
• What is the main reason for the CPU to use buffers like MAR and MDR?

  To allow the CPU to continue processing while waiting for slower memory operations.
• What are the roles of the special purpose registers in a CPU?

  • Program Counter (PC): Holds the address of the next instruction.
  • Accumulator (ACC): Holds the result of calculations.
  • Memory Address Register (MAR): Holds the address to be accessed.
  • Memory Data Register (MDR): Holds the data being transferred.
• How do the MAR and MDR work together during data transfer?

  • MAR holds the address of the memory location.
  • MDR holds the data corresponding to that address.
  • Data is read from or written to the memory location indicated by MAR.
• What is the significance of the speed difference between the CPU and main memory?

  • Main memory is slower than the CPU.
  • Buffers like MAR and MDR are used to manage this speed difference.
  • Allows the CPU to continue processing while waiting for memory operations.