The von nuemann stored program concept

• What is von neumann architecture?

  Is the design upon which many general purpose computers are based.
• What does the architecute use?

  stored program concept.
• What are the key elements of Von Neumann architecture?

  • data and instructions are both stored as binary digits
  • data and instructions are both stored together in the same RAM
  • instructions are fetched from memory one at a time and in order serially
  • the processor decodes and executes an instruction,before cycling around to fetch the next instruction
  • the cycle continues until no more instructions are available
• What are the five speical registers a processor based on von neumann architecture uses?

  • program counter - holds the memory adress of the next instruction to be fetched from main memory
  • memory address register(MAR) - holds the address of the current instruction that is to be fetched from memory,or the address in memory to which data is to be transferred
  • memory data register(MDR) - holds the contents found at the address held in the MAR,or data which is to be transferred to main memory
  • current instruction register(CIR) - holds the instruction that is currently being decoded and executed
  • accumulator(ACC) - holds the results of processing
• The fetch decode execute cycle

  1.The memory address held in the program counter is copied into the MAR.

  2. The address in the program counter is then incremented - or increased - by one. The program counter now hold the address of the next instruction to be fetched

  3. The processor sends a signal containing the address of the instruction to be fetched along the address bus to the computer's memory.

  4. The instruction held in that memory address is sent along the data bus to the MDR.

  5. The instruction held in the MDR is copied into the CIR.

  6. The instruction held in the CIR is decoded and then executed. The results of processing are stored in the ACC.

  7. The cycle then returns to step one
• Depending on the type of instruction,additional steps may be taken

  • if the instruction is to transfer held in the ACC back to the RAM,the intended memory address is copied into the MAR. The data to be transferred is copied into the MDR and then transferred to specified address using the address bus and data bus.
  • The executed instruction may require the program to jump to a different place in the porgram. In this case,the memory address of the new next instruction to be fetched is copied into the porgram counter. The proccess then restarts at step one