Von Neumann Architecture

• Key Characteristics 

  • Central Processing Unit (CPU) 
  • Single control unit (SCU) 
  • Arithmetic logic unit (ALU) 
  • Onboard cache 
  • Internal clock
• This architecture computer fetches, decodes and executes instructions 
• Programs are made up of a sequence of instructions stored in the main memory outside of the CPU – they are fetched one by one to the registers for processing 
• Fetch 

  • The program counter is checked as it holds the address of the next instruction to be executed 
  • This address is used by the MAR in order to fetch the instruction needed from RAM and bring it into MDR 
  • The address in the program counter is incremented to point at the next instruction
• Decode 

  • The instruction is in the CPU the control unit decodes the instruction to see what has to be done 
• Execute 

  • What we actually do depends on what the instruction is e.g.; 
  • We could be asked to head back to main memory and fetch some data and add it to the accumulator 
  • We could be asked to jump to another instruction 
  • We could be asked to write data held in the accumulator back into main memory
• State the data that will be placed in the memory address register (MAR)

  10010

  
• The data that will be placed in the memory data register (MDR)

  11110001

  
• The program is stored in a secondary memory device

  Data instructions are moved to main memory

  Data and instructions are stored in the same memory

  Data and instructions are moved to registers to be executed

  Instructions are fetched once at a time