4. The von Neumann model

The von Neumann model

Named after John von Neumann.

In earlier electronic computers, programming was done by connecting wires to plugs, essentially programs were written by changing the layout of the hardware.

von Neumann Model

The von Neumann model is used in modern computers, where programs are stored as a series of instructions in computer memory.

• As well as the basic components, this model specifies the capacity to carry out sequential instruction processing.
• It also specifies a single data path between the CPU and main memory (this is known as the the von Neumann bottleneck).

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von Neumann bottleneck

von Neumann Bottleneck

The von Neumann Bottleneck is that there is only a single data path between the CPU and main memory, this can slow down the processor as we are limited by how fast instructions can be passed from memory to the CPU. In most applications, this bottleneck will not slow us down.

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FDE cycle

FDE Cycle

The fetch-decode-execute cycle is the series of steps that a computer carries out when it runs a program.

• Fetch: The control unit fetches the next instruction from memory (this data comes through von Neumann bottleneck) and updates the program counter to determine where the next instruction is located. We first have to fetch an instruction from memory and place it into the IR.
• Decode: Translate the instruction into signals that the ALU can interpret and execute (it is decoded to determine what needs to be done next).
• Execute: This takes two steps.
  • Load any required data into the CPU registers. If a memory value is involved in the operation, it is retrieved and placed into the MBR.
  • Execute the instruction, placing the results in registers or memory.

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