Von Neumann architecture The name Von Neumann instantly confuses students and in modern computer architecture his name is only used as a reference. We need to tell students what Von Neumann is but only because the exam boards insist on using the term and students need to be able to identify with it. So a bit of a history lesson

John von Neumann John von Neumann was the scientist who conceived a fundamental idea that serves all modern computers - that a computer's program and the data that it processes do not have to be fed into the computer while it is working, but can be kept in the computer's memory - a notion generally referred to as the stored-program computer . In his short life, von Neumann became one of the most acclaimed and lauded scientists of the 20th century. He left an indelible mark on the fields of mathematics, quantum theory, game theory, nuclear physics, and computer science.

Computer Architecture The CPU and the Fetch Decode Execute cycle Animation

Memory Registers PC Control unit ALU Input / Output Data bus Address bus Registers PC ACC MAR MDR Control bus CIR PC or Program Counter keeps track of the memory location for the next instruction to be dealt with. In many cases it is simply incremented to the next memory location at the fetch stage of the cycle to allow the program to be executed line by line MAR or Memory Address Register, stores the location for data to be fetched from or sent to memory The Control unit is responsible for decoding instructions. it then sends out signals to control how data moves around the parts of the CPU and memory to execute the instructions Control unit ALU MDR or Memory data register stores data that has been fetched from or is waiting to be sent to memory The ALU or Arithmetic logic unit carries out calculations and logic decisions, for example addition and subtraction and comparisons such as equal to, greater than or less than Data Bus carries data between the CPU and memory ACC or Accumulator stores the results of any arithmetic or logical operations carried out by the ALC Address bus carries memory addresses for locations to be read or written to CIR or current instruction register stores the next instruction ready to be decoded Input / Output Connections to buses on the motherboard which carry data to and from devices such as disc drives and ports such as USB and VGA(video) Input / Output Registers These are memory locations within the CPU which can be accessed very quickly Main memory or RAM Random access Memory located on the motherboard Control Bus carries control signals around the CPU and memory

The Fetch-Decode-execute cycle The PC (program counter) has the address of the next instruction to fetch The value in the PC (program counter) is copied into MAR (memory address register) The CU (control unit) locates and fetches the data If it is an instruction it is placed in the CIR (Current instruction register) The PC is incremented by one The instruction in the CIR is decoded by the control unit Finally the instruction is is executed by placing any request for data into the MAR (memory address register) for the data to be collected and copied to the MDR If it is data it is placed in the MDR (memory data register)