Von Neumann Architecture

Prior to von Neumann

• Computers were built for specific tasks
• Programs were hard-wired into the computer
• You had to either build a new computer or rewire the existing one to change the program

John von Neumann

• Hungarian-American mathematician and physicist
• Member of the Manhattan Project
• Interested in how to build a general-purpose computer

Preliminary Discussion of the Logical Design of an Electronic Computing Instrument

-- Arthur W. Burks
-- Herman H. Goldstine
-- John von Neumann

Components of the von Neumann Architecture

Input/Output (I/O)

• Devices that allow the computer to interact with the outside world
• Examples:
  • IN: keyboards, mice, touchscreens, file storage (load a file, e.g. a program)
  • OUT: monitors, speakers, printers, file storage (write to a file)
• In von Neumann's time it may have included punch cards or paper tape

Memory

• Stores data and instructions that the CPU needs to operate
• Today if you hear the word "memory" we are referring to RAM. We call hard drives "storage" or "secondary storage".
• In von Neumann's time it may have included vacuum tubes

Central Processing Unit (CPU)

• The "brain" of the computer
• Where instructions are executed
• Has three main components:
  • Control Unit
  • Arithmetic Logic Unit (ALU)
  • Registers

Registers

• Small amount of storage on the CPU itself
• Extremely fast because of proximity to CPU components
• Every operation in the CPU involves fist moving data to and from registers
• There are reserved "special" registers as well as general purpose registers

Control Unit

• "Manager" of the CPU
• Manages the flow of data and instructions within the computer

Arithmetic Logic Unit (ALU)

• "Worker" of the CPU
• Performs arithmetic and logical operations
  • Arithmetic: addition, subtraction, multiplication, division
  • Logical: AND, OR, NOT, XOR
  • Comparison: greater than, less than, equal to
  • etc.

Fetch Decode Execute Cycle

The CPU will pick off one instruction at a time from memory and execute it. This is called the Fetch Decode Execute Cycle.

1. Fetch:
   • Move the instruction from memory to a special instruction register in the CPU.
   • This instruction is determined by the value in the program counter register.
   • The program counter is then incremented so that the next instruction can be fetched.
2. Decode:
   • The control unit reads the instruction and determines what to do.
   • It preps the ALU and other components for the operation that will come next.
3. Execute:
   • The ALU performs the operation on the data in the registers.
   • The intermediate result is stored in a special register called the accumulator.

By shuffling data between memory and registers, the CPU can perform all of the operations necessary to run a program.