The stored program concept

• Computer architecture


  The structure and organisation of a computer system. It specifies the components that make up a computer system and describes how these are interconnected, how they interact with each other, and how they are managed.
• In the early days of computer systems, programming was performed by manually setting the position of a large number of switches and plugs, and then entering the input data. The output was produced by determining the position of some of the switches.
• Powerful early machine


  • Colossus, a set of computers developed from 1943 to 1945 to decipher the enemy coded messages during the second world war
• Issues with early computer systems

  • Reprogramming through setting switches was very inefficient and time consuming
  • The computer system had to be reprogrammed for every new set of input data, even if it was to be used by the same calculation
• Stored program concept


  A memory unit that could store data as well as programs. Instructions could be fetched from the memory unit and executed, one after the other, by a processing unit that was designed to perform arithmetic and logical operations.
• The stored program concept paved the way for the creation of general-purpose computers such as modern-day smartphones, tablets, laptops, and desktop computers.
• First electronic computer that applied the stored program concept

  • The Manchester Baby, a computer made by the University of Manchester in 1948, which included the first type of random access memory (RAM)
• First electronic general-purpose digital computer

  • ENIAC, initially presented as a project from the University of Pennsylvania in 1946; it was improved to use a magnetic type of memory in 1953
• Von Neumann architecture


  Storing program instructions and data in main memory and moving them between memory and the processor
• Von Neumann architecture


  • Processor
  • Memory unit that can communicate directly with the processor
  • Connections for input and output devices
  • Secondary storage for saving/backing up data
• How the processor accesses instructions and data in main memory

  1. Address bus is used to identify the addressed location

  2. Data bus is used to transfer the contents to/from that location

  3. Control bus is used to synchronise and control operations
• Harvard architecture


  Keeps instructions and data in separate memories. The processor accesses these memories using separate data and address buses.
• Harvard architecture


  • Processor is connected to the 'instructions memory' using a dedicated set of address and data buses
  • Processor is connected to the 'data memory' using a different set of address and data buses
• Harvard architecture


  Minimises the issue of keeping the processor waiting while loading or saving data into memory, which in turn increases the processor performance
• Von Neumann architecture


  Instructions and data share the same pathways, which can be exploited by hackers who could disguise instructions (malware) as data that the processor may execute unknowingly
• Uses of Harvard vs Von Neumann architecture

  • Harvard architecture is commonly used in embedded systems where speed of operation is very important
  • Von Neumann architecture is commonly used in general-purpose computers that are expected to accommodate the varying needs of the end-users