What is a Computer: Inside the WITCH
Using the WITCH, the oldest working example of a First Generation computer as a practical case study, we will explore the Von Neumann Architecture model, which laid the foundations for general purpose computers.
John Von Neumann (Hungarian-American Computer Scientist)
In 1945, John Von Neumann put forward his Von Neumann Architecture model, outlining the essential components of a general purpose computer. Architecture is a design that allows instructions/data to be processed by the Central Processing Unit, which controls the computer system.
John Von Neumann and the Dawn of Digital Computing
John Von Neumann worked alongside John Mauchly and J. Presper Eckert on the US Army’s ENIAC: the first digital programmable general purpose computer. Whilst not a Von Neumann compliant machine, ENIAC inspired the development of EDSAC, which does use that architecture.
What are the Components of Von Neumann Architecture?
Von Neumann demonstrated a computer could have a basic, fixed hardware structure and still carry out computation with the correct programming. A general-purpose computer that consists of an input device, a processing unit, memory and an output device is Von Neumann compliant.
The Stored Program Computer Concept
A critical element of Von Neumann Architecture is the Stored Program Computer Concept, where a computer is able to store both data and instructions as binary digits in its primary memory. Von Neumann, among others, felt this idea was crucial to the future of computing.
The WITCH (Full Machine) (1951) by Ted Cooke-YarboroughThe National Museum of Computing
The Harwell Dekatron/The WITCH
Built in 1951, the WITCH began life as the Harwell Dekatron Computer, used to calculate logarithmic tables for the construction of the Harwell Nuclear Reactor. It is one of the earliest specialised computers to use Von Neumann Architecture.
What Makes Up a Computer?
A computer system is made up of a combination of hardware and software. Explore the WITCH's hardware below...
Punched Paper Tape Reader (3) (Pre 1951) by UnknownThe National Museum of Computing
Input
Input devices are used to feed data and instructions into a computer for processing and storage. On the WITCH, this is done using five-hole binary punched paper tape. Alternatively, simple instructions can be manually input using switches on the control panel.
The WITCH: The oldest working example of a First Generation computer in the world (2021) by The National Museum of ComputingThe National Museum of Computing
Control Unit (The WITCH) (1951) by Ted Cooke-YarboroughThe National Museum of Computing
Inside the Central Processing Unit (CPU)
The CPU is responsible for controlling the computer system and processing data. In Von Neumann Architecture, the CPU receives instructions/data from an input device or the primary memory. This is processed, then sent to an output device or secondary (external) storage.
The WITCH: The oldest working example of a First Generation computer in the world (2021) by The National Museum of ComputingThe National Museum of Computing
The Fetch, Decode, Execute Cycle
A computer that is Von Neumann compliant will use the Fetch-Decode-Execute Cycle to process an instruction. In this cycle, an instruction is fetched serially (in order), one at a time from the memory, decoded so that the computer can understand it, then executed by the CPU.
The Control Unit
The Control Unit is part of the CPU. It is responsible for moving data around the system and operates Von Neumann’s Fetch, Decode, Execute cycle. The Control Unit also transmits signals that control other hardware components within the system.
The Arithmetic and Logic Unit (ALU)
The ALU is another component that makes up the CPU of a Von Neumann compliant computer. It is responsible for performing calculations and making logical decisions. Data that is moved from the computer’s primary memory to secondary storage will always pass through the ALU.
Dekatron Memory Unit (The WITCH) (1951) by Ted Cooke-YarboroughThe National Museum of Computing
Memory
Memory is the part of a computer that stores data and instructions that are currently in use. Primary memory is located inside the computer, it plays a crucial role in the Von Neumann Architecture model, allowing the processor to access and alter the memory efficiently.
What is RAM (Random Access Memory)?
The WITCH has ⅓ Kb RAM. RAM is volatile: if the computer is switched off, data stored in the memory will be lost. RAM can easily be altered at any time. The term ‘Random Access’ refers to the fact that data can be stored/accessed with equal speed from any location in the memory.
What is a Dekatron?
The WITCH's memory is made up of 828 Dekatrons. A type of valve, a Dekatron is a small counting tube that contains a glowing charge of neon/helium. This charge can be stopped at ten possible locations around the circumference of the dekatrons face, representing the numbers 0-9.
The WITCH: The oldest working example of a First Generation computer in the world (2021) by The National Museum of ComputingThe National Museum of Computing
One Dekatron = One Decimal Digit
Each dekatron stores a single decimal digit, part of a number up to eight decimal places long. The WITCH is one of the only computers with a memory that can be read visually. The operator can see which digit is held by each Dekatron using the location of the glowing charge.
The WITCH: The oldest working example of a First Generation computer in the world (2021) by The National Museum of ComputingThe National Museum of Computing
Creed Model 75 Teleprinter (Used with the WITCH) (1958) by Creed & CompanyThe National Museum of Computing
Output
Output devices are where data leaves the computer once processed. The WITCH can be connected to an output device called a teleprinter that punches out the program results on paper tape as a series of holes. This is the final stage of the Von Neumann Architecture model.
