Questions: The Computing Revolution: From Turing to Stored-Program Computers

Question 1

What was the key innovation of the 'stored-program' computer architecture, and who is associated with formalizing it?

Model answer: In earlier machines, programs were represented in hardware — rewiring the machine to change its function. The stored-program architecture, formalized by John von Neumann in his 1945 First Draft of a Report on EDVAC, proposed that instructions (programs) should be stored in the same memory as data, allowing the machine to modify and execute programs dynamically without hardware changes. This made computers general-purpose machines. Von Neumann drew on ideas from Turing, Mauchly, Eckert, and others, and the resulting architecture — CPU, memory, I/O — is still the basis of nearly all modern computers.

The stored-program concept is perhaps the single most important architectural decision in computing history. The question of who deserves credit (Turing, von Neumann, Mauchly and Eckert) is historically contested.

Question 2

Alan Turing's 1936 paper introduced the concept of a Turing machine. What was Turing trying to answer with this mathematical construction?

• A. How fast a mechanical calculator could multiply numbers
• B. Whether it was possible to build a practical electronic computer
• C. Whether every mathematical problem has a definite procedure for solving it
• D. How to crack encrypted German military communications

Answer: (C)

Turing's 1936 paper 'On Computable Numbers, with an Application to the Entscheidungsproblem' was responding to David Hilbert's challenge: is there a definite procedure (algorithm) that can decide, for any mathematical statement, whether it is provable? Turing showed the answer was no by defining computation abstractly and proving some problems were undecidable. The Turing machine was a mathematical thought experiment, not a design for a real machine — but it established the theoretical foundations of computer science.

Question 3

What was Colossus, and why was its existence kept secret for decades?

Model answer: Colossus was an electronic programmable computer built at Bletchley Park in 1943-1944 to help crack German Lorenz cipher messages. It used vacuum tubes to process encrypted teleprinter traffic at electronic speeds. Colossus is arguably the world's first operational electronic computer. Its existence was kept secret under the British Official Secrets Act until the 1970s because the cryptographic techniques it embodied — and the fact that Britain could read German high-command communications — remained strategically sensitive. This secrecy meant that its inventors (Tommy Flowers and colleagues) received no recognition during the postwar computing boom.

The secrecy around Colossus meant that US computers like ENIAC (1945) and EDVAC were long credited as the first electronic computers, when Colossus arguably preceded them by years.

Question 4

ENIAC, completed in 1945, was the world's first electronic general-purpose computer.

• T. True
• F. False

Answer: False

ENIAC was not fully general-purpose in the modern sense — its programs were initially set by rewiring, not stored-program. Colossus preceded it by two years. And whether any particular machine deserves the title 'first computer' depends on how 'computer' is defined. What ENIAC did represent was massive scale: 18,000 vacuum tubes, 30 tons, filling a large room. The stored-program architecture came later with EDVAC and the Manchester Baby (1948).

Question 5

How did Alan Turing's wartime work at Bletchley Park connect to the development of modern computing?

Model answer: At Bletchley Park, Turing helped design the 'Bombe' — an electromechanical machine that exploited cribs (known plaintext) to narrow the search space for Enigma settings. This work was not fully electronic and is distinct from later stored-program computers, but it demonstrated the practical power of Turing's abstract computational thinking: logical analysis of cipher structure could be mechanized. The experience also convinced Turing and colleagues that electronic computation was feasible and valuable. After the war, Turing worked on the ACE (Automatic Computing Engine) at the National Physical Laboratory and contributed to the Manchester computing project.

Turing's career is a remarkable thread connecting pure mathematical logic (1936), wartime code-breaking (1939-1945), and postwar computer design (1945-1954). His prosecution and death from cyanide poisoning in 1954 — the result of chemical castration ordered by British courts after a conviction for homosexuality — came before he received any public recognition for his wartime work.