The 1947 hand-drawn flow diagram behind the first Monte Carlo simulation on ENIAC
JosephJacks_ · x · 2026-10-30
The author shares the original hand-drawn master flow diagram dated December 9, 1947 — the first automated Monte Carlo simulation, engineered by Klára Dán von Neumann with John von Neumann, Nicholas Metropolis and Stanislaw Ulam for the newly converted stored-program ENIAC.
Key facts decoded from the 24x18-inch sheet:
- ENIAC became the world's first operational stored-program computer when Klari, von Neumann, Adele Goldstine and Richard Clippinger re-architected its three 104-entry Function Tables into read-only instruction memory with a 60-opcode vocabulary, eliminating manual rewiring.
- Klari hand-wrote the entire 800+ instruction program and oversaw its 30-day execution at Aberdeen Proving Ground for Los Alamos neutron-transport simulations.
- The flow has five stages: (1) read neutron initial state from IBM punch cards into accumulators; (2) implement von Neumann's middle-square PRNG in hardware to sample exponential free-path lengths; (3) race three distances — next nuclear collision, zone boundary/escape, and time-step census; (4) branch into elastic scattering, inelastic scattering, fission or absorption, then compute post-collision kinematics; (5) map accumulators to IBM card columns and punch progeny cards.
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