Instructions live in memory

What changes when instructions and data occupy addressable memory?

Once instructions became encoded memory words, changing a program became a data-loading operation. Fast random-access memory and reusable subroutines turned the idea into an operating machine and a new software practice.

Stored control makes behavior revisable because the machine’s next transition is determined by addressable information rather than fixed wiring.

Reconstruct the mechanism

1. Fetch the encoded word at the program counter
2. Decode its operation and operand addresses
3. Transform registers or memory
4. Advance or redirect the next instruction address

Run a six-word memory image through fetch–decode–execute, then change one stored instruction and predict the new trace.

Evidence and uncertainty

Do not say von Neumann alone invented stored-program computing. Credit collaborative and parallel work and separate publication from implementation.

• Stored-program history — Computer History Museum
• EDSAC — Computer History Museum