Feedback Is Memory

The tenth intro breaks the rule of every previous level: the circuit stops forgetting. The experiment uses a latch whose output is wired back to its input.

Experiment: the mystery circuit

The circuit is two loops: switch S is on and lamp Q is lit. Now turn S off — both switches are off, yet Q keeps glowing. The reason is neither inertia nor a fault: the output is wired back to its own input, the signal runs in a circle and sustains itself.

The rule: a loop holds a bit

Every circuit before was honest: the same inputs gave the same output. This one is the first to break the rule: with S=0 and R=0 the output keeps whatever it was a second ago. A ring of two gates is called a latch — it stores one bit. Turning R on dims Q, and the circuit remembers that event instead.

Transfer: where memory is needed

Where is such a circuit most useful? In a calculator's memory: to keep the entered number while you type the next one. Addition is the adder's job, and the adder has no memory; holding a value between key presses is the latch's job.

Where it is used

On this level you will build such a latch yourself — from two gates cross-connected. Then memory grows: latches stack into registers, registers into RAM cells. Every file and every photo is billions of such rings.