Pencil on paper showing fundamental building block of computer memory

The fundamental building block of memory is a bistable element, an element with two stable states. This diagram shows a simple bistable element consisting of a pair of inverters connected in a loop. The inverters are cross-coupled, meaning that the input of I1 is the output of I2 and vice versa. The circuit has no inputs, but it does have two outputs, Q and Q’. Q depends on Q’ and Q’ depends on Q. 
Both Q = 0 and Q = 1 are stable (hence bistable), and the subtle point is that the circuit has a third possible state, a metastable state, with both outputs approximately halfway between 0 and 1. This allows 1 bit memory to be stored when power is applied.

Part of God is Meditating: Still, Golden Thread Gallery, Belfast, 15th January - 19th February 2022
Photo by Simon Mills