They discovered a quantized version of Newton’s law because their processor had no floating point unit. Pretty creative.

> S(0)=0 (reference position) S(1)=v₀-½g (initial velocity) S(t+2)=2S(t+1)-S(t)-g

Thus, the value of the previous screen frame (t+1) is added twice, the value of the screen frame before that one (t) is subtracted, and the constant g is subtracted.