This script demonstrates the basic algorithm of determining whether a number is prime or not. It uses the the method of the wrong preposition, where the goal is to determine that the number is not prime. It uses a boolean variable for that.
var x,i:integer; IsPrime: boolean; begin write ('Type x: '); readln (x); IsPrime:=true; for i:=2 to x div 2-1 do if x mod i = 0 then IsPrime:=false; if IsPrime then write (x,' is prime.') else write (x,' is not prime.'); readln; end.