CHKCHR (Function)

is a function that takes two parameters: the x-coordinate for a character location on the console screen, and the y-coordinate. The value it returns is the  value of the character at those coordinates on the console (top screen text display). If the coordinates are out of range, i.e. x is not between 0 and 31 inclusive or y is not between 0 and 23 inclusive, the function returns -1, which is not the  value of any character.

After a,   on any location on the screen will give 0. It is impossible to tell if a character location has remained blank since the last, or if that character location has had    ed to it.