Add floor division to basm

[rgiot]

I have too often numeric issue in computation

   ` char_y = {line_y}/8 ; BUG => result is a float !!!`

vs

  `  char_y = {line_y}>>3 ; OK => real /8 ..`


 ` char_y = {line_y}//8 ; BUG => result is a float !!!`

could be the solution