Constant SMALL_FIRST_ROW 

pub const SMALL_FIRST_ROW: u8 = 32;

The small-text plane: text-buffer rows SMALL_FIRST_ROW..SMALL_FIRST_ROW + SMALL_ROWS, of which columns 0..SMALL_COLS hold cells. PUT, FILL and READ address them like any row; host::WINDOW puts them on screen.