D $AC32 "unknown31": This is a temporary variable used by the following routines: #R$BBCC, ...

D $BC6C Don't allow the 'S' cursor to go into the top blue border.
C $BC6C Decrement the y-coordinate.
C $BC6D Fine if it's not zero, just jump to #R$BC70.
C $BC6F Otherwise that would put the 'S' cursor into the top blue border so increment the y-coordinate to keep the 'S' cursor at the top of the playing field.
C $BC70 Call #R$BC83 to return.