Here's a partial, probably wrong answer, completely in pseudo-pseudo code:

define playfield (define borders of screen)
set up ball object
position ball object in center of screen
read joypad
  test direction of joypad press
    if pressed left, move ball left + down
    if pressed right, move ball right + down
compare position of ball to borders
  if ball = bottom border, move up + (left or right)
  if ball = top border, move down + (left or right)
  if ball = right border, move left + (up or down)
  if ball = left border, move right + (up or down)

I'll try to work out some actual code for this, provided I'm on the right track here.