Pretty basic notion, but I'm falling over on this...
With a sprite that has a starting X,Y and a speed, I'd like the sprite to remain on screen.

However:

-- locks sprite off screen after passing the screen boundary
-- if btn(0)
--and x<128
-- then//move sprite
-- if x < 128
-- then
-- x=x-speed
-- end
-- end

-- locks sprite centre screen
-- while x < 128 do
-- if btn(0) then
-- x=x-speed
-- end
-- end