Actionscript Border Limits

[Imago]

I'm currently programming my own game in flash and all is going well but! I need to make it so that the 'spaceship' that you control cannot go off screen. I have no idea what the code is and couldn't guess so if anyone knows could you help me out?

Player controlled symbol/instance name: Ship
[and just incase] border image symbol/instance name: Border

If you could help me out, it would be much appreciated.