veiwport help required

I've been playing around with viewports for an idea for a new game, and have hit a bit of a problem. the code does exactly what i want it to do, but what would be the best way of stopping the viewport from going past the edges? what i have tried so far hasn't been successful. what would you guys suggest?

also, i know that there is an issue with viewports on some low end machines. this is not an issue in this case.

SuperStrict
Graphics 800, 600, 0, 60

Local p1:player = New player.Create()
p1.x = 1000;p1.y = 900

While Not KeyHit(KEY_ESCAPE)
	
	SetOrigin 400 - p1.x, 300 - p1.y
	SetViewport 400 - p1.x, 300 - p1.y, 1600, 1200
	
	For Local i:Int = 0 To 11
		For Local o:Int = 0 To 12
			If i Mod 2 = 0
				If o Mod 2 = 0
					SetColor 0, 0, 0
				Else
					SetColor 255, 255, 255
				End If
			Else
				If o Mod 2 = 0
					SetColor 255, 255, 255
				Else
					SetColor 0, 0, 0
				End If
			End If
			
			DrawRect (o * 200) , (i * 200), 200, 200
			
		Next
	Next

	p1.update()
	p1.draw()
	
	Flip
	Cls
Wend

Type player

	Field easing:Float = 0.10;
	Field dx:Float;
	Field dy:Float;
	Field x:Float
	Field y:Float

	Function Create:player()
		Local n:player = New player;
		Return n;
	End Function
	
	Method update() 
			
		Local mx:Int = MouseX() * 2
		Local my:Int = MouseY() * 2
   		
		dx = mx - x;
   		dy = my - y;
   
  	 	dx:*easing:Float;
   		dy:*easing;
   
   		x:+dx;
   		y:+dy;
		
	End Method
   
	Method draw() 
		SetColor 0, 0, 255
		DrawRect x, y, 10, 10
	End Method
	
End Type

cheers
Charlie