HLSL Shaders

LOL shaders wtf.

I cant manipulate my texture coords properly.
float2 UV0 : TEXCOORD0;
float2 changedUV = (UV0.x - Scrollu,UV0.y - Scrollv);
Col = tex2D(g_DiffuseSampler, changedUV );                   
fairly strightforward, but not.
So..... basics time. what does TEXCOORD0 pass into 'UV0'

Hmm... Maybe


(define UV0 TEXCOORD0)
(define changedUV (list (- (prop UV0 x) Scrollu) (- (prop UV0 y) Scrollv)))
(set! Col (tex2D g_DiffuseSampler changedUV))

?