upwind differencing 2 dimensional FVM

On a 8 x 8 cell grid, work out the values of ϕ that are obtained by discretizing the equation u*dϕ/dx+v*dϕ/dy= 0, where u = 0.5 and v = 0.5 using upwind differencing. The values of ϕ at x = 0, are unity and at y = 0, are 0. Show how the sharp front gets smeared by upwinding.

[Torsten]

What have you done so far ?