Forming a windowing display in a frame buffer

A windowing display using deferred drawing commands operates by processing the drawing commands that write to a tile 22 of a frame buffer 30 to form one or more new pixel values are stored within a tile memory 40. Dirty pixel data indicative of which pixels within the tile memory are dirty pixels storing new pixel values and which pixels within the tile memory are clean pixels not storing new pixel values is also formed. In dependence upon the dirty pixel data, the new pixel value stored within the tile memory are written to the frame buffer memory. Pixels stored within the frame buffer memory corresponding to clean pixels within the tile memory remain unaltered as they are not written.