1. A method of operating a graphics processing system that includes a vertex shading stage which processes values of input attributes to the vertex shading stage to generate values of output attributes of the vertex shading stage for use by a subsequent stage or stages of the graphics processing system, the method comprising: associating with one or more attributes of the vertex shading stage, meta information to be used when loading or storing values of the one or more attributes wherein the meta-information indicates a technique to use for, and any data necessary for, deriving an index to be used to load or store a value of the one or more attributes.