1. A computer-implemented method that enables a user to specify via an application programming interface (API) of a computing system at least one instruction to a procedural shader computational subunit of a graphics subsystem of the computing system, whereby said procedural shader computational subunit performs according to said at least one instruction, the method including: the user using the API to specify at least one indexed instruction that refers to-at least one indexable readable, writable and/or variable input and/or output register that stores non-constant graphics data of the procedural shader computational subunit and said at least one indexed instruction causing said procedural shader computational subunit to at least one of read from or write to the at least one indexable readable, writable and/or variable input and/or output register, as specified by an index of the at least one indexed instruction, whereby operations affecting multiple input and/or output registers storing non-constant graphics data are specified with said index of said indexed instruction.