1. Apparatus for processing data, said apparatus comprising: a stack memory; instruction decoder circuitry responsive to program instructions to generate control signals; and data processing circuitry coupled to said stack memory and controlled by said control signals to perform data processing operations upon data values stored in said stack memory; wherein said instruction decoder circuitry is responsive to a null check instruction to generate control signals that control said data processing circuitry to perform a dedicated null checking operation comprising: (i) reading a data value from said stack memory; (ii) comparing said data value with a null value; (iii) if said data value matches said null value, then jumping to execution of a null value exception handler stored at an address independent of said null check instruction; and (iv) if said data value does not match said null value, then proceeding to a next program instruction.