1. A data processing apparatus for performing a reciprocal operation on an input value ‘d’ to produce a result value ‘X’, the reciprocal operation involving iterative execution of a refinement step to converge on the result value, the refinement step performing the computation: 
 X i =X i−1 *M where X i is an estimate of the result value for an i-th iteration of the refinement step, and M is a value determined by a portion of the refinement step, the data processing apparatus comprising: a register data store having a plurality of registers operable to store data; processing logic operable to execute instructions to perform data processing operations on data held in the register data store; the processing logic being responsive to a single refinement instruction to implement said portion of the refinement step by performing at least a multiply-accumulate operation in which the input value, a value derived from a previous estimate of the result value and a constant are used as inputs of the multiply-accumulate operation, the constant being determined from the single refinement instruction without reference to the register data store.