1. A conjugate gradient solver apparatus for generating data defining a solution vector x for a linear system represented by Ax=b where A is a predetermined matrix and b is a predetermined vector, the apparatus comprising: solver circuitry for processing input data, defining said matrix A and vector b, in accordance with an iterative conjugate gradient method to generate said data defining the solution vector x, the solver circuitry processing data items, corresponding to vectors used in said iterative conjugate gradient method, having a variable fixed-point data format; and a precision controller for determining the fixed-point data formats of respective said data items adaptively during progress of the iterative conjugate gradient method in the solver circuitry.