1. A method of forming an executable program from a plurality of object code modules, each object code module comprising section data and relaxation instructions, at least one of said object code modules comprising a library module of predefined section data and relaxation instructions, the executable program to be run on a target processor having a selected endianness, the method comprising: in response to a relaxation instruction, loading a library module into temporary storage; retrieving the value of a first variable, said first variable denoting the selected endianness of the target processor; comparing the value of said first variable with the endianness of the section data and relaxation instructions of the library module; in response to the result of the comparing step not being a match, converting the endianness of the section data of the library module to that of said first variable; processing the relaxation instructions and converted section data to form part of the executable program.