A data processing system replicates the operation of a target graphics processor under test by making use of a native graphics processor of the data processing system. The native graphics processor is provided with executable program instructions that replicate some or all of the fixed function operations of the target graphics processor. The programmable processing circuitry of the graphics processor of the data processing system can then generate an output by executing the executable program instructions, rather than by using fixed function processing circuitry of the graphics processor of the data processing system. The data processing system can provide a “bit-exact” testing environment when replicating the operation of the target graphics processor using the native graphics processor, for example where the target graphics processor and native graphics processor are configured very differently from one another.