Selective coherency control

A data processing system is provided with a general purpose programmable processor and an accelerator processor. Coherency control circuitry manages data coherence between data items which may be stored within a cache memory and/or a further memory. Memory access requests from the accelerator processor are received by a memory request switching circuitry which is responsive to a signal from the accelerator processor to direct the memory access request either via coherency control circuit or directly to the further memory.