1. A circuit for providing an instruction stream to a processing device, comprising: an input to receive an external instruction stream for provision of a first set of instruction values; a memory adapted to contain a second set of instruction values; two or more outputs for provision of output instruction streams to the processing device; a control input; and a selection means adapted to distribute the first set of instruction values and the second set of instruction values between the two or more outputs according to the control input, the second set of instruction values not being provided by the external instruction stream.