1. Apparatus for processing data comprising: instruction decoder circuitry responsive to a program instruction to generate one or more control signals; processing circuitry responsive to said one or more control signals to perform a data processing operation specified by said program instruction; and a flow control register configured to store a programmable flow control value; wherein said instruction decoder is responsive to a marker instruction indicative of an end of a current sequence of program instructions being executed to read said programmable flow control value from said flow control register and in dependence upon said programmable flow control value generate said one or more control signals to trigger after completion of said current sequence of program instructions one of: (i) processing of a target sequence of program instructions starting from a target program instruction; and (ii) entry to an idle state awaiting a new processing task to be initiated.