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