Abstract: In a processor, instructions of an instruction stream are supplied to an execution unit which executes the supplied instructions according to respective execution modes. A control unit recognizes a user-defined instruction sequence (UDIS) in the instruction stream. The UDIS is associated with a UDIS definition provided in-line and/or as contents of machine-state registers (MSRs), and specifying, at least in part, a start, optionally an end, and a particular execution mode for the UDIS. Subsequently, ones of the instructions of the UDIS are executed in accordance with the particular execution mode, such as by optionally altering recognition of asynchronous events. For example, disabling hardware interrupts during the executing results in apparent atomic execution. Fetching, decoding, issuing, and/or caching of the instructions of the UDIS are optionally dependent on the particular execution mode. MSRs optionally specify a maximum length and/or execution time.