1. An interrupt control function adapted to control the execution of interrupt requests of differing criticality by a processor which is required to execute tasks of differing criticality under the control of a computer operating system; the interrupt control function being adapted to recognise critical and non-critical interrupt requests originating from different interrupt sources, and to recognise when the processor is required to execute each of critical and non-critical tasks; the interrupt control function being further adapted to pass critical interrupt requests to the processor for execution in preference to non-critical interrupt requests, to block non-critical interrupt requests to the processor when they coexist with critical interrupt requests or the processor is required to execute critical tasks, and to pass non-critical interrupt requests to the processor when they do not coexist with any critical interrupt requests and the processor has no critical tasks to be executed.