A programmable timer is disclosed for use in conjunction with a microcontroller circuit. The timer is used as part of a time slice arbiter in a real time operating system, which arbiter manages device routines by allocating them to distinct code time slices executable by such microcontroller. The set up of time slices, including their number, sequence, duration, etc., can be configured and optimized to achieve a desired system performance level based on characteristics of an associated system bus, devices on the bus, etc. The timer operates as a hardware controller to direct the interrupt handler to various entry points in the corresponding routines associated with interrupt based devices on a system bus.