Instruction Level Timing Mechanisms for Accurate Real-Time Task Scheduling

The scheduling of timed tasks is generally based, at the hardware level, upon the use of time intervals. For example, most microprocessor families provide their only hardware support for timing control in the form of a programmable interval timer chip accessible as an I/O device over the system bus. In this paper we will argue that a more natural and elegant solution bases timing on a local (to a particular CPU) absolute timer. Furthermore, we will show that the desired timing functions can be provided by simple extensions to existing CPU architectures. The widespread use of the ``time interval'' view has also influenced, in a negative way, the design of many programming languages. An important example is Ada, a language designed with real-time multitasking explicitly in mind. We will describe the difficulty with the current timing methods used in Ada, and present a method for overcoming the timing weakness by using the proposed timing mechanisms, while still remaining within the definition of the Ada language.