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Keil RTX5 and RTX FuSa RTS time-determinism


I'm trying to compare Keil RTX5 and RTX FuSa RTS in the domain of time-determinism. They both state that they are "fully deterministic" RTOSes. But what does that mean in technical terms ?

What I understand is that all the functions provided by the RTOS are time-bounded (i.e. deterministic). Which means that in the case we do not use interrupts at all, we know exactly what the RTOS is doing at each moment.

However it's also stated in RTX FuSa RTS that : 

  • Time-deterministic interrupt execution: RTX utilizes the LDEX/STEX instruction available on most Cortex-M processors and therefore user interrupts are never disabled

In what way are interrupts execution time-deterministic ? What are the LDEX/STEX instructions used for ? Are there some performance number or documentation about interrupt latency that are available ?

Thanks, best regards.