I know Linux does not have any application on the '51, but I think this does apply as to general information on what the RTOS people consider "real"

MontaVista has reduced the number of places in the stock Linux kernel where interrupts are disabled by two orders of magnitude, leaving only about 100 critical regions where interrupts must be disabled. With the changes, a typical 300-MHz 32-bit processor has a worst-case interrupt disable latency on the order of 180 µs.

I have had to get it under 2uS for a real real time app on a 51 which, for sure is not a "300-MHz 32-bit processor".

http://www.elecdesign.com/Articles/...10975.html

Erik