... read about TINI OS, which is one of the available multitasking environments on 51 derivatives.

Note that the Dallas derivatives where it is implemented are behemoths with lots of code in internal ROM and a relatively big external RAM attached. Don't expect anything near AT89S52.

While the idea - writing independent code running simultaneously - sounds fine, it in reality takes a lot of work and resources to accomplish it, and is not veri plausible for a microcontroller (you might be fooled by the modern 32-bit microcontrollers such as the ARMs, often running some multitasking of one form or other; but those are in fact not microCONTROLLERS but microPROCESSORS with lots of memories etc. integrated on one chip - the difference is subtle but substantial).

In the word of microcontrollers, multitasking-like ideas are often hidden behind the buzzword (buzz-expression) "Real Time Operating System", RTOS. Try to search on this topic, too.

Even when you have some multitasking OS up ad running, programs written for it tend to have various limitations and quirks, so it's not that simple than just taking 3 different standalone '51 programs, pour it together with an OS kernel, stir and enjoy.

You might also want to read about the alternative(s), presented on his 8052.com homepage by a seasoned '51 developer, Ian Bell.

And, of course, searching this site (or the whole net) with the related keywords will give you tons of material to study.

JW