In the field of embedded software, bare running and RTOS are always eternal after dinner. In fact, software programming is like the way of governing the country. I have never heard of any provinces, municipalities and autonomous regions like Singapore. It is also like starting a company to do business. Small companies are bosses who have the final say, even if they are sub-sectors, it doesn't make much sense. The same is true for writing programs. Bare running is like a big module. When the program is small, the programmer can control everything. The interrupt + large loop is the most efficient and intuitive. When the program is large, the logical relationship between them is complicated. If you use a large module, it will become difficult to control and easily cause bugs. At this time, it is necessary to divide the module and divide the module, and it is necessary to deal with coordination among modules, resource allocation, and reasonable allocation of CPU time. These problems will become simpler with the participation of RTOS. In fact, even without rtos, a savvy programmer can write a management program tailored to the order, in fact, this management program can be counted as a simple "operating system." If the program is further complicated and the code is further large, this simple management program will not work. Choosing a rtos is the smartest choice.
So, when the program scale is large, what is the operating system? This cannot be generalized. Some code, very long, but the logic is very simple, some code, although the amount of code is not large, but the logic is very complicated. Personal experience, the critical point is between 10,000 and 20,000 lines. 
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