In most embedded systems, an initialization sequence executes to set up the system before the main task is executed.

Figure 2.5 shows the default initialization sequence.

At a high level, the initialization sequence can be divided into three functional blocks. __main branches directly to __scatterload. __scatterload is responsible for setting the runtime image memory map, whereas __rt_entry (runtime entry) is responsible for initializing the C library.

__scatterload carries out code and data copying, decompression of RW data if necessary, and zeroing of ZI data.

__scatterload branches to __rt_entry. This sets up the application stack and heap, initializes library functions and their static data, and calls any constructors of globally declared objects (C++ only).

__rt_entry then branches to main(), the entry to your application. When the main application has finished executing, __rt_entry shuts down the library, then hands control back to the debugger.

The function label main() has a special significance. The presence of a main() function forces the linker to link in the initialization code in __main and __rt_entry. Without a function labeled main() the initialization sequence is not linked in, and as a result, some standard C library functionality is not supported.