- 先运行systeminit函数，其功能：初始化时钟（SYSCLK, HCLK, PCLK2 and PCLK1 prescalers）、配置中断向量表（中断向量表的定位是在flash还是SRAM）
- 再运行__main函数：
- 作用：Initialization of the execution environment and execution of the application
- You can customize execution intialization by defining your own __main that branches to __rt_entry.
- The entry point of a program is at __main in the C library where library code:
- Copies non-root (RO（不会拷贝，官方提供和实际实践有出入） and RW) execution regions from their load addresses to their execution addresses. Also, if any data sections are compressed, they are decompressed from the load address to the execution address.
- Zeroes ZI regions.
- Branches to __rt_entry.
- If you do not want the library to perform these actions, you can define your own __main that branches to __rt_entry。
- 再运行The library function __rt_entry() runs the program as follows:
- Sets up the stack and the heap by one of a number of means that include calling __user_setup_stackheap(), calling __rt_stackheap_init(), or loading the absolute addresses of scatter-loaded regions.
- Calls __rt_lib_init() to initialize referenced library functions, initialize the locale and, if necessary, set up argc and argv for main().This function is called immediately after__rt_stackheap_init() and is passed an initial chunk of memory to use as a heap. This function is the standard ARM C library initialization function and it must not be reimplemented.
- Calls main(), the user-level root of the application.
- From main(), your program might call, among other things, library functions.
- Calls exit() with the value returned by main().
因为库函数里面的 ___main函数 和 ____rt_entry函数是弱函数，所有我们可以自己编写__main和__rt_entry
