Unresolved Symbol ?C_STARTUP，见：https://developer.arm.com/documentation/ka003779/1-0
For a project in the SMALL memory model that uses no floating point arithmetic, C51S.LIB is required.
For a project in the LARGE memory model that uses floating-point, C51FPL.LIB and C51L.LIB are required.
If a project uses floating arithmetic, it is important that C51FPx.LIB is included before the standard library file C51x.LIB. Otherwise printf and scanf will not work with floating-point numbers.
keil 编译 C 时，先生成
- Generate Assembler SRC File
- Assembler SRC File
C run-time libraries，因此必须手动添加
STARTUP.A51，链接器会链接默认的 C run-time 库）
When you compile all your C modules with the SRC directive and the assembler, the linker does not detect the required C run-time libraries (because the files were assembled). In this case, you must add the libraries to your project manually.