startup files in Keil

The IDE does not use any startup file.

It's the programs you develop that are using startup files.

When you write programs for a PC, there are also startup files. But since the tools then knows the target is a PC, the startup file can have a fixed contents. So it is hidden inside the language runtime library. And some of that "startup file" is actually moved all the way to the BIOS when the computer boots. Every single motherboard have a custom "startup file"/BIOS explicitly adapted to the available hardware. An to that, you also have a custom hardware driver layer used by the OS.

For embedded projects, each target hardware is so different that only you, the developer, can know what is needed to prepare the hardware in a way that the C runtime library can then be used.

Each C program should start from main() - that is a bit of a simplification. If you look at the C language standard, you will see that there are numbe of rules that must be fulfilled before the program reaches main(). For example - all global variables should be zero-initialized. And main() must be directly able to call other functions, so some magic must have set up the stack.

A C program is linked with a C RTL - C Runtime Library. And it must have been properly initialized before the program reaches main().

The linker configuration file contains information about what object files that should be linked first.

And the startup file (normally in assembler, but could be in C for Cortex-class processors) contains information like:

                AREA    RESET, CODE, READONLY

to inform the linker that a generated code block should match the requirements for the linker configuration file.

The IDE does not use any startup file.

It's the programs you develop that are using startup files.

When you write programs for a PC, there are also startup files. But since the tools then knows the target is a PC, the startup file can have a fixed contents. So it is hidden inside the language runtime library. And some of that "startup file" is actually moved all the way to the BIOS when the computer boots. Every single motherboard have a custom "startup file"/BIOS explicitly adapted to the available hardware. An to that, you also have a custom hardware driver layer used by the OS.

For embedded projects, each target hardware is so different that only you, the developer, can know what is needed to prepare the hardware in a way that the C runtime library can then be used.

Each C program should start from main() - that is a bit of a simplification. If you look at the C language standard, you will see that there are numbe of rules that must be fulfilled before the program reaches main(). For example - all global variables should be zero-initialized. And main() must be directly able to call other functions, so some magic must have set up the stack.

A C program is linked with a C RTL - C Runtime Library. And it must have been properly initialized before the program reaches main().

The linker configuration file contains information about what object files that should be linked first.

And the startup file (normally in assembler, but could be in C for Cortex-class processors) contains information like:

                AREA    RESET, CODE, READONLY

to inform the linker that a generated code block should match the requirements for the linker configuration file.

But how is it happen, for example in Keil, that the linker know (or compiller) that before main() it sholuld execute the code contained in startup file although the file can have different name? In other words what makes that the startup file is execute first?

In other words what makes that the startup file is execute firs

The location (mostly, in flash) of its binary image!

As I mentioned, there is a linker configuration file telling the linker how to order things.

It may look like:

; *************************************************************
; *** Scatter-Loading Description File generated by uVision ***
; *************************************************************

LR_IROM1 0x00000000 0x00001000  {    ; load region size_region
  ER_IROM1 0x00000000 0x00001000  {  ; load address = execution address
   *.o (RESET, +First)
   *(InRoot$$Sections)
   .ANY (+RO)
  }
  RW_IRAM1 0x40000000 0x00008000  {  ; RW data
   .ANY (+RW +ZI)
  }
  RW_IRAM2 0x7FE00000 0x00004000  {
    adc.o (+ZI)
    fram.o (+ZI)
   .ANY (+RW +ZI)
  }
}

LR_IROM2 0x00003000 0x0003D000  {
  ER_IROM2 0x00003000 0x0003D000  {  ; load address = execution address
   .ANY (+RO)
  }
}

Notice the "(RESET, +First)" part?

Match that up with the line in the startup assembler file:

                AREA    RESET, CODE, READONLY

So it doesn't matter what the startup file is named - if it contains an area "RESET" it will match the region placement of the linker configuration file.

But you can use the IDE to specify segment namings for C files too, and add other sorting orders in the linker configuration (scatter) file.

All that is required, is that you have a suitable startup code located where the physical processor initially jumps or reads a reset vector.

You want to say that is in some way independent. Startup is compiled its code is put in flash according to the given location and beside it jumps to main() function?
I think I have missed something

Per reply explains it all. The common reference in the startup file and the scatter file to RESET places the startup at the entry address to internal flash.