STM32 CDC Bootloader

[RussDuino]

I apologize for I am new at learning bootloader and I am trying to create a custom bootloader for stm32f103 custom dev board. First, I just want to know how can create a compatible binary file to be flashed at existing bootloader.

https://github.com/stevstrong/STM32-CDC-Bootloader

I installed the bootloader from the repo link above and I have managed to flash an app but it has overwritten the bootloader.

I have already edited the linker file of my application to start at the address wherein it wouldn't touch the memory allocated for the bootloader.

/* Memories definition */
MEMORY
{
  RAM    (xrw)    : ORIGIN = 0x20000000,   LENGTH = 20K
  FLASH    (rx)    : ORIGIN = 0x8001000,   LENGTH = 20K
}

the binary file of the repo has a 4kb size and I follow the 4*1024 =4096
decimal to hex = 1000 for memory addressing. Has anyone have been able to create an app binary file which is fit at the bootloader program?
What is the best way to solve this?

[Uwe Bonnes]

If you want to debug, attach a debugger and read out memory and compare what is really written in flash. E.g. gdb compare-sections command will compare flash against the elf file.

Otherwise I propose to switch to a DFU bootloader. E.g. the blackmagic probe  has a small DFU bootloader for F1. If your device already has a DFU bootloader in system memory, you do not need your own bootloader at all, you just start the system bootloader.