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embeddedbootbootstrapping

Developing a simple bootloader for an Embedded system

I've been tasked in developing a simple bootloader for an embedded system. We are not running any OS or RTOS so I want it to be really simple.

This code will be stored in a ROM and the processor will begin execution at power on.

My goal is to have a first part written in ASM which would take care of the following operations:

  • Initialize the processor
  • Copy the .data segment from ROM to RAM
  • Clear the .bss segment in RAM
  • Call main

Main would be obviously written in C and perform higher level operations like self-test etc...

Now what I really don't know how to do is combine these two programs into a single one. I found a crappy tool that basically uses objcopy to gather the .text and .data sections from executables and appends some asm in front but this seem to be a really ugly way to do it and I was wondering if someone could point me in the right direction?

Solution

  • You can (in principle) link the object file generated from the assembler code like you would link any object from your program.

    The catch is that you need to lay out the generated executable so that your startup code is in the beginning. If you use GNU ld, the way to do that is a linker script.

    Primitive setup (not checked for syntax errors):

    MEMORY
{
    FLASH (RX) : ORIGIN = 0,          LENGTH = 256K
    RAM (RWX)  : ORIGIN = 0x40000000, LENGTH = 4M
}

SECTIONS
{
   .bootloader 0 : AT(0) { bootloader.o(.text) } >FLASH AT>FLASH
   .text : { _stext = .; *(.text .text.* .rodata .rodata.*); _etext = . } >FLASH AT>FLASH
   .data : { _sdata = .; *(.data .data.*); _edata = .; _sdata_load = LOADADDR(.data) } >RAM AT>FLASH
   .bss (NOLOAD) { _sbss = .; *(.bss .bss.*); _ebss = . } >RAM
}

    The basic idea is to give the linker a rough idea of the memory map, then assign sections from the input files to sections in the final program.

    The linker keeps the distinction between the "virtual" and the "load" address for every output section, so you can tell it to generate a binary where the code is relocated for the final addresses, but the layout in the executable is different (here, I tell it to place the .data section in RAM, but append it to the .text section in flash).

    Your bootloader can then use the symbols provided (_sdata, _edata, _sdata_load) to find the data section both in RAM and in flash, and copy it.

    Final caveat: if your program uses static constructors, you also need a constructor table, and the bootloader needs to call the static constructors.