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I started writing kernel based on Bare Bones tutorial, and currently I am planning how to write basic kernel memory allocator.
In order to write allocator first I must found out which memory regions are free to use.
I am using memory map provided by GRUB to get available memory regions.
Also it is possible that GRUB marks some regions as free, but those same regions overlap with .data, .bss, etc. sections.

My idea was to put symbol at end of the SECTIONS command, to which I will refer in C code, to find out first address not used by kernel.
All memory regions reported by GRUB which are declared as "available" and which are located after all sections should be free?

Researching I found out several basic kernels do this, but is this guaranteed to work.
Also in linker script provided in Bare Bones tutorial at the end of SECTIONS command there is a comment:



So are these sections placed before my symbol, or what?

Basically, it doesn't matter really. Just add symbols to the end of you're SECTIONS.

You need to add 2 symbols to identify where the kernel starts and ends into you're linker script. So, your linker script should be something like:

Then in your main code, add:

Hello,

Please note that you want the address from the _kernel_start and _kernel_end variables, so pseudocode would look like this:

Now the kernel_start and kernel_end variables contain the adressess of the start and end of the kernel.

-thomtl