When linking the kernel you may receive the following linker warning:
/bin/ld: warning: .note.gnu.build-id section discarded, --build-id ignored.
This is safe to ignore. The linker script instructs the linker to discard this section as it is a) not needed and b) bloats the kernel binary, which has a bad side effect of causing the Multiboot header to be placed at an offset >8K.
The kernel requires a minimum of 2 MB of memory available to boot. This is simply a technical requirement as the kernel is mapped to physical address 1 MB at boot (and should take up no more than 1 MB of space).
If required, this can be changed by altering the linker script and (possibly)
the stack setup code in boot.s.
The kernel uses a flat memory model addressable up to 4 GB physical. There is no concept of a virtual memory system, and all memory is directly accessible by its physical address.
Physical memory is divided into two segments: kernel code (ro) and kernel data (rw). This is enforced by the CPU through the GDT.