Read this article to learn about common pitfals.
The multiboot specification contains a memory map. The memory map has some characteristics that are hard to understand.
A trick for debugging is to type c when GRUB shows the selection screen to enter a console mode. Once in console mode, Type lsmmap to tell GRUB to print the multiboot memory map.
The Memory Map Contains More Addresses than are Physically Available!
The memory map describes more addresses than are actually available. Some of the addresses in the memory map are not backed by physical memory! Those areas are marked as reserved.
For example if you start qemu and emulate a machine with 1G of RAM,
qemu-system-i386 -m 1G -cdrom image.iso
the memory map will contain entries with addresses for up to 4G. The memory areas in the memory map that are not backed by physical memory are marked as reserved. That means if you ignore all reserved areas in the memory map and only concentrate on the available memory areas, you will have no problems. The areas marked as available are always backed by physical memory as far is I can tell from my personal experiments.
Available Memory Is Not The Same As Free Memory!
The kernel can read and use that memory map to find available regions of physical memory. Available means usable by the the BIOS, the bootloader and the operating system! All these components make use of the available memory, so some of the available memory will be occupied by the time your operating system takes over control from the bootloader! Available and “Not Occupied” or “Free” are two different things! The information that the multiboot memory map gives you is about theoretically available memory not about free memory. That memory might already be filled with important data!
Lower 1 Megabyte
For GRUB, a memory area is available, if you do have access to it and can write into it. The lower MB of RAM typically contains things like the Video RAM at 0xB8000 placed there by the BIOS. You can choose to store data in this section of the memory but being video memory, it will write those characters to the screen! You should just not use this part of the memory although it is available RAM! GRUB does not care! GRUB will just tell you that this Area of memory is available.
The lowest 1 Megabyte also contains interupt vectors amongst other things.
A good approach would be to treat the first MB of RAM is memory that is in use and to just start using the available memory starting from 1MB.
Dealing with memory occupied by GRUB Modules
If you tell GRUB to load modules, those modules are loaded into available memory areas by GRUB. GRUB still says those memory areas are available! You can decide to reuse that space and just write new data over the modules rendering the modules unusable. GRUB does not care!
That means you have to take a close look at the modules section of the multiboot information data struct to learn about which locations in available memory are occupied by the modules in order to not accidentely write data into those sections..