E820

e820 is shorthand for the facility by which the BIOS of an x86-based computer system reports the memory map to the operating system or boot loader.
It is accessed via the int 15h BIOS [interrupt call|call], by setting the register to value E820 in hexadecimal.
It reports which memory address ranges are usable and which are reserved for use by the BIOS.
BIOS-e820 is often the first thing reported by a booting Linux kernel, and it can also be seen with the dmesg command.