e820 is shorthand to refer to the facility by which the BIOS of x86-based computer systems reports the memory map to the operating system or boot loader.
It is accessed via the int 15h call, by setting the AX register to value E820 in hexadecimal. It reports which memory address ranges are usable and which are reserved for use by the BIOS.
You will often see BIOS-e820 as the first thing reported by a Linux kernel booting, and it can also be seen with the dmesg command.
Sometimes the BIOS is buggy and incorrectly reports the reserved memory. This can cause memory testing software, like Memtest, to report errors.
출처 : Wikipedia
http://en.wikipedia.org/wiki/E820